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Equipment Discussions >> Reflectors

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Jason D
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Concise thread about autocollimators+improvements
      #3532750 - 01/01/10 12:21 AM

Towards the end of this lengthy thread, I promised to start a new concise thread, and hopefully less intimidating, about conventional autocollimators and improvements (2-pupil and CAM).

The autocollimator is by far the most misunderstood collimation tool. Not only misunderstood by many users but also misunderstood by some of the vendors who manufacture them. One vendor explains that achieving a darkened background implies perfect collimation – very wrong. Another vendor explains how you can achieve perfect collimation by a final tweak to the secondary mirror to “stack” all reflections – very wrong. Many believe miscollimation errors are magnified which each additional round trip reflections between the autocollimator and primary mirrors – very wrong.

Reflections formed when light bounces back and forth between a flat mirror and a concaved mirror are very different from reflections formed when light bounces back and forth between two flat mirrors which seems to be one source of the confusion. Understanding how autocollimator reflections are formed and figuring out how to get the most out of an autocollimator is not intuitive. Fortunately, most of the autocollimator secrets have already been revealed by the work of Nils Olof Carlin, Vic Menard, and Jim Fly. Nils Olof Carlin was the first to publish a thorough mathematical analysis of the autocollimator reflections (link). Vic Menard was the first to publish the optimal procedure for using autocollimators (link). Jim Fly was the first to simulate and publish autocollimator images and animations to improve understanding of the tool.

The objective of this thread is to explain the autocollimator and improvements with many supporting photos, illustrations, and animations. It took quite a bit of time and effort in my part to complete this thread.


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Jason D
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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532755 - 01/01/10 12:22 AM

Let us start off with reviewing the fundamentals of collimation:

Collimation consists of:
1- Secondary mirror alignment: Optimizes illumination distribution within the FOV – less critical
2- Axial alignment : Minimizes “coma” and brings the whole FOV to focus – more critical

Secondary mirror alignment involves centering and rounding the secondary mirror under the focuser. Doing so will optimized the 100% illumination field placement within the eyepiece FOV. Any star located within the 100% illumination area will have its entire reflected light cone intercepted by the secondary mirror. Star located outside the 100% illumination area will have their light cone partially eclipsed by the secondary mirror edge; subsequently, these star images will underutilize the primary mirror aperture. The following animation shows how a centered star and an adjacent one have their entire reflected light cone intercepted by the secondary mirror. However, a third star which is way off-axis has its reflected cone eclipsed by the secondary mirror edge.



Referring to the next animation, centering the secondary mirror under the focuser will center the 100% illumination field within the FOV – an optimal setup.



The following illustration explains the “good”, the “bad”, and the “ugly” for secondary mirror alignment.
1- The “good” diagram shows a centered 100% illumination in the FOV – ideal.
2- The “bad” diagram shows an off-centered 100% illumination in the FOV. Visual observers can live with it but not astrophotographers.
3- The “ugly” diagram shows an off -FOV 100% illumination. The reflected light-cone for the on-axis star will be partially eclipsed by the secondary mirror edge. This setup will not utilize the primary aperture to its fullest.



The autocollimator, which is the main subject of this thread, is NOT the proper tool to align the secondary mirror. That is the job of a quality sight-tube or a quality holographic laser collimator. Refer to the following thread for more insight about secondary mirror alignment – just the first few pages of the thread.


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Jason D
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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532757 - 01/01/10 12:24 AM

Secondary mirror misalignment as was described in the previous post (even the “ugly” case) will not introduce the infamous on-axis view killer known as coma. Axial misalignment is the main culptrit for on-axis coma. Attaining axial alignment using an autocollimator will be the main subject for the remainder of this thread.

Axial alignment involves:
1- Coinciding the focal points of the primary mirror and the eyepiece
2- Coinciding the focal planes of the primary mirror and the eyepiece

If we could see the focal points and focal planes of the primary mirror and the eyepiece then axial alignment would be a breeze. Unfortunately, we can’t. We use indirect means to achieve axial alignment. Coinciding the primary mirror and the focuser axes when using a quality focuser implies the eyepiece’s focal plane/point will be well-aligned with the primary’s mirror focal plane/point.

Refer to the following:
The “Good” diagram is ideal. Both the eyepiece and primary mirror focal points and planes coincide.
The “Bad” diagram might be acceptable for visual observers but might not be for astrophotographers. Both eyepiece/primary focal points coincide but the focal planes are at an angle (or tilted)
The “Ugly” diagram shows a bad case where the focal points of the eyepiece and primary are separated laterally. Even a 1mm separation is enough to ruin the view of a fast reflector.



Different collimation tools are available to guide us to coincide both the focuser and primary axes. There are four fundamental collimation axial errors targeted by different collimation tools:

CAE (COC Axial Error) corresponds to the distance between the focuser/primary axes at the COC plane. COC stands for Center Of Curvature. COC point is located on the primary mirror axis at twice the focal length distance. It is the virtual center of the sphere which the primary mirror surface curvature follows.

PAE (Primary Axial Error) corresponds to the distance between the focuser/primary axes at the focal plane.

FAE (Focuser Axial Error) corresponds to the distance between the focuser/primary axes at the primary mirror surface.

LAE (paralleL Axial Error) corresponds to the distance between a parallel projection to the focuser axis starting from the COC point and the center of the primary mirror. It is a measure of parallelism between the focuser and primary axes. If LAE=0 then both the focuser and primary axes are parallel and possibly coincident.



Every known collimation tools is designed to provide visual cue to eliminate at least one of mentioned four errors. For example, the cheshire is designed to eliminate PAE. Unbarlowed laser is designed eliminate FAE and CAE. Barlowed laser is designed to eliminate PAE. The conventional autocollimator is capable of eliminating CAE, FAE, and potentially PAE. The enhanced autocollimator (to be discussed later) is capable of eliminating CAE, LAE, FAE, and potentially PAE.

Eliminating any two of the 4 errors is enough to achieve axial alignment. That is, eliminating any two if the 4 errors ensures that both the focuser and primary axes coincide. Furthermore, eliminating any two of the 4 errors also implies the remaining two errors will also be eliminated. For example, if CAE and FAE are zeros then LAE and PAE must be zeros.


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532758 - 01/01/10 12:24 AM


(courtesy of Catseye)

A conventional autocollimator (AC) with a single/centered pupil consists of a perforated mirror mounted inside an eyepiece. Looking through the autocollimator pupil hole after the scope has gone through rough collimation will reveal several reflections of the primary mirror center spot. Typically, four reflections are seen. Identifying each reflection and interpreting its relative location is important.

Reflection P: Sharp and bright. It is the only reflection that does not move when you wiggle the AC in the focuser.

Reflection 1: Has the same orientation as P but could look somewhat fuzzy and bloated.

Reflection 2: Sharp and rotated 180 degrees with respect to P.

Reflection 3: Rotated 180 degrees with respect to P but could look somewhat fuzzy and bloated. It is the dimmest reflection and the hardest to see.

In addition to the above 4 reflections, there is the reflection of the AC pupil which will always be located at the center of the AC foreground reflection.



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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532761 - 01/01/10 12:25 AM

Stacking/aligning/centering reflection “P” against each of the other reflections mentioned in the previous post will tell us something about how the focuser axis is positioned in relation to the primary axis:

Aligning P+pupil means the focuser axis intersects the primary axis at the focal point.
Aligning P+1 means the focuser axis intersects the primary axis at the COC point
Aligning P+2 means the focuser and the primary axes are parallel
Aligning P+3 means the focuser axis intersects the primary axis at the primary mirror center



The apparent distance between P and the pupil reflection equals two times PAE
The apparent distance between P and reflection 1 equals two times CAE
The apparent distance between P and reflection 2 equals two times LAE
The apparent distance between P and reflection 3 equals two times FAE

But how can we make use of the above information?

Mathematically, any two lines that share two distinct points are coincident. Using the same principle, if we can ensure the focuser and primary axes share at least two distinct points then both axes have to be coincident -- which is the definition of axial alignment.

Each of the four shown alignments can be accomplished by either adjusting the secondary mirror or the primary mirror with only one exception which is P+3 alignment. Adjusting the primary mirror will not alter reflection 3 relative position to reflection P. Only secondary mirror adjustment is capable of stacking P+3. We will take advantage of this exception later.


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532763 - 01/01/10 12:26 AM

In theory, if we align “P” with any two of the four reflections (pupil, 1, 2, 3) then we would achieve axial alignment. But which two should we select?

As it turns out, the best two alignments to use are P+1 and P+3.

P+pupil alignment is not a good choice because it is hard to discern the black pupil refection against the darkened background.

P+2 alignment is not a good choice either because reflection 2 disappears right before axial alignment is reached. Each frame of the following animation represents a case where the focuser and primary axes are parallel (P+2 is aligned in each frame). The only difference between the frames is the distance between both axes. As the parallel focuser axis gets closer to the primary axis, reflection 2 will start to fade away gradually until it disappears when both axes are within 0.5mm from each other. Also note how it is hard to discern the pupil reflection against the darkened background



P+1 is a good choice because reflection 1 does not disappear when the final axial alignment is reached

P+3 is another good choice because it is the only alignment that is insensitive to the primary mirror adjustment. Only adjusting the secondary mirror will move reflection 3 in reference to reflection P. The remaining (P+pupil, P+1, P+2) alignments are sensitive to both the secondary and the primary mirrors adjustments. P+3 insensitivity to the primary mirror adjustment allows us to isolate FAE elimination from others – very handy!!!!


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532764 - 01/01/10 12:26 AM

How to can we apply the knowledge presented thus far to achieve axial alignment?

The following are the recommended steps for using the autocollimator as described by Vic Menard.

Step 1: Use other collimation tools to achieve rough collimation. The objective of this step is to bring collimation close enough to be able to see most of the center spot reflections from the autocollimator pupil. Without this step, it would be almost impossible to make use of the autocollimator.

Step 2: Start off with stacking P+3 by ONLY adjusting the secondary mirror. Stacking P+3 means the focuser and the primary axes will intersect at the primary mirror center (FAE=0). You will need to adjust (decollimate) the primary mirror a little to move reflections 1 and 2 out of the way to be able to accurately stack P+3. This is our first intersection point between the two axes. Note: Make sure you are stacking P+3 reflections in this step – not P+2 reflections.



Step 3: Follow the above step with stacking P+1 by ONLY adjusting the primary mirror to intersect the focuser axis with the primary axis at the COC point (CAE~0). This is our second intersection point between the two axes. Since adjusting the primary mirror will NOT impact P+3 stack, step 3 is independent of step 2. That is, stacking P+1 by adjusting the primary mirror will not undo the P+3 alignment achieved in step 2.

After the third step, both the focuser and primary axes will have two intersection points which mathematically implies both axes are coincident. Since CAE and FAE are ~zeros, PAE and LAE should also be ~zeros.

Again, the following two images are key to using the conventional single-pupil autocollimator. The left photo represents P+3 (FAE=0) case and the right photo represents P+1 (CAE=0) case.



NOTE: Step#2 (P+3) is called CDP (Carefully Decollimated Primary) collimation protocol introduced by Vic Minard based on the mathematical analysis published by Nils Olof Carlin. For more details, refer to the following link


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532765 - 01/01/10 12:27 AM

There are two pitfalls which autocollimator users need to watch out for. Here is the first pitfall:



Some would stop when the final view looks similar to the left photo thinking a “centered” hexagram represents stacked reflections. After all, reflection 2 is 180 degrees rotated which should form a hexagram when stacked on the top of reflection P.

Seeing a hexagram on the final autocollimator view via the central pupil is incorrect.

The final view should look similar to the right photo. Only a single reflection against darkened background. The reason we do not see a hexagram is because when proper collimation is achieved, both reflections 2 and 3 (both are 180 degree rotated reflections) will disappear. The technical reason is beyond the scope of this thread but it has to do with the central pupil hole.

I will cover the second pitfall in a later post.


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532766 - 01/01/10 12:27 AM

So far, all the information presented is based on the conventional autocollimator with a single/centered pupil.

But, can the conventional autocollimator be further improved? If yes, what kind of shortcomings the conventional autocollimator has and how can it be improved?

If you followed the steps outlined earlier, you should end up with an outstanding axial alignment. However, there is a potential of persisting residual errors introduced by:
1- Reflections 1 and 3 fuzziness gets worse with shorter focal length scopes. It is harder to accurately stack reflections with fuzzy edges; therefore, the accuracy of stacking P+1 and P+3 might degrade for scopes with shorter focal lengths. The technical reason for why reflections 1 and 3 get fuzzier with shorter focal length primary mirrors is beyond the scope of this thread.
2- Stacking P+1 and P+3 are susceptible to parallax though this issue can be minimized by reducing the size of the AC pupil and pulling the eye slightly away from the pupil.
3- The final autocollimator view will not reveal residual PAE and FAE but rather only residual CAE. The remaining of this post and the next few ones are dedicated to explain this point.

Consider the following scenario:

Let us say you’ve been invited to a star party with your friends, Adam and Brian. Both collimate their identical scopes using their own collimation tools and methods. Both believe to have achieved perfect collimation. They ask you to assess their collimation; however, they emphasize that you are not allowed to touch any adjustment screw because they want to preserve their so-called “perfect” collimation. You insert your quality and trusted conventional autocollimator to assess both scopes. You get the views shown below.



Do both scopes have “perfect” collimation?

Both views look quite similar, don’t they? A single center spot reflection against darkened background. But is that enough to declare both scopes have “perfect” collimation?


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532767 - 01/01/10 12:28 AM

The final view of a conventional single-pupil autocollimator could be somewhat deceiving. The single center spot reflection against darkened background does not necessarily mean axial alignment has been achieved. All it means is that both focuser/primary axes intersect at the COC point – the same as P+1 alignment. In other words, the final view tells us that CAE is zero but it does not tell us if we have residual PAE or FAE.

Back to Adam’s and Brian’s scenario: Since both views include a single center spot reflection against darkened background, the only fact we can deduce is that the focuser axis intersects the primary axis at the COC point (CAE~0). But that is only one intersection point. In order to verify axial alignment, we need to assess whether both axes intersect at a different point. Unfortunately, examining both autocollimator views does not provide enough accurate information to assess a second intersection point:
P+pupil can’t be easily seen (PAE??)
P+2 can’t be evaluated due to the disappearance of reflection 2 (LAE??)
P+3 can’t be evaluated due to the disappearance of reflection 3 (FAE??). It is possible to decollimate the primary mirror until reflection 3 appears but this an intrusive method. We need a good method to assess final collimation without the need to touch any collimation adjustment screw.

One solution is to use another quality collimation tool which evaluates a different intersection point – other than CAE. Using a quality cheshire is a good choice. Using a quality barlowed laser is another good choice – both evaluate PAE.

Continuing with Brian’s and Adam’s scenario, to assess a second intersection point you remove the autocollimator and replace it with a quality cheshire. Now it is clear who has an excellent collimation and who does not. In fact, Adam’s scope is WAY OFF. It has a PAE of 1mm which is enough to show coma for F5 scopes. If CAE~0 and PAE=1mm then FAE=2mm.



Brian’s scope has an excellent axial alignment because:
1- The autocollimator tells us both focuser/primary axes intersect at the COC point (CAE~0)
2- The cheshire tells us both focuser/primary axes intersect at the focal point (PAE=0)
Since both axes share two distinct points then both axes must be coincident.

Adam’s scope is axially misaligned because:
1- Though the autocollimator tells us both focuser/primary axes intersect at the COC point (CAE~0)
2- Unfortunately, the cheshire tells us that both axes are 1mm apart at the focal plane (PAE=1mm)
Since both axes share only one point then both axes are not coincident. In other words, as long as we can show at least one of the four errors (PAE, FAE, LAE, or CAE) is non-zero then our scope is axially misaligned.

In the photo loaded in the previous post, I purposely obscured the pupil reflection of the autocollimator because the camera can see it but the unaided eye can’t. In the photo of this post, I did not obscure the pupil reflection. You can see how the cheshire and pupil reflection agree. If we could only see the pupil reflection with the clarity depicted in the photo, then we would not need to use the cheshire.

IMPORTANT: As was shown in Brian’s and Adam’s scenario, the autocollimator view of Adam’s scope did not catch the miscollimation issue but the cheshire view easily caught it. HOWEVER, NO one should draw the conclusion that cheshires are more accurate than autocollimators. Both tools are NOT competing tools but rather complementary tools. In Adam’s setup, CAE~0 and PAE>0 which favors the cheshire. I could easily concoct an opposite scenario where the cheshire view will not catch a miscollimation issue but the autocollimator view would. That scenario will include an axial relationship where PAE=0 and CAE > 0.

Bottom line: It is highly recommended to use a complementary tool with the autocollimators. The word “complementary” refers to a quality collimation tool that aligns/assess an axial intersection point other than the COC point (CAE). Quality cheshires and barlowed lasers are good choices which evaluate (PAE).


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532769 - 01/01/10 12:29 AM

In the previous scenario it was shown how the final view of an autocollimator consisting of a single center spot reflection against darkened background is NOT enough to constitute axial alignment. All it means is that the focuser/autocollimator axes and the primary axis intercept at the COC point (CAE~0) – same as P+1. Refer to the photo below. Both photos show a single center spot reflection against darkened background but the left photo has a whopping 4mm PAE and 8mm FAE – that is a HUGE error.



THIS IS THE SECOND PITFALL OF USING AN AUTOCOLLIMATOR. Many are under the impression that getting darkened background means axial alignment has been met -- this is far from the truth.

In fact, consider the following hypothetical experiment. Suspend a conventional autocollimator from a string attached to the COC point then lightly swing the autocollimator. The autocollimator axis will always run through the COC point (CAE=0), hence, it will always have a darkened background with a single center spot reflection showing.

Note: Hold off on the view depicted in the lower left corner of the animation for now.



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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532772 - 01/01/10 12:29 AM

Can the autocollimator be improved to flag collimation problems – similar to the one described in Adam’s scope -- without the use of a cheshire?

The answer is YES.

First, here is a quick review: Adam’s final autocollimator view only told us that the focuser and primary axes intersect at the COC point (CAE~0). That is all we can deduce from the autocollimator final view. We need to be able to evaluate the proximity of the focuser and primary axes at a different point to determine whether Adam’s scope has reached axial alignment (PAE, FAE, or LAE). Unfortunately, that final view of the autocollimator does not provide the ability to assess other points since the autocollimator other alignments (P+2 LAE, and P+3 FAE) are not visible when focuser/primary intersect at the COC point and since (P+pupil PAE) in not easily discernable. That is why we had to resort to a different collimation tool such as the cheshire.

As it turns out, it is possible to solve the visibility of P+2 alignment (LAE) even when the focuser and primary axes intersect at the COC point (CAE~0).


(Courtesy of Catseye)

By utilizing the simple solution of adding an offset pupil to the conventional autocollimator, the P+2 disappearance problem is solved!!!! Solving P+2 disappearance will improve the ability to assess the final axial alignment.

We can use the central pupil to evaluate the proximity of the focuser/primary axes at the COC point (CAE) then use the offset pupil to evaluate parallelism of the two axes (LAE). If the central pupil tells us that both the focuser/primary axes intersect at the COC point (CAE~0) and the offset pupil tells us that both axes are parallel (LAE=0), then mathematically both axes are coincident and we have attained axial alignment.

Refer to the following photos:



Top row: Brian’s scope. Left photo via the central pupil indicates the focuser axis intersecting the primary axis at the COC point (CAE~0). Right photo via the offset pupil indicates parallel focuser/primary axes (LAE=0). Conclusion: Brian’s scope attained axial alignment. Two different alignments are zeros (CAE and LAE)

Middle row: Adam’s scope. Left photo via the central pupil indicates the focuser axis intersecting the primary axis at the COC point (CAE~0). Right photo via the offset pupil indicates unparallel focuser/primary axes. Conclusion: Adam’s scope has NOT attained axial alignment since one of the 4 alignments is non-zero, namely, LAE > 0.

Bottom row is for a scope with gross miscollimation: PAE=4mm and FAE=8mm despite CAE~0. The left photo is via the central. It shows a single center spot reflection against darkened background because the focuser axis runs through the COC point (CAE~0). If we could only see reflection 2 via the central pupil, then it will be apparent how grossly reflections are unstacked but we can’t. If you look carefully you can actually see a very dim reflection 2 via the central pupil detected by a long exposure camera shot – not visible via the unaided eye. However, the offset pupil shows reflection 2 very clearly. The relative positions of reflection 2 to reflection P is the same from both pupils; however, it is only visible from the offset pupil when CAE~0 – see the following animation. The animation also shows how both reflections P and 2 relative positions are parallax-free. Even when you move your eye by as much as 0.5” across pupils, the relative positions remain intact -- the AC mirror has to be located within few millimeters from the focal plane.



Below are the same photos but re-arranged for clarity to show how the second pupil manifests Adam's scope miscollimation.



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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532773 - 01/01/10 12:30 AM

Can the autocollimator even be improved further?

The answer is YES.

When the focuser axis intersects the primary axis at the COC point (CAE~0), then we end up with the familiar view of a single center spot reflection against darkened background. Well, both axes do not really need to precisely intersect at the COC point to end up with the familiar view. In reality, when the focuser axis is approximately within 0.5mm from the COC point, we will see the familiar view (CAE <= 0.5mm). The ramification is that the 2-pupil autocollimator has a maximum residual error of 0.5mm. FAE, PAE, and CAE are <= 0.5mm for the 2-pupil autocollimator though LAE will be zero.

Refer to the following:



The top row is for a perfectly collimated scope (CAE=0,PAE=0,FAE=0,LAE=0).

The middle row is for a scope with the focuser axis parallel to the primary axis (LAE=0); however, a shift of 0.5mm was purposely introduced between both axes (PAE=0.5mm, CAE=0.5mm, FAE=0.5mm). Note how the offset pupil view indicates axial parallelism (P+2 are stacked) but the central pupil view shows only a single center spot reflection against darkened background even though CAE=0.5mm. The views from both pupils look quite similar for both cases. The 0.5mm is the maximum error that can be introduced – typical errors will be less than 0.5mm for the 2-pupil autocollimator.

The bottom row is for a scope with parallel axes just like the top/mid setups; however, the separation between the focuser and primary axes is “slightly” greater then 0.5mm. Note how reflection 2 started to appear via the central pupil and how P+1 unstacking is getting clearer. Therefore, any errors above 0.5mm for either CAE, PAE or FAE will be detected by the 2-pupil autocollimator.

It is possible to reduce that maximum 0.5mm error by adding a CAM to the 2-pupil autocollimator. The CAM which stands for (COC Alignment Mask) consists of two small rings positioned exactly opposite to each other. One ring is dark and the other is reflective. The backside of both rings is darkened to avoid internal light scatter.



When collimation is close, two reflections of the CAM will be seen: A foreground reflection on the top of a background reflection. The background reflection is always 180 degree rotated. Both reflections will stack only when the focuser axis precisely intersects the primary axis at the COC point (CAE=0). There is no longer a 0.5mm error to deal with.



Stacking the CAM is precise because it involves stacking two reflections that reside on the same visual plane which means both reflections will look sharp and parallax-free. In comparison, stacking P+1 reflections involves the somewhat fuzzy reflection 1 which is also susceptible to parallax because reflections P and 1 do not reside on the same visual plane.

When the CAM foreground reflection stacks precisely over the background reflection, the foreground dark ring will completely eclipse the reflective ring background reflection. Eclipsing the reflective ring with the dark ring is visually clearer, hence, more precise.

IMPORTANT: The central pupil can’t be used to stack the CAM for the same reasons the central pupil can’t be used to stack P+2. The background reflection will virtually disappear when CAE <= 0.5mm. If has to be done via the offset pupil.

Refer to the following photos:



Top row: Focuser axis is parallel to the primary axis but both axes are 0.5mm apart (CAE=0.5mm, PAE=0.5mm, FAE=0.5mm, LAE=0). The view from the central and offset pupils will not flag the 0.5mm residual error – using the 2 pupil autocollimator

Middle row: Uses the same 2-pupil autocollimator on above setup but with an added CAM. By illuminating the bright ring, it is clear that the CAM reflections are not perfectly stacked. The 0.5mm error is easily flagged.

Bottom row: When the CAM reflections (CAE=0) are perfectly stacked and P+2 reflections are also perfectly stacked (LAE=0), then we know we have achieved perfect collimation.


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532774 - 01/01/10 12:31 AM

SUMMARY:

1- Axial alignment is important to optimize the quality of your view. Axial alignment occurs when both the focuser and primary axes coincide. More accurately, when the eyepiece and primary axes coincide.

2- To align/assess axial alignment, at least two different points of intersection between the focuser/primary axes need to be evaluated.

3- There are 4 different focuser/primary alignments (or errors) that can be evaluated by typical collimator tools:
a. Distance between both axes at the COC point (CAE)
b. Distance between both axes at the focal point (PAE)
c. Distance between both axes at the primary center (FAE)
d. Parallelism between both axes (LAE)

4- If you can prove that at least two of the above errors are eliminated, then your scope is axially alignment. On the other hand, if you could show at least one of the above four errors is non-zero then your scope is axially misalignment.

5- The proper use of the conventional single pupil autocollimator (AC) involves first eliminating FAE by stacking P+3 then eliminating CAE by stacking P+1.

6- The main shortcomings of the conventional single pupil AC which could introduce residual errors are:
a. The final conventional autocollimator view evaluates only CAE. That is only one evaluation point which is not enough to ensure both focuser/primary axes are coincident. FAE can’t be evaluated based on the AC final view.
b. P+3 and P+1 stacking is susceptible to parallax and could involve fuzzy reflections which in turn could impact the accuracy of stacking.

7- The above shortcomings can be solved by adding a second pupil to the AC which will enable an additional error evaluation, namely, parallelism (LAE) by stacking P+2. P+2 stacking involves two parallax-free and sharp reflections. That makes P+2 less susceptible to errors. In addition, the offset pupil along with the central pupil will allow more precise evaluation of the final collimation setup.

8- Adding a CAM will further enhance the autocollimator by replacing P+1 stacking with CAM stacking. The latter, just like P+2, involves sharp and parallax-free reflections. In addition, CAM stacking is more readable compared to P+1.

9- In terms of residual errors -- the bottom-line:
a. If you do not follow the single-pupil autocollimator protocol as outlined by Vic Menard including the use of CDP and a cheshire, then you might end up with a relatively large error -- as much as 1mm to 2mm PAE and FAE. This is enough to ruin your view.
b. With the dual-pupil autocollimator, the maximum residual error you might end up with is 0.5mm PAE and FAE without the use of CDP or a cheshire.
c. With the dual-pupil autocollimator and CAM, residual error is virtually eliminated without the use of CDP and a cheshire. Actually, you do not even need to use the central pupil. Having said that, the use of CDP will reduce the number of iterations; therefore, it is highly recommended to use CDP if it is feasible.


Edited by Jason D (01/01/10 06:00 PM)


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532776 - 01/01/10 12:31 AM

How to evaluate the quality of an autocollimator

First, you need to have a good quality focuser. That is, if your drawtube has a noticeable slop then using an expensive quality autocollimator might not help much because the additional collimation precision achieved by the autocollimator will be overshadowed by the focuser slop error.

Assuming you have a quality focuser, roughly collimate your scope then insert the autocollimator. Adjust the secondary mirror (or the primary mirror) until reflections P and 2 overlap – not stacked. Rotate the auto collimator and monitor the following via the CENTRAL pupil:

1- If you can discern the pupil reflection, make sure it remains at a fixed position relative to reflection P. If the pupil reflection rotates, then the autocollimator perforation is not centered. This would be a concern.
2- Note the shape of reflection 2. If it is deformed and changes shape as you rotate/tilt the autocollimator, then the autocollimator mirror is either non-flat or mounted under stress. This would be a concern.
3- Note the relative positions of reflections P and 2. If both reflections remain stationary as you rotate the autocollimator then you have a good autocollimator. If reflection 2 jitters a little, that is OK. This is a very sensitive test. However, if reflection 2 noticeably shifts back and forth then this would be a concern.
4- Look for reflection 3. If you can’t see it, then you will be unable to perform CDP; however, if this is the only issue, the autocollimator can still be used to produce accurate collimation. In this case, you will need to re-iterate between the autocollimator and a quality cheshire or a quality barlowed laser -- unless you have the 2-pupil autocollimator. Not having high reflectivity autocollimator mirror will contribute to dimmer reflection 3. Furthermore, scopes with short focal length will increase the fuzziness of reflection 3 – sometimes beyond recognition.


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532779 - 01/01/10 12:32 AM

Understanding which axial error each collimation tool eliminates is important. For example, someone with a quality scope and quality collimation tools might find that the cheshire agrees with the barlowed laser but both disagree with the autocollimator. This could happen because both the cheshire and barlowed laser check for PAE but the final view through a conventional autocollimator checks for CAE.

Understand which axial errors are checked for by your collimation tools.
Ensure that at least two axial errors are covered; otherwise, you will not be able to achieve good axial alignment.

The following table is a good reference. It tells you what each collimation tool practically checks for.




Edited by Jason D (01/04/10 12:25 AM)


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3532780 - 01/01/10 12:32 AM

Links for further reading…

The Autocollimator and its reflections
Addendum to the Fifth Edition of New Perspectives on Newtonian Collimation
Passive Tool Collimation and the Newtonian
Catseye vs. Howie Glatter and Blug??
New idea to improve the autocollimator (AC) tool
Questions RE the use of the Infinity XLK A/C?
Collimate with 1-1/4" or 2" laser???
Autocollimation question
The Catseye XLK 2-pupil collimation tool
Collimaton Tools Don't Agree
The INFINITY XLKTM Collimation Procedure
Vic Menard's Carefully Decollimated Primary (CDP) Collimation Protocol


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3533099 - 01/01/10 08:31 AM

Jason,

This series is no doubt a "Masterpiece" of elegance, detail, clarity and tutorial excellence! Bravo, my friend! With the additional insight you bring to the table completing the final chapter in the quest for AC reflection understanding, no doubt you have have earned your place amoung the ranks of the "elite few" who comprehend this special niche of optical physics. Your special talents of novel spacial/mathematical analysis and graphic presentation artistry have been beautifully blended synergistically here in your best work to date.



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Re: Concise thread about autocollimators+improvements new [Re: CatseyeMan]
      #3533177 - 01/01/10 09:48 AM

Hear! Hear!

Edited by rockethead26 (01/01/10 09:57 AM)


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Re: Concise thread about autocollimators+improvements new [Re: rockethead26]
      #3533313 - 01/01/10 11:08 AM

Wow that's a lot of reading. Unfortunately for me it will take reading this thread quite a few more times before I can begin comprehend it all.

But I applaud you for taking the time to help newbies like myself!

THANKS!


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3533387 - 01/01/10 11:57 AM

Great work! I've saved this as a favorite thread. Would love to see this in article/pdf format.

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Re: Concise thread about autocollimators+improvements new [Re: turtle86]
      #3533575 - 01/01/10 01:56 PM

Thank you everyone for the kind words.

This thread is a labor of

I do realize that there is so much material in this thread and it is inevitable that some of the presented information is unclear and would need more clarification. Please feel free to ask questions and to challenge some of the concepts presented. If you have questions but you are hesitant to post them because you think they are too basic to ask, be assured that many others will have the same questions in mind but are also hesitant to post them for the same reason. Just post your questions no matter how basic they are.

Jason


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3533965 - 01/01/10 05:56 PM

Stickey! Stickey!

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Re: Concise thread about autocollimators+improvements new [Re: Tim L]
      #3533998 - 01/01/10 06:18 PM

I will put this in with the Best of Reflectors list.

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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3534327 - 01/01/10 10:45 PM

Jason, I've got some work ahead of me to understand all the detail of your explanation. Thanks for the lessons from a guru...

c/s,


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3534575 - 01/02/10 06:38 AM

Kudos to Jason for not only identifying the potential, previously unknown (or at least ignored) problems with the AC:

* the vanishing reflections (that can be seen in the offset pupil),
* the defocus problems (solved by the CAM),
* the consequences of having the AC mirror away from the focal plane

but also solving them!

Nils Olof


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Re: Concise thread about autocollimators+improvements new [Re: Nils Olof Carlin]
      #3534714 - 01/02/10 09:33 AM

The pictures and explanations as this topic progresses are truly excellent, Jason. Nice work.

Then we get to the table (what many readers are going to take away from this), and I have to wonder about a few things there:

1) Minor quibble, the cheshire in a combo tool can also be calibrated to a primary mirror mark, so could also earn an Excellent.

2) Was interested to see that there are no Excellent FAE tools(?)

3) Was interested to see that the CAM AC did not get promoted into the PAE row at all, since I thought its intent, one of them, anyway, was to make a cheshire completely redundant.

4) There ought to be some reemphasis that PAE and FAE are really the only two collimation axes. CAE and LAE are interesting to help talk about AC theory only.

Mike


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Re: Concise thread about autocollimators+improvements new [Re: Nils Olof Carlin]
      #3535104 - 01/02/10 01:15 PM

Hi Nils Olof,
My work is based on the strong foundation laid out by your mathematical analysis of the autocollimator
Jason


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Re: Concise thread about autocollimators+improvements new [Re: hudson_yak]
      #3535203 - 01/02/10 02:12 PM

Quote:

The pictures and explanations as this topic progresses are truly excellent, Jason. Nice work.




Thank you, Mike

Quote:

Then we get to the table (what many readers are going to take away from this), and I have to wonder about a few things there:




For the record, I did include a disclaimer explaining that the table is subjective and based on my own opinion.

Quote:


1) Minor quibble, the cheshire in a combo tool can also be calibrated to a primary mirror mark, so could also earn an Excellent.



My grading is relative. A “Good” rating should not be construed as a bad rating just because there is an “Excellent” rating. Using a cheshire like Catseye’s with a triangle and a ring does add more reading accuracy than the sight-tube/combo tool with the distracting cross-hairs. I am speaking from experience since I have used both tools extensively.

Quote:

2) Was interested to see that there are no Excellent FAE tools(?)



Correct. Getting the “Excellent” rating has high standards. It should involve visual cues that are parallax-free, sharp, and highly-readable. Stacking P+1 does not meet this criteria. A laser collimator does not either.
Let me put it this way, a “Good” rating provides accuracy to within ~0.5mm. An “Excellent” rating provides accuracy to within ~0.2mm.

Quote:

3) Was interested to see that the CAM AC did not get promoted into the PAE row at all, since I thought its intent, one of them, anyway, was to make a cheshire completely redundant.



The table is meant to rate “direct” and “explicit” alignments – not “indirect” alignments. The CAM AC does not get a “rating” for PAE because it is impractical to “directly” align the pupil reflection against reflection “P”. HOWEVER, as long as a tool gets “direct” excellent rating for two error elimination, it implies “indirectly” the remaining two error elimination get the same rating.
OK, Mike I'll grant you that table could be confusing. I need to think about how to update that post later today.

Quote:

4) There ought to be some reemphasis that PAE and FAE are really the only two collimation axes. CAE and LAE are interesting to help talk about AC theory only.



I disagree with this line of thinking. PAE, FAE, LAE, and CAE are all means to an end which is “Axial Alignment”. I will follow whichever path that gets me to my axial alignment destination with the greatest accuracy whether it is PAE, FAE, LAE, CAE or even XAE -- whatever that is

Jason


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3535247 - 01/02/10 02:43 PM

Quote:

Quote:

2) Was interested to see that there are no Excellent FAE tools(?)



Correct. Getting the “Excellent” rating has high standards. It should involve visual cues that are parallax-free, sharp, and highly-readable. Stacking P+1 does not meet this criteria....

Jason




What about stacking P & 3 in the CDP?


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3535266 - 01/02/10 02:55 PM

Jason
Incredible drawings again! You really know how to make a blind man see!


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Re: Concise thread about autocollimators+improvements new [Re: hudson_yak]
      #3535285 - 01/02/10 03:08 PM

Since you invite comments, Jason, in #3533575, here are some on account of Mike's comments:
Quote:

There ought to be some reemphasis that PAE and FAE are really the only two collimation axes. CAE and LAE are interesting to help talk about AC theory only.




Indeed true - IIRC CAE=2*PAE+FAE and LAE=2*PAE+2*FAE (vector sums), and thus, the CAE and LAE should perhaps not be seen as "fundamental" in the sense of being independent of the others (you could, in principle, derive PAE and FAE from CAE and LAE, but this would be pointless).

It could also be made clearer that the PAE and FAE are indeed independent of each other - you may have accurate collimation of one axis and still have some miscollimation of the other (even if that's what you try to avoid). It is only the PAE that causes coma in the center of the FOV, the FAE will cause a tilt of the focal planes but no error in the center of the FOV.

Nils Olof


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Re: Concise thread about autocollimators+improvements new [Re: CatseyeMan]
      #3535335 - 01/02/10 03:38 PM

Quote:

What about stacking P & 3 in the CDP?




It depends on the focal length of the scope. For my 1200mm scope, it is hard to see reflection 3 primarily because of its fuzziness.

The left photos are what I see and the right ones is with the camera focused at infinity.



Those with longer focal lengths scopes, reflection 3 will be clearer.

For a reference to others, the following posts explain the relationship between focal length and reflections 1&3 fuzziness:
post1
post2
post3

But the trend seems to be moving towards scopes with shorter focal lengths (< F3.0 scope). Not to mention that imaging scopes tend to have a short focal length – the very scopes that require more accurate focuser axial alignment.

Jason

Edited by Jason D (01/03/10 04:58 PM)


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3535338 - 01/02/10 03:39 PM

One interesting thing, very well illustrated in Jason's post #3532773 (and perhaps easier seen in photos than directly by eye), see the lower set of images:

The reflections occur at different apparent distances from the pupil.
P and 2 are seen one (mirror) focal length away, the AC and CAM reflections at infinity, and reflections 1 and 3 at minus one focal length (that is, behind your head).

When the camera is focused on P and 2, 1 and 3 are obviously fuzzy. On the CAM images, the camera is focused at infinity and both pairs are equally fuzzy (while the CAM rings are sharp), but only half as much as in the previous case. If the camera lens could be moved inwards from the infinity setting, it would be possible to get 1 and 3 sharp - but "normal" cameras do not allow it (a weak negative add-on lens might do the job).

(The thought of seeing images occuring behind your head may be strange - but those images are virtual, and it wasn't before I realized that they are indeed behind the observer's head that I could even begin to make sense of all the reflections.)

Nils Olof


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Re: Concise thread about autocollimators+improvements new [Re: Nils Olof Carlin]
      #3535379 - 01/02/10 04:06 PM

Just check the post submitted 1 minute before your post. The photo and included links explain the unintuitive concept of seeing an image behind our heads. Of course, you were the first to have explained it to me, Nils Olof. When the camera focuses at infinity then P&2 images are formed behind the retina and 1&3 images are formed in front of the retina by about the same distance – that is why both sets of images would look similar in fuzziness. For scopes with longer lengths, the distance between the four images and the retina is reduced which brings them all closer to focus (retina).
But the eye has the tendency to focus on P&2 on the expense of 1&3 since the eye is designed to see images in front of the eye – not behind.


Jason


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Re: Concise thread about autocollimators+improvements new [Re: Nils Olof Carlin]
      #3535404 - 01/02/10 04:20 PM

Quote:

Indeed true - IIRC CAE=2*PAE+FAE and LAE=2*PAE+2*FAE (vector sums), and thus, the CAE and LAE should perhaps not be seen as "fundamental" in the sense of being independent of the others (you could, in principle, derive PAE and FAE from CAE and LAE, but this would be pointless).

It could also be made clearer that the PAE and FAE are indeed independent of each other - you may have accurate collimation of one axis and still have some miscollimation of the other (even if that's what you try to avoid). It is only the PAE that causes coma in the center of the FOV, the FAE will cause a tilt of the focal planes but no error in the center of the FOV.




I covered the PAE/FAE relevant importance in the 3rd post of this thread.

What helped me to think out-of-the-box is thinking of axial alignment as what it takes to coincide two lines – the focuser axial line with the primary axial line. I was not thinking in terms of PAE and FAE – that is how CAE and LAE got more attention from me. Since the objective of this thread is to attain the most possible accurate axial alignment, there is no distinction between the importance of eliminating one residual error over another. The objective is to eliminate all errors: PAE, FAE, LAE, and CAE. Of course, eliminating two is enough to eliminate all four.

Jason


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3535412 - 01/02/10 04:23 PM

Quote:

Quote:

What about stacking P & 3 in the CDP?




It depends on the focal length of the scope. For my 1200mm scope, it is hard to see reflection 3 primarily because of its fuzziness.

The left photo is what I see and the right one is with the camera focused at infinity.


Those with longer focal lengths scopes, reflection 3 will be clearer.

For a reference to others, the following posts explain the relationship between focal length and reflections 1&3 fuzziness:
post1
post2
post3

But the trend seems to be moving towards scopes with shorter focal lengths (< F3.0 scope). Not to mention that imaging scopes tend to have a short focal length – the very scopes that require more accurate focuser axial alignment.

Jason




... so the question is: What other tool/methodology is out there that is "better" than stacking P & 3 in the AC for insuring no significant FAE?

I'm curious as to how the "diffraction ring" effect (using the Glatter 1 mm aperture option) of the return beam around the Barlow Screen exit pupil stacks up on accuracy? (Vic has mentioned this phenomenon)?


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Re: Concise thread about autocollimators+improvements new [Re: CatseyeMan]
      #3535431 - 01/02/10 04:35 PM

Jim,
FAE, PAE, LAE, and CAE are not independent. Eliminating two out of the four with great accuracy also implies that remaining two are eliminated with the same level of high accuracy. This is a mathematical fact.
Therefore, eliminating LAE with P+2 stacking and CAE with the CAM with great accuracy implies both PAE and FAE are also eliminated with the same great accuracy.
Having said the above, it is highly recommended to start off the steps with CDP. If reflection 3 is clear enough to eliminate all or most of FAE then the rest of the collimation steps will coverage quicker. If reflection 3 is not clear enough to accurately execute CDP then the same accurate axial alignment can still be achieved but it might require and additional iteration or two.
I can't comment about the diffraction ring affect of the laser collimator because I have never tried it.
Jason


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3535469 - 01/02/10 05:01 PM

Quote:

.... Therefore, eliminating LAE with P+2 stacking and CAE with the CAM with great accuracy implies both PAE and FAE are also eliminated with the same great accuracy.




Sounds like the practical CAM option needs to happen sooner than later stay tuned....


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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3535550 - 01/02/10 05:40 PM

Quote:

Using a cheshire like Catseye’s with a triangle and a ring does add more reading accuracy than the sight-tube/combo tool with the distracting cross-hairs. I am speaking from experience since I have used both tools extensively.




I agree the xhairs do get in the way, in fact I took mine out for that reason, so mine isn't quite as combo as it used to be. Regardless, it is possible to make a mirror mark that matches up well with the cheshire in a combo tool. For example I made a 8mm diameter circle mark which made an easy eccentricity read within the 9.5mm cheshire dark circle of the combo tool.

Just a general comment on the table, it's a good summary of the foregoing posts in the thread, in the context of your intent to discuss AC theory. I was mostly thinking about a relative novice dropping in here, skipping over all that stuff, seeing the table and thinking ah, here's what I need to buy, and perhaps getting a bit mislead as to how best to start. The disclaimer that the table is just your opinion doesn't really help in that respect.

One way to avoid this would have been to not include the table at all.

Mike


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Re: Concise thread about autocollimators+improvements new [Re: hudson_yak]
      #3535621 - 01/02/10 06:28 PM

I see your point, Mike
I edited the post. I removed all ratings and replaced them by "check" marks. OK, I left two checks for what I consider to be highly accurate alignment but I did not explain it.
I did not want to delete the post because understanding which errors each tool checks for is important.
Jason

Edited by Jason D (01/02/10 09:03 PM)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3536982 - 01/03/10 12:17 PM

Jason, I have read over your info and find it quite interesting. I have never used a AC to collimate but do use a site tube to align the secondary and a barlowed laser to align the primary. I double check with a cheshire and all seems well. My question is how easy is the AC to use in the dark? The reason I ask is because once my entire scope reaches ambient temp the collimation does shift a small amount. My Obsession will do it and yes it is repeatable in the same direction all the time. My Discovery also did this as well. I collimate just after setting up then have to recheck the primary after 3-4 hours. There is usually a temp. drop of 20 degrees + during this time.

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Re: Concise thread about autocollimators+improveme new [Re: demiles]
      #3537114 - 01/03/10 01:15 PM Attachment (169 downloads)

Quote:

My question is how easy is the AC to use in the dark?



I use a clip-on light source as shown in the attachment. To preserve eye adaptation, I added few red translucent layers to the light source. You could also clip the light source to the vanes.
Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3537227 - 01/03/10 02:17 PM

Excellent idea. Do you use something like red brake light tape over the light?

Quote:

Quote:

My question is how easy is the AC to use in the dark?



I use a clip-on light source as shown in the attachment. To preserve eye adaptation, I added few red translucent layers to the light source. You could also clip the light source to the vanes.
Jason




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Re: Concise thread about autocollimators+improveme new [Re: demiles]
      #3537257 - 01/03/10 02:29 PM

Quote:

Jason, I have read over your info and find it quite interesting. I have never used a AC to collimate but do use a site tube to align the secondary and a barlowed laser to align the primary. I double check with a cheshire and all seems well. My question is how easy is the AC to use in the dark? The reason I ask is because once my entire scope reaches ambient temp the collimation does shift a small amount. My Obsession will do it and yes it is repeatable in the same direction all the time. My Discovery also did this as well. I collimate just after setting up then have to recheck the primary after 3-4 hours. There is usually a temp. drop of 20 degrees + during this time.



I have found that if I collimate perfectly when the telescope is at ambient temperature in the afternoon, I can watch the collimation changing all over the place as the telescope cools. But once it has cooled, the collimation returns. How fast this happens depends on the scope's materials, pole thicknesses, etc.


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mark Jimenez
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Re: Concise thread about autocollimators+improveme new [Re: Starman1]
      #3537401 - 01/03/10 03:27 PM Attachment (189 downloads)

Hello,
I just wanted to show how Jim at Catseye helped me get the AC mirror located at the focal plane on my imaging newt. On mine, and many others that are designed for use with a coma corrector, the focal plane often resides above the top of the focuser.
This is a two pupil (XLK) autocollimator with 1.5" extension built into the barrel. Works great.
Also, I find these tools very easy to use in the dark with a small red LED flashlight.
-Mark


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: mark Jimenez]
      #3537585 - 01/03/10 05:03 PM

Nice!!!!!!!!!
Thanks for posting the photo.
Imager enthusiasts will definitely be interested in this option.
Jason


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: turtle86]
      #3537603 - 01/03/10 05:14 PM

Rob, I taped pieces of red translucent wrap. Here is an old photo to show you how it would look like at night. The CAM in the photo is an old makeshift CAM.



With the XLK+CAM I use two source lights: One to illuminate the center spot and another to illuminate the CAM. The production CAM will not require the CAM source light – this is what Jim is trying to resolve.

Photos below are without the red translucent wrap.


Jason


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turtle86
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3537809 - 01/03/10 07:15 PM

Thanks! Looks like just the ticket for using an autocollimator in the dark.

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CatseyeMan
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3537936 - 01/03/10 08:46 PM Attachment (160 downloads)

Here's what I use and recommend:

Coast Cutlery #TT75331CP LED clip light retrofitted with a bright 3000 MCD Red LED (Gilway E184 found here) and clipped to the spider; I get the best illumination with the light pulled in close to the Secondary as shown in the pics.


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Don W
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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3537954 - 01/03/10 08:57 PM

Slick. Me likey.

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turtle86
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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3538021 - 01/03/10 09:41 PM

Thanks, Jim. Looks great!

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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3538022 - 01/03/10 09:42 PM

Quote:

Here's what I use and recommend:

Coast Cutlery #TT75331CP LED clip light retrofitted with a bright 3000 MCD Red LED (Gilway E184 found here) and clipped to the spider; I get the best illumination with the light pulled in close to the Secondary as shown in the pics.




Ummm, I wonder if these points of light on the side have enough brightness to illuminate the CAM?


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Spaced
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3538771 - 01/04/10 11:28 AM

Jason, my compliments on another superbly written and illustrated how-to manual. This is just excellent! Thanks so much for sharing your knowledge!

I'm currently using only a Glatter laser + Blug to collimate my f/4.5. My secondary is properly positioned with a Catseye sight tube. Assuming I'm using these tools properly and with care, am I able to achieve sufficiently accurate collimation, such that improvement with an AC wouldn't provide noticeably better views? To make your answer of more general interest, what do you think is the fastest focal ratio for which just a good barlowed laser + sight tube provides sufficient collimating accuracy for visual use?

Thanks much.


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Re: Concise thread about autocollimators+improveme new [Re: Spaced]
      #3538843 - 01/04/10 12:11 PM

First, thank you for the kind words, Mike.

You asked good questions. Here is my take:

I believe the combination of:
- Quality dual-pupil autocollimator and calibrated cheshire
OR
- Dual-pupil autocollimator+CAM
will give the most accurate collimation available today. We are talking sub 0.2mm accuracy for both the focuser and primary axial alignment.

The above is factual and measureable.

What is not as factual and measurable is how accurate collimation should be? Will the use of a sub 0.2mm collimation accuracy for FAE/PAE produce noticeable difference at the EP compared to sub 0.5mm collimation accuracy? Answering this question is somewhat subjective – after all, we are not talking about the difference between 1mm and 0.2mm accuracy but between 0.5mm and 0.2mm. The scope specs, average seeing conditions, observing experience, and whether the setup is meant for imaging or visual can sway the answer from one end of the spectrum to the other. Therefore, I can’t say that the additional accuracy provided by the dual-pupil autocollimator + (cheshire or CAM) will make a noticeable difference for everyone. But what I can tell you is that you have quality collimation tools that are capable of delivering superb collimation.

Jason


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Starman1
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3538897 - 01/04/10 12:38 PM

One way to look at it is that collimation is an envelope of accuracy.
If you are inside of the envelope, your collimation is probably good enough. You don't have to be dead center in that envelope to see good images.
However, telescopes are not static devices--they flex, sag, change shape with temperature changes.
You want to make certain your telescope stays within the envelope of good collimation--the greater accuracy you achieve, the more likely will your telescope stay within that envelope as it sags, twists, and contracts.

Plus, achieving a superior accuracy in collimation when you first set up may eliminate any necessity to check collimation during the night.

Last is the benefit of being able to see tiny changes in collimation as the scope moves up and down. Tracking down sources of sag and flexure and fixing them means the scope is better able to hold its collimation after you've spent the time to make it good.

So do you NEED to collimate to such a high precision? It depends on the scope, its targets, and its conditions of use. But, there are never any deleterious effects from collimating better than is required. After all, developing the skills of collimating using accurate tools will pay off when you get that 20" f/3 scope that uses a Paracorr.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3539029 - 01/04/10 02:03 PM

Quote:


I believe the combination of:
- Quality dual-pupil autocollimator and calibrated cheshire
OR
- Dual-pupil autocollimator+CAM
will give the most accurate collimation available today. We are talking sub 0.2mm accuracy for both the focuser and primary axial alignment.



Sub 0.2mm? I can read a calibrated Barlowed laser (PAE) to about 0.01-inch (about 0.005-inch PAE) pretty consistently--that's a direct read of PAE with no parallax error. Taking registration into consideration I would add +/-0.005 to 0.01-inch (about 0.01- to 0.015-inch PAE). Assuming you're interpreting PAE from CAE and LAE, what do you consider the read accuracy of the individual alignments--or are you assuming both can be consistently read to better than 0.1mm accuracy (without detailed examination of a closeup image)? I know that both errors (CAE and LAE) contain PAE and FAE components, but balancing one against the other doesn't necessarily improve the resolution of the interpreted errors. What do you feel are the best, and worst case scenarios?

If Mike is using a Glatter laser and Blug to align the axes of his 14.5-inch f/4.5 optic, he should have no problem keeping the FAE in tolerance (+/-0.4-inch), even if he's using a Paracorr (+/-0.07-inch). The PAE high magnification tolerance (+/-0.018-inch) should also be manageable with a Barlowed laser solution (although I would recommend the 1mm Glatter aperture stop over the Blug for the best precision).

I apologize for coming into the discussion this late (three pages already). It was a busy holiday with lots of family get-togethers.

Twenty-plus years ago, when I first persuaded Tom Clark to include the "complicated" autocollimator in the 3-tool Tectron arsenal, I never imagined it would be studied in such detail and modified to utilize the new CAE and LAE alignments! If you continue on this path, the autocollimator will eventually have to either auto collimate or make coffee--but I'm not sure which the end user would demand first!

Great post!


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Spaced
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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3539073 - 01/04/10 02:31 PM

Quote:

The scope specs, average seeing conditions, observing experience, and whether the setup is meant for imaging or visual can sway the answer from one end of the spectrum to the other. Therefore, I can’t say that the additional accuracy provided by the dual-pupil autocollimator + (cheshire or CAM) will make a noticeable difference for everyone. But what I can tell you is that you have quality collimation tools that are capable of delivering superb collimation.




I understand that too many variables prevent a concise answer. I infer that your answer to my first, specific question is, "You're probably doing just fine without the AC."

Quote:

One way to look at it is that collimation is an envelope of accuracy.
If you are inside of the envelope, your collimation is probably good enough. You don't have to be dead center in that envelope to see good images.




That description resonates.

Quote:

So do you NEED to collimate to such a high precision? It depends on the scope, its targets, and its conditions of use. But, there are never any deleterious effects from collimating better than is required.




You just won't make a decision easy, will you?

Quote:

After all, developing the skills of collimating using accurate tools will pay off when you get that 20" f/3 scope that uses a Paracorr.




Oh yeah, I'd better start practicing right away!

Quote:

If Mike is using a Glatter laser and Blug to align the axes of his 14.5-inch f/4.5 optic, he should have no problem keeping the FAE in tolerance (+/-0.4-inch), even if he's using a Paracorr (+/-0.07-inch). The PAE high magnification tolerance (+/-0.018-inch) should also be manageable with a Barlowed laser solution (although I would recommend the 1mm Glatter aperture stop over the Blug for the best precision).




That's reassuring.


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3539147 - 01/04/10 03:19 PM

Vic, Jason (and all)
As I go through the steps (using the Glatter laser [w/1mm aperture stop] and Blug) collimation cannot be complete without the AC. But using the offset pupil seems to have eliminated two previously used procedures, the first being the center AC pupil. Apart from showing the disappearing act, cannot all AC collimating procedures be accomplish without the center AC pupil and just using the offset AC pupil? Secondly, doesn't the AC offset pupil eliminate CDP?

dave


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Re: Concise thread about autocollimators+improveme new [Re: nsldvd]
      #3539269 - 01/04/10 04:19 PM

Quote:

Vic, Jason (and all)
As I go through the steps (using the Glatter laser [w/1mm aperture stop] and Blug) collimation cannot be complete without the AC.



Why not? What is the aperture and focal ratio of your Newtonian? And are you using a coma corrector? I also suggest that you verify the Blug alignment against the self-Barlow attachment to be sure that the axial alignment with normal focuser registration is the same as the self centering alignment achieved at the bottom of the focuser drawtube.

Quote:

But using the offset pupil seems to have eliminated two previously used procedures, the first being the center AC pupil. Apart from showing the disappearing act, cannot all AC collimating procedures be accomplish without the center AC pupil and just using the offset AC pupil? Secondly, doesn't the AC offset pupil eliminate CDP?



Stacking P-3 via CDP is only useful from the center pupil, allowing a direct read of FAE (magnified 2X), which may be critical in some applications. As Jason has already noted, you can iterate between CAE and LAE and achieve precise axial alignment only limited by the read precision of the signature alignments (CAM alignment and P-2 alignment from the offset pupil). But the iterative procedure may require several repetitions if the necessary precision adjustment isn't easily effected.

I think, perhaps, the more significant question is, if one finds a discrepancy between the calibrated Barlowed laser and the autocollimator using the offset pupil to assess CAE and LAE, should one trust the calibrated Barlowed laser or the autocollimator when correcting PAE? (Or should one anticipate that the residual error is exclusively FAE, since FAE is a component of both CAE and LAE?)


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3539475 - 01/04/10 06:04 PM

Vic,

With XLK+CAM, I consistently achieved as good of PAE reduction as a cheshire – I would say even better. However, I am more experienced and knowledgeable about autocollimators and collimation than an average user. It won’t be fair to extrapolate my success to others. The 0.5mm and 0.2mm quotes are maximum errors for average users. The true average errors will have to be dedicated by others.

Here is a thought. If CDP can be performed with great accuracy which puts our FAE at ~0 then the CAM will magnify PAE by 4X as opposed to only 2X by the cheshire. Not only that but the CAM readability where a bright ring gets eclipsed by a dark ring is easier to discern than centering a triangle within a larger ring – of course, I am referring to the cheshire. In other words, starting with FAE=0 as our initial point, I do not see how the cheshire can outperform the CAM. Actually, I do not know of any tool that magnifies PAE by 4X other than the scenario I described. Of course, the catch here is that FAE needs to be eliminated. But the objective of the XLK+CAM is to eliminate all residual errors. Those who do not mind living with few millimeters of FAE should not use any kind of autocollimators, PERIOD. Actually, the very same users can be happy with a cheaper mass produced laser with small miscollimation error. They can use it to achieve an FAE with few millimeters error then use it again with a barlow to eliminate PAE. I just happened to belong to the camp who strives to eliminate all collimation errors for the reasons Don mentioned in his last post. I will not tolerate any FAE. That is why I am interested and a strong supporter of the autocollimator tool. That is my belief which others might not agree with. But that is OK. After all, this is only a hobby where people are free to do whatever they want and spent their money whichever way they like. There is no right or wrong.

By the way, here is an old animation that shows readability difference between a cheshire and a CAM. The CAM used in the animation is an old makeshift CAM that I build at home.

click here to view the animation

In general, let us compare the conventional steps of the single-pupil AC + cheshire to dual-pupil AC + cheshire. How can the first even be at the same level as the second? Not only everything in the first tool combination is covered by the second combination, but here is what you get with the second tool combination:
1- You get the CAM alignment which is more accurate than P+1 stack because it involves reflections located at the same plane which means both are sharp and parallax-free – not to mention the CAM has better readability than P+1
2- You get P+2 alignment which again involves sharp and parallax-free reflections.
For those who own the XLK and a chechire, if you need to iterate at the end then you should iterate between the cheshire and P+2 stack via the offset pupil as opposed to iterating between the cheshire and P+1 via the central pupil. For future XLK+CAM owners, assuming such product makes it to market, then the cheshire becomes optional. However, for those who are not interested in eliminating FAE then the cheshire would be a requirement even for the XLK+CAM. As I mentioned earlier, those who are not interested in eliminating FAE then they should not use any kind of autocollimator because it will not work.

Jason

Edited by Jason D (01/04/10 08:22 PM)


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Re: Concise thread about autocollimators+improveme new [Re: nsldvd]
      #3539498 - 01/04/10 06:23 PM

Quote:

Vic, Jason (and all)
As I go through the steps (using the Glatter laser [w/1mm aperture stop] and Blug) collimation cannot be complete without the AC. But using the offset pupil seems to have eliminated two previously used procedures, the first being the center AC pupil. Apart from showing the disappearing act, cannot all AC collimating procedures be accomplish without the center AC pupil and just using the offset AC pupil? Secondly, doesn't the AC offset pupil eliminate CDP?

dave




Dave, if you have an autocollimator with only an offset pupil (no central pupil) equiped with the CAM, then you can achieve highly accurate collimation without a cheshire -- or the need for a central pupil. That is how I collimate.
However, without the CAM you will need either a cheshire or the central pupil. As Vic mentioned, if you start off with P+3 stack (CDP) via the central pupil then you will cut down on the number of iterations. That is why I recommend to start off with CDP provided reflection 3 can be clearly discerned.
Jason


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Re: Concise thread about autocollimators+improveme new [Re: Spaced]
      #3539549 - 01/04/10 06:45 PM

Quote:

I understand that too many variables prevent a concise answer. I infer that your answer to my first, specific question is, "You're probably doing just fine without the AC."



My point is that an autocollimator will provide more accuracy; however, I can't tell you that the additional accuracy will always translate to a discernable improvement at the EP. This is akin to premium optics. In many nights, you will not easily discern the difference between good mass produced optics and guaranteed premium optics. But in those few nights when everything is perfect to get the most out of your premium optics then the additional collimation accuracy will make a difference.
Sorry, I just can’t make a general statement that the autocollimator will always produce discernable improvement at the EP all the time especially when you are using a quality laser collimator.
However, as the trend towards sub F3.0 reflectors continues, I can see more importance to the autocollimator in the future.
Jason


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3539709 - 01/04/10 07:54 PM

Quote:


..................If you continue on this path, the autocollimator will eventually have to either auto collimate or make coffee--but I'm not sure which the end user would demand first!

Great post!




Vic, there you go, limiting your expectations: let's have one that 'auto' collimates AND makes coffee!.

This is a great thread


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Re: Concise thread about autocollimators+improveme new [Re: Moggi1964]
      #3539743 - 01/04/10 08:14 PM

I am working on it. The central pupil will dispense caffeinated coffee and the offset pupil will dispense decaffeinated. This feature will help keeping you awake in case you spend the whole evening collimating.

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erick
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3539800 - 01/04/10 08:46 PM

Quote:

This feature will help keeping you awake in case you spend the whole evening collimating.




Right, that's the collimation done. Phew! What's that to the east? Drat! Dawn!!

Thanks for the thread. Time I learnt how to use my autocollimator on my coming scope.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3539851 - 01/04/10 09:10 PM

Quote:

The 0.5mm and 0.2mm quotes are maximum errors for average users. The true average errors will have to be dedicated by others.



If 0.2mm is the maximum error, then you are expecting accuracies better than 0.1mm for CAE and LAE, is that correct?

Quote:

If CDP can be performed with great accuracy which puts our FAE at ~0 then the CAM will magnify PAE by 4X as opposed to only 2X by the cheshire.



I understood CAE to be the same as P-1, which is 4X for FAE and 2X for PAE (assuming the axes intersect at the COC). But P-2 magnifies both FAE and PAE 4X, so if FAE is zero, then P-2 would magnify PAE 4X. But getting FAE perfect, there's the rub. I've tried using the Barlowed laser to minimize PAE and then followed with FAE correction at the secondary mirror to minimize P-2, but the FAE correction creates a PAE error, so reiteration is necessary to achieve maximum precision.

Quote:

Not only that but the CAM readability where a bright ring gets eclipsed by a dark ring is easier to discern than centering a triangle within a larger ring – of course, I am referring to the cheshire.



I'm typically not using the Cheshire--I use a variation of the Barlowed laser (1mm aperture stop) with a concentric diffraction pattern that reads to very high precision (like the CAM, only up close where I can use reading glasses!)

Quote:

In other words, starting with FAE=0 as our initial point, I do not see how the cheshire can perform better than the CAM.



But you have to perfect the FAE first. Even with a CDP, P-3 only magnifies FAE 2X, and there are focus and parallax issues. No doubt, P-3 is better than a sight tube cross hair alignment, and possibly better than laser alignment with a 1mm aperture stop (curiously, also performed with a CDP). But I'm not ready to rely on P-3 as the reference alignment PAE will be collimated to.

Quote:

Actually, I do not know if any tool that magnifies PAE by 4X other than the scenario I just described.



Me either.

Quote:

Of course, the catch here is that FAE needs to be eliminated.



Yup.

Quote:

But the objective of the XLK+CAM is to eliminate all residual errors.



That's a worthy goal, but PAE and FAE are the only ones that matter. I don't know about eliminating all residual errors (especially considering registration tolerances), but reducing errors, if possible to the resolution of the tool, may be necessary for some applications. Using CAE and LAE to reduce PAE and FAE is the goal I see.

Quote:

Those who do not mind living with few millimeters of FAE should not use any kind of autocollimators PERIOD.



But what about those f/3 Dobs? A 20-inch f/3 can tolerate 2.5mm of FAE, with a Paracorr, but requires a PAE correction of 0.13mm (0.005-inch)! That's assuming, of course, that an f/3 scope is capable of high magnification performance.

Quote:

...I just happened to belong to the camp who strives to eliminate all collimation errors for the reasons Don mentioned in his last post. I will not tolerate any FAE. That is why I am interested and a strong supporter of the autocollimator tool.



I'm sure you know you're preaching to the choir. But there are tolerances, and there is also useful readability. Of course, you can carefully examine an image of the alignment and extrapolate a fairly reliable PAE (usually the more critical axial alignment). I know you know what I mean and the implications.

Quote:

...let us compare the conventional steps of the single-pupil AC + cheshire to dual-pupil AC + cheshire.



Instead of a Cheshire, let's compare the two autocollimators combined with a calibrated Barlowed laser.

Quote:

How can the first even be at the same level as the second?



If precise PAE is the ultimate goal, unless you can find a way to zero FAE first, the calibrated Barlowed laser has some real advantages for many users. If precise FAE is the goal, P-2 in the offset pupil should eventually (after several iterations) deliver an FAE nearly as precise as the PAE delivered by the Barlowed laser.

Quote:

...here is what you get with the second tool combination:
1- You get the CAM alignment which is more accurate than P+1 stack because it involves reflections located at the same plane which means both are sharp and parallax-free – not to mention the CAM has better readability than P+1



That's true, but a perfect CAM alignment does not in and of itself indicate perfect PAE alignment (neither does a perfect P-2 alignment in the offset pupil). Because both errors (PAE and FAE) can be present in both alignments (CAE and LAE), PAE and FAE must be extrapolated from the CAE and LAE alignments. Of course, if you can align CAE and LAE perfectly (whatever the read accuracy actually is), then PAE and FAE must be very good, if not perfect.

Quote:

For future XLK+CAM owners, assuming such product makes it to market, then the cheshire becomes optional.



Like you, I want the best alignment possible (for my f/4 primary). I tend to rely on redundant verification when possible. So while the XLK+CAM may indeed allow me to derive the PAE correction I need, I'll keep my Barlowed laser nearby so I can actually look at the PAE correction.

Quote:

As I mentioned earlier, those who are not interested in eliminating FAE then no autocollimator derivative tool should be used.



I disagree, because I prefer redundant verification. My autocollimator is still the quality control inspector for my axial alignment (even though my FAE tolerance, with Paracorr, is 2.8mm).


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3540163 - 01/05/10 12:58 AM

Quote:

If 0.2mm is the maximum error, then you are expecting accuracies better than 0.1mm for CAE and LAE, is that correct?



The 0.2mm is an estimate for the maximum error an average user might run into. It is my estimate. Real life users will determine what this number should be. I can do much better but I am not an average user.

Quote:

I've tried using the Barlowed laser to minimize PAE and then followed with FAE correction at the secondary mirror to minimize P-2, but the FAE correction creates a PAE error, so reiteration is necessary to achieve maximum precision.



For the benefit of others, Vic just described the strength of the XLK. Using XLK+Cheshire might require additional iterations but the extra effort should not be viewed as negative. See, those who use XL+cheshire will get to a point where all visual cues will indicate perfect collimation and that would be the end of collimation. However, those who use XLK+cheshire might flag additional unforeseen residual errors which would require additional iterations. Those who decide not to perform the additional iterations will end up with collimation as good as the XL+cheshire. It is a choice.

Quote:

That's a worthy goal, but PAE and FAE are the only ones that matter. I don't know about eliminating all residual errors (especially considering registration tolerances), but reducing errors, if possible to the resolution of the tool, may be necessary for some applications. Using CAE and LAE to reduce PAE and FAE is the goal I see.



I prefer to treat all FAE, PAE, LAE, and CAE with the same importance. After all, my goal is to eliminate all four. Mathematically, the goodness of the axial alignment is determined by the smallest two residual errors out of the mentioned four. Since LAE and CAE can be reduced with the most accuracy using the XLK+CAM, they should provide the smallest PAE. It is just a different paradigm of the way I view axial alignment.
Having said all the above, owning a cheshire with XLK+CAM is not a bad idea. Actually I would recommend it. Not only it would server as a redundent check but in case the owner knows he/she has an residual FAE but do want want to take the time to correct it then a cheshire will be handy to eliminate PAE and live with the residual FAE.

Quote:

But what about those f/3 Dobs? A 20-inch f/3 can tolerate 2.5mm of FAE, with a Paracorr, but requires a PAE correction of 0.13mm (0.005-inch)! That's assuming, of course, that an f/3 scope is capable of high magnification performance.



Vic, you know how I feel about the 0.034D FAE tolerance derivation. In my opinion, the premise behind the formula is too loose. I am still of the opinion to use the edge of the field stop as the proper reference to calculate the FAE tolerance – not the edge of the coma “free” zone. According to my premise, FAE tolerance will even be tighter than the current tolerance calculation for a paracorr.

However, let us proceed with the 2.5mm FAE and 0.13mm PAE scenario you have described. Getting below 2.5mm FAE is not a problem. Even a tuned mass produced laser collimator can bring FAE below 2.5mm. A quality calibrated cheshire (or a quality barlowed laser) used by an experienced user can bring PAE at 0.13mm. Then my question is why should an autocollimator be used? After all, we can bring such a setup within tolerance using a stock laser collimator and a quality cheshire!!

Quote:

Quote:

As I mentioned earlier, those who are not interested in eliminating FAE then no autocollimator derivative tool should be used.


I disagree, because I prefer redundant verification. My autocollimator is still the quality control inspector for my axial alignment (even though my FAE tolerance, with Paracorr, is 2.8mm).



I guess what I am saying is that those who accept the 0.034D FAE tolerance formula will not need an autocollimator. On the other hand, those who strive to eliminate as much as of their FAE as their PAE then an autocollimator would be of great benefit.

Consider this: Someone who accepts the 0.034D formula stops when FAE=2mm and PAE=0mm. How would that someone use the autocollimator? If that someone inserts a quality single-pupil autocollimator, unstacked reflections will be seen. How would that be helpful? I guess I am having hard time reconciling the 0.034D formula with the use of an autocollimator. I do not know how an autocollimator can be used to eliminate PAE yet tolerate few millimeters of FAE -- it can't be done. What is your take, Vic?

Jason


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CatseyeMan
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3540402 - 01/05/10 07:56 AM

Quote:

... Consider this: Someone who accepts the 0.034D formula stops when FAE=2mm and PAE=0mm.




I think this is an unrealistic scenario. What tool out there reads PAE independently of FAE permitting the user to obtain a PAE of "zero" in the presence of 2mm FAE ? In the presence of a 2mm FAE, a "perfect" Chesire or Barlowed laser read will leave undetected PAE. The beauty of the P-3 stack of the CDP is that it DOES allow an independent read and subsequent correction of FAE prior to addressing PAE thus maximizing the effectiveness of the PAE tools to optimally reduce PAE within the tool(s) capability.


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Vic Menard
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3540497 - 01/05/10 09:07 AM

Quote:

The 0.2mm is an estimate for the maximum error an average user might run into. It is my estimate. Real life users will determine what this number should be. I can do much better but I am not an average user.



I don't consider myself an average user, but I have real problems keeping CAE and LAE less than 0.1mm (4 thousandths of an inch read at 88-inches) with my XLK+CAM autocollimator. I suspect most average users will have a hard enough time achieving a 0.2mm (0.008-inch) read for both signatures at 50-inches, which, worst case scenario, could deliver a PAE about twice that amount.

Quote:

...my goal is to eliminate all four...Since LAE and CAE can be reduced with the most accuracy using the XLK+CAM, they should provide the smallest PAE.



If you're limited to using just the XLK+CAM. The question I posed earlier, "...if the "perfect" XLK+CAM alignment differs from the calibrated Barlowed laser alignment, which one should the end user choose?", is the reason for the discussion. My concern is that a "better than average" user with a moderately large aperture, short focus optic will have difficulty achieving the correction resolution you're expecting. While I find that errors in the LAE and CAE signatures are useful indicators that there may be a problem with PAE and/or FAE, I'm not willing to use what I consider to be good LAE and CAE signatures as the final arbiter of best PAE alignment. But I do feel it's important to have a tool that is sensitive enough to verify the axial alignment I can achieve with my PAE alignment tool, and even cause a PAE reevaluation when necessary.

Quote:

...a tuned mass produced laser collimator can bring FAE below 2.5mm. A quality calibrated cheshire (or a quality barlowed laser) used by an experienced user can bring PAE at 0.13mm. Then my question is why should an autocollimator be used? After all, we can bring such a setup within tolerance using a stock laser collimator and a quality cheshire!!



While your closing comment is essentially correct, I think I've answered your question already. The autocollimator is simply too good at what it does not to be used! And for a scope with such demanding PAE tolerances, it's worth the effort to reduce FAE far below the tolerance threshold to be able to see the other alignment signatures and prove the alignment accuracy of the other tools!


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sixela
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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3540503 - 01/05/10 09:09 AM

Quote:

Quote:

... Consider this: Someone who accepts the 0.034D formula stops when FAE=2mm and PAE=0mm.




I think this is an unrealistic scenario. What tool out there reads PAE independently of FAE permitting the user to obtain a PAE of "zero" in the presence of 2mm FAE




You're selling it - a BlackCat placed precisely so that the focal plane is between the pupil and the ring.


Quote:

? In the presence of a 2mm FAE, a "perfect" Chesire or Barlowed laser read will leave undetected PAE.



Nope. If placed properly, it directly detects PAE (and nothing else).

Of course, many people don't know where to place the tool, but I do .

Yeah, I'm only precise in placement to about 3mm, so I guess 2mm FAE will translate into one micron extra PAE...I can read the BlackCat well, but not that well (assuming "zero" with apologetical quoting means as little as you can detect with the tools at hand).


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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3540548 - 01/05/10 09:29 AM

Quote:

Quote:

... Consider this: Someone who accepts the 0.034D formula stops when FAE=2mm and PAE=0mm.




I think this is an unrealistic scenario. What tool out there reads PAE independently of FAE permitting the user to obtain a PAE of "zero" in the presence of 2mm FAE ? In the presence of a 2mm FAE, a "perfect" Chesire or Barlowed laser read will leave undetected PAE. The beauty of the P-3 stack of the CDP is that it DOES allow an independent read and subsequent correction of FAE prior to addressing PAE thus maximizing the effectiveness of the PAE tools to optimally reduce PAE within the tool(s) capability.



I get what you're saying, but I don't agree with it entirely. I think claiming a PAE of "zero" is unrealistic--it's certainly possible but the read accuracy is going to place a limit on what you can actually see. And while Cheshire derivatives and Barlowed lasers are relatively insensitive to residual FAE, a 2mm defect at 60-inches (about 0.001-inch lateral displacement +/-0.75-inch from the focal plane--assuming collimation takes place at or near the focal plane and the focuser travel is linear) is unlikely to impact the corrected PAE enough to be cause for concern at or near the focal plane.

But I do agree that it's best to address FAE first (when possible), and always make the final tweaks to correct PAE last.


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3540960 - 01/05/10 12:59 PM Attachment (142 downloads)

Quote:

I don't consider myself an average user, but I have real problems keeping CAE and LAE less than 0.1mm (4 thousandths of an inch read at 88-inches) with my XLK+CAM autocollimator. I suspect most average users will have a hard enough time achieving a 0.2mm (0.008-inch) read for both signatures at 50-inches, which, worst case scenario, could deliver a PAE about twice that amount.



My XT10 has a focal length of 47.6-inches and I do not have an issue seeing 0.2mm LAE and CAE errors. Attached is a representation of how the 0.2mm LAE and CAE would look like. But I’ll grant you that going below 0.2mm to around 0.1mm then experience, illumination, and vision acuity play a large factor. That is why I quoted the 0.2mm maximum error. Of course typical errors are less than that.
But there is always advantage for owning and double checking with a cheshire. Not only as a redundancy check but also as an alternative method to quickly collimate a scope without worrying about FAE elimination and the additional work needed to eliminate FAE.

Quote:

"...if the "perfect" XLK+CAM alignment differs from the calibrated Barlowed laser alignment, which one should the end user choose?",



There is no simple answer. If that happens to me, I would analyze both tools using my camera to determine the root cause of the discrepancy. Is it the quality of the tools? Is it readability? Is it consistency? Or is it something else? Of course, this kind of analysis will be done only once – not with each collimation session.

Quote:

While your closing comment is essentially correct, I think I've answered your question already. The autocollimator is simply too good at what it does not to be used! And for a scope with such demanding PAE tolerances, it's worth the effort to reduce FAE far below the tolerance threshold to be able to see the other alignment signatures and prove the alignment accuracy of the other tools!



Then for those who intend to use the autocollimator should discard the 0.034D FAE tolerance formula and should tackle the FAE reduction with the same vigor as PAE reduction. That is, FAE becomes as important as PAE when an autocollimator is used. Of course, elevating FAE importance to the same level as PAE also implies elevating both CAE and LAE to the same high standards. I agree with this approach. Those who intend to get the most out of their scopes should not compromise any part of collimation including FAE elimination.

Jason


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3540981 - 01/05/10 01:11 PM Attachment (143 downloads)

And a 0.2mm PAE representation using a cheshire

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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541019 - 01/05/10 01:32 PM

Here's where I wound up with this, based on the particular scope I have.

After adding tension to the spider vanes and making sure the truss clamping was holding, the scope maintains good FAE through a normal range of elevations, by good I mean holding it within 1-2mm drift. On the assumption this was good enough, I set FAE once after scope setup and don't mess with it the rest of the evening, using a decent laser that I have verified for repeatable results. Since FAE isn't going to be exactly 0 much of the time, and I don't care that it isn't, I don't see any need for better accuracy than that.

However, that minor FAE drift does cause a similar PAE drift, and, given the tighter tolerance for PAE, I do care about making sure it is close to correct several times during an evening, particularly when I want to look at a planet. Another issue I see sometimes is that the mirror seems to shift a bit in the cell during an evening.

It helps that, unlike the secondary's closely-spaced adjustment screws and flexy spider vanes, the primary's widely-spaced screws with large thumb knobs lend themselves to making precise adjustments easily and quickly.

For a few months I used a cheshire for this, and decided the best tool would be one that is self-illuminated, remotely readable so that primary adjustment can be made while observing in real time, similar in weight to an eyepiece so that the scope wouldn't move up or down when switching between eyepiece and tool, accurate, didn't require FAE to be zeroed out first, and convenient. Only one tool fit all these requirements, the 1.25" Glatter tuBlug.

Most of the time, only a small tweak, if any, is necessary, which also tells me nothing bad has happened to the focuser axis. I can also tell that by racking the focuser in and out to make sure the reading doesn't change too much. If something does seem to be seriously amiss, it probably means a truss shifted and it is time to revisit the FAE.

This is why I haven't found the AC to be worth getting. That could change if I ever got a bigger coma-corrector-equipped scope capable of holding FAE more precisely.

Mike


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demiles
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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541041 - 01/05/10 01:41 PM

Guys, I don't mind hearing a little theory, but alot of us here are already unsure about wether we should use the AC or not. The last 2 pages of this thread for me are filed in the "What the H___ did he just say?" category. It seems to me that the AC can be so precise that any instability in the telescope at all can will simply drive one crazy trying to align it or keep it there. In the perfect world with stabile temps and perfect seeing I can maybe see its advantage. Are we not using the true potential of our scopes by not using the AC to collimate to the tolerances you guys are talking about? How far do we really need to go with this?

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Re: Concise thread about autocollimators+improveme new [Re: demiles]
      #3541061 - 01/05/10 01:50 PM

Quote:

Guys, I don't mind hearing a little theory, but alot of us here are already unsure about wether we should use the AC or not. The last 2 pages of this thread for me are filed in the "What the H___ did he just say?" category. It seems to me that the AC can be so precise that any instability in the telescope at all can will simply drive one crazy trying to align it or keep it there. In the perfect world with stabile temps and perfect seeing I can maybe see its advantage. Are we not using the true potential of our scopes by not using the AC to collimate to the tolerances you guys are talking about? How far do we really need to go with this?




Dwayne, I gave my answer to your question in post #3539549
The AC does provide more accuracy but whether the additional accuracy can be seen at the EP or whether the additional accuracy can be maintained by the scope is your call. The AC is just like other components in this hobby. Some swear that they can see a huge difference between an Ethos and a Nagler when others do not see the difference. Some could swear that their primary optics is significantly better than stock optics when others can't see the difference under typical sky conditions.
Jason


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Starman1
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3541074 - 01/05/10 01:54 PM

Coincidentally, Jason's illustrations show why I prefer circles, to collimate with, to triangles.
I find it much easier to assess the inaccuracy of the overlapped circles than the small visible error in the overlapped triangles--especially in the offset hole in the XLK autocollimator.
The issue with a circle occurs with the cheshire--is it easier to assess concentricity of the dark and light circles visible in the cheshire, or to assess the presence of the points of a triangle on the edge of the dark center in the cheshire?
I found it easier to assess concentricity than to tell when the points hit the edge of the dark center. You have to have really good vision to see the points on the triangle, but only mediocre vision to assess concentricity. Both circles can be slightly out of focus and concentricity can still be assessed.
We have to figure on visual acuity when assessing the efficacy of a tool.
I think this is an area where the XLK + CAM may provide an easier-to-assess verification of PAE correction than does the cheshire, if triangular primary marks are used.


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: Starman1]
      #3541085 - 01/05/10 01:59 PM Attachment (154 downloads)

Quote:

Coincidentally, Jason's illustrations show why I prefer circles, to collimate with, to triangles.
I find it much easier to assess the inaccuracy of the overlapped circles than the small visible error in the overlapped triangles--especially in the offset hole in the XLK autocollimator.




Don, here is a suggestion for a different center spot shape.

Edited by Don W (01/25/10 04:46 PM)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3541092 - 01/05/10 02:02 PM

Quote:

...Then for those who intend to use the autocollimator should discard the 0.034D FAE tolerance formula and should tackle the FAE reduction with the same vigor as PAE reduction. That is, FAE becomes as important as PAE when an autocollimator is used.



I see no reason to discard a performance tolerance. There's nothing that says you can't do better than the tolerance, but it's important to know specifically what needs to be accomplished. If the optic has no coma correction, then it's important to keep your eye on the PAE. But if the optic is an 8-inch f/4 astrograph with coma correction, the FAE tolerance (0.04-inch) may be as important as the moderate magnification PAE tolerance (about 0.025-inch). Again, there's nothing that says you can't do better than the prescribed tolerances, and if the goal is high resolution digital imaging, those 8-inch f/4 axial tolerances may demand the precision of an autocollimator.

Quote:

Of course, elevating FAE importance to the same level as PAE also implies elevating both CAE and LAE to the same high standards.



I don't see it that way because I approach precision alignment redundantly. CAE and LAE signatures are useful if they can help me improve PAE (for most visual applications) or PAE and FAE (for some imaging applications). In visual applications (in particular when f/ is less than 4.5 and fl is greater than 60- or 70-inches), I would be loathe to substitute better precision CAE and LAE over a calibrated Barlowed laser PAE. I'll settle for good CAE and LAE, knowing the PAE is very good and the residual error affecting CAE and LAE is probably FAE that is also likely to be inside the tolerance window. And I honestly don't see that as a compromise.


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541137 - 01/05/10 02:16 PM

Quote:

I see no reason to discard a performance tolerance. There's nothing that says you can't do better than the tolerance, but it's important to know specifically what needs to be accomplished.



Vic, you missed my point. My point is that if you use an autocollimator then you have to treat FAE with the same importance as PAE; otherwise, the tool will not work. Elevating the importance of FAE when using an autocollimator has nothing to do with FAE tolerance but it has to do with the autocollimator.

Quote:

I'll settle for good CAE and LAE, knowing the PAE is very good and the residual error affecting CAE and LAE is probably FAE that is also likely to be inside the tolerance window. And I honestly don't see that as a compromise.



I would treat all PAE, LAE, CAE, and FAE with the same importance. If all agree and all show zero errors then I am done. If at least one shows inconsistency then I investigate.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Starman1]
      #3541151 - 01/05/10 02:21 PM

Quote:

...I found it easier to assess concentricity than to tell when the points hit the edge of the dark center.



I agree. When I use the Glatter laser with the 1mm aperture stop, I can see the silhouette of the triangle and the perforation embedded in the concentric rings diffraction pattern. I get the best of both alignment signatures--concentricity for accuracy and triangular edges that make the silhouette stand out the concentric rings.

Having used notebook paper reinforcement rings before (and still collimating other scopes with them regularly), I'm not ready to give up my perforated triangle yet...

Quote:

I think this is an area where the XLK + CAM may provide an easier-to-assess verification of PAE correction than does the cheshire, if triangular primary marks are used.



Having used the XLK+CAM, I wonder if the white CAM ring shouldn't be about 1mm (diameter) larger than the black CAM ring (and the white perforation 1mm smaller). This would make the alignment pattern a thin (0.02-inch) annulus--instead of a black ring with no bright background. (What do you think, Jim?)


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demiles
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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541183 - 01/05/10 02:32 PM

Jason, I will be getting a AC to give this a try. Does the center spot have to be reflective, or can it be a solid black circle. Can I lay a reflective triangle over the circle? Any preference over a 1 1/4 vs 2 in AC and cheshire?

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Re: Concise thread about autocollimators+improveme new [Re: demiles]
      #3541201 - 01/05/10 02:41 PM

Quote:

Guys, I don't mind hearing a little theory, but alot of us here are already unsure about whether we should use the AC or not.



It depends. Is your 15-inch Obsession a Classic or UC? At f/4.5, the Classic is borderline if you're adept with the other axial alignment tools. At f/4.2, the UC is less forgiving--and the UTA secondary mirror alignment can be a puzzler as well! Understanding your mechanicals and using precision alignment tools will help you to maintain the high performance standards your optics demand.

Quote:

It seems to me that the AC can be so precise that any instability in the telescope at all can will simply drive one crazy trying to align it or keep it there.



There's some truth to that, especially for the UC. That's the price you pay for that kind of portability. But the fact is that you must use precision tools and you can maintain the prescribed tolerances, whether your scope is a Classic or a UC.

Quote:

In the perfect world with stable temps and perfect seeing I can maybe see its advantage. Are we not using the true potential of our scopes by not using the AC to collimate to the tolerances you guys are talking about? How far do we really need to go with this?



Well, if your seeing never steadies out and your mirror never reaches thermal equilibrium, then you won't be using high magnifications and you can collimate to a broader tolerance. In Florida, I'd say we get about 80- to 90-percent excellent observing nights for 15-inch apertures where you can push the magnification well over 500X. Come to the Winter Star Party, you'll be stunned how well your scope will perform!


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3541240 - 01/05/10 02:55 PM

Quote:

I would treat all PAE, LAE, CAE, and FAE with the same importance. If all agree and all show zero errors then I am done. If at least one shows inconsistency then I investigate.



But you can't see FAE with an autocollimator without decollimating the primary mirror, and you can't really see PAE with any clarity. And you can't see either if you don't use the center pupil, which you suggested earlier. Which makes me think that you consider LAE and CAE to be more important than FAE or PAE.

Edited by Vic Menard (01/05/10 03:10 PM)


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demiles
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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541241 - 01/05/10 02:57 PM

Vic, I have been able to push the mag to 500+ on Mars, Jupiter and Saturn with good results when the seeing allowed. The opportunity just doesn't happen very often. FYI, I have the 15 in. classic.

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sixela
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Re: Concise thread about autocollimators+improveme new [Re: hudson_yak]
      #3541257 - 01/05/10 03:04 PM

Quote:


However, that minor FAE drift does cause a similar PAE drift,



I disagree slightly with the wording. The FAE drift doesn't cause PAE drift. It's just that a sagging secondary generates both an FAE *and* a PAE, but typically a lot less PAE because the path from secondary to focuser is shorter than the path from secondary to primary (on my scope, by a ratio of rougly 1/4).

I used to have the same behaviour, but with my new Dob structure, the secondary and truss structure sag is zero. Zilch, nada, nothing.

When the temperature differences are brutal I do have to recollimate once the scope is cooled, though.


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Re: Concise thread about autocollimators+improveme new [Re: demiles]
      #3541264 - 01/05/10 03:06 PM

Dwane, what collimation tools are you currently using (and are you using a Paracorr or similar coma correction)?

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sixela
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Re: Concise thread about autocollimators+improveme new [Re: Starman1]
      #3541270 - 01/05/10 03:08 PM

Quote:

Coincidentally, Jason's illustrations show why I prefer circles, to collimate with, to triangles.
I find it much easier to assess the inaccuracy of the overlapped circles than the small visible error in the overlapped triangles--especially in the offset hole in the XLK autocollimator.



For a Cheshire I have a different experience. But to judge placement of the triangle it's helpful not only to look at the points of the triangle, but also at the relative size of the three circular segments between the triangle sides and the BlackCat Cheshire ring.


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Re: Concise thread about autocollimators+improveme new [Re: sixela]
      #3541290 - 01/05/10 03:15 PM

Quote:

For a Cheshire I have a different experience. But to judge placement of the triangle it's helpful not only to look at the points of the triangle, but also at the relative size of the three circular segments between the triangle sides and the BlackCat Cheshire ring.



Good point. I also used the three segments to get a better read when I was using my BlackCat Cheshire more regularly. (I do keep my Cheshire readily available just in case my laser acts up.)


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Re: Concise thread about autocollimators+improveme new [Re: sixela]
      #3541467 - 01/05/10 04:27 PM

Quote:

I disagree slightly with the wording. The FAE drift doesn't cause PAE drift.




Yeah, I could have worded it differently, what I meant was both drifts often happen due to the same reason. And with the LB, and probably most mass-produced truss or strut scopes (the new Orion 12-14" models are probably better than most), it's not just secondary holder sag but UTA sag. This appears to cause about the same drift in both FAE and PAE. Of the same order, anyway, though I haven't attempted to really look at the geometry of it. All I know is the PAE gets out further than I want it to when moving the scope up and down, and the 2x PAE read makes it quite obvious.

Mike


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541475 - 01/05/10 04:31 PM

Vic, I use a site tube for secondary position. A AstroSystems laser in single beam mode for secondary tilt/barlowed for primary adjustment. Yes I do use a Paracorr all the time.

Edited by demiles (01/05/10 04:35 PM)


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Re: Concise thread about autocollimators+improveme new [Re: demiles]
      #3541542 - 01/05/10 05:03 PM

In the early '90s I used a single beam AstroSystems laser with my homebuilt 20-inch f/6.2 Dobsonian. I upgraded to a Glatter with a Barlow attachment when I purchased a 22-inch f/4.1 StarMaster about ten years ago.

From your commentary (500X, good detail(?) on Mars and Jupiter) it sounds to me like you're getting reasonably good collimation. But Don Pensack would note that, "The seeing seems to get better with improved collimation." The fact that you're considering an autocollimator is an indication that something is amiss.

I refer to the autocollimator as the ultimate quality control inspector. It will tell you whether or not you're doing a good job with your other axial alignment tools. It will almost certainly help you to improve FAE, and possibly PAE too. When both axial alignments are well corrected, they deliver snap focus.

I would also suggest replacing your primary mirror center spot if you decide to purchase an autocollimator--I like the large reflective white triangle with 3/16th-inch perf (1/4-inch perf is also excellent, but fragile before it's affixed to the primary). The XLK+CAM is still a beta product, but the XLK will get the job done. CatsEye autocollimators are only available in 2-inch.

If you find that the autocollimator is always finding significant residual axial errors no matter how carefully you use your AstroSystems laser, you may want to upgrade your laser too. Using the Glatter with 1mm aperture stop I can get the axial alignment so close that the autocollimator is almost always nearly perfect before I even attempt a minor tweak.


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541553 - 01/05/10 05:08 PM

Quote:

Having used the XLK+CAM, I wonder if the white CAM ring shouldn't be about 1mm (diameter) larger than the black CAM ring (and the white perforation 1mm smaller). This would make the alignment pattern a thin (0.02-inch) annulus--instead of a black ring with no bright background. (What do you think, Jim?)




Been there...


reference post

I even built a similar one

reference post

And the pupil views. Note how the CAM is barely visible from the central pupil when it is apparent from the offset pupil. Another reaffirmation why the CAM can’t be stacked from the central pupil. The AC background reflection disappears for the same reasons reflections 1 & 3 disappear from the central pupil.

reference post

Having spent countless hours analyzing the CAM, I like it in its current form with two equally sized rings. The problem with the above is that the eye can discern the annulus ring but it is very hard to discern the thickness.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: demiles]
      #3541588 - 01/05/10 05:26 PM Attachment (130 downloads)

Quote:

Jason, I will be getting a AC to give this a try. Does the center spot have to be reflective, or can it be a solid black circle. Can I lay a reflective triangle over the circle? Any preference over a 1 1/4 vs 2 in AC and cheshire?




- Reflective center spot is highly recommended
- I do not see harm with laying the triangle over the existing donut though it might look somewhat awkward
- You got to go with the 2" autocollimator. I have both sizes and the 1.25" autocollimator is a pain. The 2" gives you a wider view to locate and work with all reflections. Just see the attachment. You can see how the field view of the 1.25" is very constraint – right photo.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541634 - 01/05/10 05:48 PM

Quote:

Quote:

I would treat all PAE, LAE, CAE, and FAE with the same importance. If all agree and all show zero errors then I am done. If at least one shows inconsistency then I investigate.



But you can't see FAE with an autocollimator without decollimating the primary mirror, and you can't really see PAE with any clarity. And you can't see either if you don't use the center pupil, which you suggested earlier. Which makes me think that you consider LAE and CAE to be more important than FAE or PAE.




I do not evaluate PAE using the central pupil. I use the cheshire which agrees with the initial XLK+CAM collimation work. If someone has a cheshire in their case it is highly recommended to use it to confirm the final collimation work.
As far as FAE, I do not check for it because I can't without decollimation. However, during my initial analysis work I would decollimate then take a photo to evaluate P+3 stack to ensure my flow/tools did in fact yield ~0 FAE.
The point is that if you build confidence by experimentation that PAE and FAE always agree with LAE and CAE then PAE/FAE checking becomes optional. If they don't then you investigate.

Actually, I ran into situations when things did not agree. I investigated. This is how I unraveled how placing the AC away from the focal plane will impact results. Normally, the difference is small -- sometimes too small to notice by the eye. This is why I took photos and analyzed them on my computer to accurately measure discrepancies. Of course I do all that as part of my research and I do not expect others to do the same.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3541635 - 01/05/10 05:48 PM

Vic, Getting a more accurate FAE would be my main reason for trying the AC.

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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541665 - 01/05/10 06:02 PM

My centermark ring, which is 3M white reflective material, appears 90% as large as the black cheshire center to my eye. It is very easy to assess the evenness of the black ring around the white centermark when collimating. Any differences in thickness of the black ring are easily seen.
I tried the triangle (albeit red) and found the 4 marks hard to see compared to white.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3541727 - 01/05/10 06:33 PM

Quote:

Been there...



Well, half way there. I would like to try one with the annulus visible on the outside diameter too. I think there's a good chance at getting better illumination with inner and outer annuluses visible.


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541779 - 01/05/10 07:04 PM

Quote:

Quote:

Been there...



Well, half way there. I would like to try one with the annulus visible on the outside diameter too. I think there's a good chance at getting better illumination with inner and outer annuluses visible.




I believe I wanted to do that but did not find the proper washer sizes. But I did something close -- right CAM in the photo. Gosh, I tried so many things and did so much analysis to the point I can recall everything I did.
The right CAM as depicted in the photo has the same outer diameter but smaller inner diameter. The idea here is to balance the interior illumination and eliminate the outer one.


reference post

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3541781 - 01/05/10 07:04 PM

Quote:

Which makes me think that you consider LAE and CAE to be more important than FAE or PAE.

Quote:

I do not evaluate PAE... As far as FAE, I do not check for it...






You could have just said "yes".

Quote:

However, during my initial analysis work I would decollimate then take a photo to evaluate P+3 stack to ensure my flow/tools did in fact yield ~0 FAE.
The point is that if you build confidence by experimentation that PAE and FAE always agree with LAE and CAE then PAE/FAE checking becomes optional...I do not expect others to do the same.



I keep reading this synopsis but I just can't imagine aligning my scope using just the offset pupil in the XLK+CAM, never looking at PAE and FAE! I'm either not sufficiently adept making the required iterative (and possibly "intuitive") adjustments with the XLK+CAM (probable), or there's something at a fundamental level that's causing me to subliminally reach for my laser when I try to consider limiting myself to alignments other than PAE and FAE (almost certainly). I just don't have your level of confidence--and that might be because we have different visual acuity, different scopes, and/or different thinking when it comes to assessing and correcting axial errors.


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541813 - 01/05/10 07:18 PM

Quote:

I just don't have your level of confidence--and that might be because we have different visual acuity, different scopes, and/or different thinking when it comes to assessing and correcting axial errors.



That is not the reason. After I collimated my scope 100s of times over few weeks and got consistent results, that is when I build my confidence. However, I still reach out to my cheshire just because it is there and it is always gratifying to bag another reaffirmation
Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3541832 - 01/05/10 07:24 PM

Quote:

...The...CAM as depicted in the photo has the same outer diameter but smaller inner diameter. The idea here is to balance the interior illumination and eliminate the outer one.



The problem I have with the (current beta) CAM is that it's difficult to illuminate properly. If the illumination comes from the side, it looks like there is a residual crescent from the background reflection when in fact it's just the illuminated edge of the washer. I guess it might still be problematic with two annuluses showing, but at least there would be more brightness in the alignment side of the CAM, and since the offset illumination would affect opposite sides of the inside and outside anuluses, it might improve the read.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3541922 - 01/05/10 08:06 PM

Quote:

Quote:

I just don't have your level of confidence--and that might be because we have different visual acuity, different scopes, and/or different thinking when it comes to assessing and correcting axial errors.



That is not the reason...



I wasn't talking about your level of confidence--I was talking about mine--and I suspect those are the reasons. I actually have issues reading the alignments to the accuracies you've described, and I know that's at least partially due to my visual acuity (it's also the reason I rely so much on my Glatter). I also know that while my longer focal length helps me to see P-3 more focused, the distance I must observe P-3 (or P-2) from limits the read somewhat (I'm sure visual acuity is at least partly to blame here as well). Finally, I can't get my mind around ~zero FAE, PAE, or any XAE. I have a reasonable understanding of functional or usable read accuracies and how they apply to tolerances and mechanical registrations. And while I understand CAE and LAE, I still think a direct read of PAE and FAE is better than an interpreted read of PAE and FAE, and this is almost certainly because of my experience with the XLK+CAM.

And I'm not a big advocate of reiteration--recollimating over and over again. It's a requirement for good (enough) secondary mirror alignment, but it doesn't have to be for (very) good axial alignment--even considering mechanical registration, etc. With the XLK, I found that if P-2 was suffieciently misaligned to be obvious in the 22 f/4, it was quicker (for me) to just reassess P-3 with the autocollimator and PAE with the Glatter (and be particularly careful) to arrive at a better P-2 alignment in the XLK. I found this worked consistently, and almost always to a "sufficient" precision (P-2 looked like a "perfect" hexagram) on the second try.


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541949 - 01/05/10 08:17 PM

Quote:

it looks like there is a residual crescent from the background reflection when in fact it's just the illuminated edge of the washer.




Ummm, this is what I get. Left photo is for a slightly misaligned CAM and the right photo for a perfectly aligned CAM.

The barley seen trace of light around the dark ring of the right photo is only seen by the camera. When I align the CAM, it looks pitch black. I do not know why it is different for you.
Jason


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3541974 - 01/05/10 08:30 PM

Vic, something sounds amiss. I suspect your XLK+CAM might be slightly misaligned or maybe your scope with different specs than mine is manifesting a new issue that I have not run into with my scope. I certainly explained all the discrepancies I have run intowith my scope and the newly gained knowledge helped me to achieve collimation with consistent visual cues from all reflections. Maybe we need to follow up offline with Jim. Maybe Jim can ship you another XLK+CAM or I can get hold of yours temporarily to analyze it.

As far as your re-iteration comment, I am also not a fan of it. If P+3 stack and a cheshire agree but both disagree with P+2 and the CAM then we can either stop or re-iterate to get all four visual cues to agree. If we do not re-iterate then we end up with the best collimation an XL+Cheshire could provide. If we decide to re-iterate then we improve our collimation.
In other words, I see the required re-iteration required by the XLK+CAM as above and beyond what the XL+Cheshire provides.
Jason


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Re: Concise thread about autocollimators+improveme new [Re: sixela]
      #3542244 - 01/05/10 10:32 PM

Quote:


... You're selling it - a BlackCat placed precisely so that the focal plane is between the pupil and the ring.

...If placed properly, it directly detects PAE (and nothing else).





Touche'

Actually, I think I realize the disconnect after reading the later posts you participated in about Secondary Sag affecting both PAE and FAE.

Here's the point I was trying to make:

Starting from theoretical perfect axial alignment with PAE=0 and FAE=0, if the Secondary is then tilted to cause an FAE of 2mm at the Primary, I don't consider PAE to have been introduced. In other words, it's the Secondary movement that has caused the problem - not a shift of the Primary. If one inserts a Cheshire into this Scenario, they will see a "pseudo" PAE error and unless they realize that it is a Secondary tilt issue, they will be misled into adjusting the tilt of the Primary to recenter the spot in the Cheshire ring.

This is exactly why (in lieu of the CDP) to achieve axial alignment that the iterative process is required alternating the use of the Cheshire (or Barlowed laser) to adjust the Primary tilt (effecting PAE) and the autocollimator to adjust the Secondary tilt/rotation (effecting FAE). Fortunately in this method, the errors diminish to negligibility after 2-4 iterations.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3542258 - 01/05/10 10:39 PM

Quote:

Vic, something sounds amiss.




I would suspect Vic's illumination method is the likely difference. I don't have another CAM unit built (other than my beta which I will keep here) so perhaps Vic will lend his to you for comparison to yours.


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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3542679 - 01/06/10 05:35 AM

Quote:

Starting from theoretical perfect axial alignment with PAE=0 and FAE=0, if the Secondary is then tilted to cause an FAE of 2mm at the Primary, I don't consider PAE to have been introduced.




Jim, I don't quite see how you think here. The primary's axis between the primary and the secondary won't change, of course, but the reflected axis reaching the actual focal plane will - less than the FAE, as much as the distance focal plane-secondary is less than the distance secondary-primary, but still in this case some 0.5 mm or a bit less.
If you had moved the FAE by tilting the focuser, the PAE introduced would be very much smaller.

Nils Olof


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3542712 - 01/06/10 06:47 AM

Concerning CAM illumination, would it not be possible to coat the desired area of the CAM with a phosphorescent coating (http://www.glonation.com/)? After holding it next to a light source it should "glow" on its own for some time afterwards.

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Re: Concise thread about autocollimators+improveme new [Re: nsldvd]
      #3542728 - 01/06/10 06:58 AM

Quote:

Concerning CAM illumination, would it not be possible to coat the desired area of the CAM with a phosphorescent coating (http://www.glonation.com/)? After holding it next to a light source it should "glow" on its own for some time afterwards.




Great minds think alike . I've actually used a phosphorescent plastic media for the bright CAM element in preliminary beta development work - It works great for the first 5-10 seconds after charging but becomes too dim too quickly for practical use. I plan to try some alternate materials to test for longer half-life - thanks for the link.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3542732 - 01/06/10 06:59 AM

Quote:

Quote:

it looks like there is a residual crescent from the background reflection when in fact it's just the illuminated edge of the washer.




...this is what I get...I do not know why it is different for you.



I think I know now. Your image clearly shows a directed beam that illuminates only the white washer. The black washer is carefully shielded from any extraneous illumination. I was clearly seeing some off axis illumination on the black washer from my diffuse light source (this even happened with Jim's test setup at PSSG with a "glow in the dark" white washer--must have been getting light from somewhere else). I'll need to try again when I get the 22 out for the next observing session.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3542764 - 01/06/10 07:46 AM

Quote:

Vic, something sounds amiss. I suspect your XLK+CAM might be slightly misaligned or maybe your scope with different specs than mine is manifesting a new issue that I have not run into with my scope. I certainly explained all the discrepancies I have run into with my scope and the newly gained knowledge helped me to achieve collimation with consistent visual cues from all reflections...



I don't think it's a misaligned CAM (I was having the same issues with Jim's beta at PSSG), more than likely it's just a lack of persistence (almost certainly attributable to the round trips back and forth from the 22-inch primary mirror to the focuser, coupled with pushing the limits of the read), although I think I was expecting a quicker resolution on Jim's much shorter test rig with no intervening secondary mirror.

Quote:

If P+3 stack and a cheshire agree...



...but they always agree since they are direct reads...

Quote:

...but both disagree with P+2 and the CAM then we can either stop or re-iterate to get all four visual cues to agree.



That's the rub. If after several iterations I still can't get all four (or just the CAE and LAE alignments) aligned to the resolution of the signature reads, I feel compelled to finish with a precision PAE alignment.

Quote:

...I see the required re-iteration required by the XLK+CAM as above and beyond what the XL+Cheshire provides.



It's certainly above and beyond, but until I achieve your level of confidence regarding the derived PAE precision, and the persistence to push through the necessary reiterations to bring all four signatures into alignment, I'll be keeping a Cheshire (or Barlowed laser) handy for a quick look at (or tweak to) the primary axial correction.


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Re: Concise thread about autocollimators+improveme new [Re: Nils Olof Carlin]
      #3542770 - 01/06/10 07:49 AM

Agreed Nils.

See my clarification post above to Sixela.

In the Cheshire/AC iterative process of axial alignment, for conceptual simplicity I promote the concept of adjusting the Secondary tilt/rotation to minimize FAE and adjusting the Primary to minimize PAE when in fact as you state, adjusting the Secondary in the process minimizes both FAE and PAE when restacking the images in the central pupil of the AC after Cheshire use. To me it's less confusing to limit the Secondary to only FAE effect for tutorial purposes when conveying the logistics of optical component adjustment sequence with this method.

As for routine adjustment of the focuser tilt to correct FAE, I discourage this practice. Once reasonable focuser squareness has been achieved and initial axial collimation has been accomplished, I attribute any subsequent FAE introduction to Secondary support mechanics shifts which IMHO need to be reversed rather than compensated for via focuser tilting. That said, I also understand that it's sometimes easier to change the tilt of the focuser axis than to re-adjust the Secondary, but at some point if the problem persists with creeping Secondary changes, the original source of the error (the Secondary) may need to be addressed.

Regards,


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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3543150 - 01/06/10 11:45 AM

Jim, it makes sense indeed to get the focuser axis right before fine-tuning the primary.
One possibility that kind of suggests itself is to put a very thin reflective annulus around the AC pupil - say, 1/4" outer dia (and a mm wide or so?), matching the center hole on the primary marker. This would show the PAE directly, in the same way as the cheshire does, with no penalty to any of the other indicators, and possibly help shortening the procedure some.

Nils Olof


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3543156 - 01/06/10 11:51 AM

Quote:

I think I know now. Your image clearly shows a directed beam that illuminates only the white washer. The black washer is carefully shielded from any extraneous illumination.




I use two clip-on light sources as depicted in the photo of post #3537603. One meant for the CAM and another for the center spot.
I carefully place the first light source clipped on to one of the spider vanes to illuminate the CAM’s reflective ring. The bottom of the focuser shields light from the dark ring. I use the second light source for the center spot. By only keeping the first light source on, I further enhance the CAM contrast (right photo).



By the way, the above photo shows a CAE of 0.1mm

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Nils Olof Carlin]
      #3543189 - 01/06/10 12:10 PM

Quote:

Jim, it makes sense indeed to get the focuser axis right before fine-tuning the primary.
One possibility that kind of suggests itself is to put a very thin reflective annulus around the AC pupil - say, 1/4" outer dia (and a mm wide or so?), matching the center hole on the primary marker. This would show the PAE directly, in the same way as the cheshire does, with no penalty to any of the other indicators, and possibly help shortening the procedure some.

Nils Olof




Jim and Vic experimented with this idea years ago. Jim called it the AutoCat. Check out the following post
AutoCat

Here is another post

I have expressed my concerns about the idea in this post

The area around the central pupil is highly sensitive because of the “G4” focal point which is basically the background AC reflection for the central pupil. The “G4” naming comes from your autocollimator analysis page, Nils Olof. If any part of the G4 hits the reflective annulus ring, the whole background will lighted up which would complicate and interfere with the autocollimator operation which is what Vic indicated when he experimented with it.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3543240 - 01/06/10 12:31 PM

Quote:

Quote:

If P+3 stack and a cheshire agree...



...but they always agree since they are direct reads...



With "agree", I meant both "agree" that alignment has been met which is indicative of axial alignment. It is certainly possible to have misaligned P+3 view but aligned cheshire view. As you know, both are independent of each other.

Quote:

Quote:

...but both disagree with P+2 and the CAM then we can either stop or re-iterate to get all four visual cues to agree.



That's the rub. If after several iterations I still can't get all four (or just the CAE and LAE alignments) aligned to the resolution of the signature reads, I feel compelled to finish with a precision PAE alignment.



My point is that if you can't get all views (P+2, CAM, P+1, and cheshire) to agree at least once then you need to investigate to root cause the issue. If you know your scope and tools are capable to getting all views aligned but you decide at the moment not to take the time to align all and rely on the cheshire then that is a different story. I am trying to differentiate between: “Yes I can do it if I take the time” versus “No it can’t be done”

Quote:

Quote:

...I see the required re-iteration required by the XLK+CAM as above and beyond what the XL+Cheshire provides.



It's certainly above and beyond, but until I achieve your level of confidence regarding the derived PAE precision, and the persistence to push through the necessary reiterations to bring all four signatures into alignment, I'll be keeping a Cheshire (or Barlowed laser) handy for a quick look at (or tweak to) the primary axial correction.



It comes down to the trust level of your tools. I aligned with the cheshire and taken photos. I aligned with the XLK+CAM and taken photos. I’ve taken 100s of photos. I found out that the XLK+CAM has consistently at least matched the cheshire results. That is part of confidence building and trusting the tools. That is why I feel comfortable with the XLK+CAM. Having said that, I still reach out for my cheshire because it is there and to keep my XLK+CAM confidence high.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3543402 - 01/06/10 01:43 PM

Quote:

...I am trying to differentiate between: “Yes I can do it if I take the time” versus “No it can’t be done”...It comes down to the trust level of your tools.



I'm sure it can be done, but for me it comes down to my ability to accurately read the signature alignments, and then make the best available correction. I'm pretty good at reading concentricities and hexagrams, but trying to balance the two against each other (CAM and LAE) became frustrating (for me) several iterations before I reached the full read precision. That said, I feel I can read P-3 to almost the same resolution as P-2 (given my 88-inch focal length), but I know I can't guarantee zero FAE with my best P-3 read (P-2 proves that). In that respect, I can't fully trust my read of P-2 to guarantee "perfect" parallelism. And since the CAM puts the alignment reference point (what P-2 is parallel to and the only actual "point" on the alignment axis when CAM and LAE are used in tandem) a focal length away from the focal plane, I just feel more comfortable with another point that's actually on the alignment axis.


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3543611 - 01/06/10 03:17 PM

Vic's comments about the readability of tools brings to mind my first uses of the XLK. The two stacks are P-2 and 1-3. P-2 is clear and sharp, while 1-3 is a little "fuzzy".
So differentiating which group is which should be easy, right?
Wrong.
With my glasses on, P-2 is fuzzy and 1-3 is sharp. With my glasses off, P-2 is sharp and 1-3 is fuzzy. At first, I couldn't tell which stack was which because I wasn't certain whether glasses on or glasses off gave the correct read. I use glasses for the sight tube (to see the wires), and no glasses for the cheshire (because I can focus that far away).
Then Jason mentioned the P-2 stack didn't suffer from parallax as much as 1-3. By moving my head back and forth it became obvious which stack was P-2, because 1-3 would stack and unstack while P-2 remained nearly constant. Now I know that I always put the XLK into the focuser one way (I have a mark on the body) and NOT use glasses.

So when it comes to readability of the tools, you have to experiment a bit to adapt to the tool use for your circumstances (focal length, vision, light levels).

I'd still like to see the commercial development of the Krupa collimator. It would be a lot easier to read than the bottom of a barlowed laser or a BLUG, and it could be parallax-free.


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Re: Concise thread about autocollimators+improveme new [Re: Starman1]
      #3543890 - 01/06/10 05:29 PM

Vic, Don, points well-taken...

Don, I found it interesting how your eye-glasses make a difference. I do not wear glasses (not yet) so I never had a similar experience. But your experience made me think and I came up with an explanation. You must be long sighted (OK I know this is a personal matter; therefore, you do not have to answer). When the eye is long sighted then the lens focuses images behind the retina. Which means that reflections P and 2 will form behind the retina; therefore, they will look fuzzy. Now, reflections 1 and 3 by the virtue of their location behind our heads will always form in front of reflections P and 2. For what is considered health eyes, P&2 will end up on the retina (sharp images) and 1&3 will end up in front of the retina (fuzzy images). But for long sighted eyes, the whole thing will move away from the eye lens; therefore, P&2 will end up behind the retina (fuzzy image) and the 1&3 will end up ion the retina (sharp images). If this is true, then short sighted eyes are out of luck for all four reflections since none of the reflections will form on the retina.

Talking about readability, I am still interested to hear your opinion about the following center spot shape.



Left: The suggested center spot
Middle: View when it is used with the cheshire. The outer edge of the newly suggested center spot will be easier to align with the cheshire.
Right: View with P+2 stacked. The P+2 circle will make it easier to stack

And the newly suggested center spot will give the impression your mirror is radioactive

Jason


Edited by Don W (01/25/10 04:46 PM)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3543933 - 01/06/10 05:56 PM

Jason, I think you should start the "now really concise, I promise" autocollimator thread .

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Re: Concise thread about autocollimators+improveme new [Re: sixela]
      #3543943 - 01/06/10 06:02 PM

Quote:

Jason, I think you should start the "now really concise, I promise" autocollimator thread .




Ummm, may be I shall call it the "mother of all concisions" thread


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3543969 - 01/06/10 06:22 PM

Quote:

Ummm, may be I shall call it the "mother of all concisions" thread




That would be fine. Post the images that we need to see through the tools, then LOCK the thread.


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Re: Concise thread about autocollimators+improveme new [Re: Galaxyhunter]
      #3544103 - 01/06/10 07:42 PM

Before this thread is locked a question:- It seems to me that the suggestion is that a 2" AC would be more useful than a 1.25" AC. In order to clearly identify the reflections, the wider AC mirror is better? True?

I have a 1.25" AC, but have never had a mechanically stable scope to use it seriously on - but I have a quality scope coming shortly.


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Re: Concise thread about autocollimators+improveme new [Re: sixela]
      #3544213 - 01/06/10 08:44 PM

Quote:

Jason, I think you should start the "now really concise, I promise" autocollimator thread .



I'm sure Jason has an article planned.
We can discuss it here after it's released!

(Has there ever been a concise thread about autocollimators?)


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Re: Concise thread about autocollimators+improveme new [Re: erick]
      #3544221 - 01/06/10 08:47 PM

Quote:

...a question:- It seems to me that the suggestion is that a 2" AC would be more useful than a 1.25" AC. In order to clearly identify the reflections, the wider AC mirror is better? True?



Yes.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3544233 - 01/06/10 08:56 PM

Quote:

...I am still interested to hear your opinion about the following center spot shape.



Seems to combine the benefits of a triangle and a ring--and should work well with a laser diffraction pattern too.

Edited by Don W (01/25/10 07:56 PM)


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Re: Concise thread about autocollimators+improveme new [Re: erick]
      #3544243 - 01/06/10 09:00 PM Attachment (152 downloads)

With respect to 2" versus 1.25" AC, refer to the attachment.

Top left photo represents P+3 (CDP) using a 2" AC -- actual unaltered photo.

Top right photo (A) is the same photo but altered to represent the same view via a 1.25” AC. Note the following:
1- The view field is significantly narrower – even part of reflection P is missing.
2- The AC background reflection is now getting on the way.

Well, photo (A) is not what you would really see – I lied. What you will see is represented by photo (B) -- a dark background with only reflections P&1 showing. None of the other reflections or even the AC background will be showing. Why? The reason is technical but I will go ahead and explain it. In order for all other reflections to show, the AC pupil background reflection (shown by the red arrow) which is the center point of the AC background reflection (shown in yellow) has to fall within the AC foreground reflection (shown in cyan). In the bottom left photo, the red arrow did fall within the AC foreground reflection in case of the 2” AC. However, in case of the 1.25” AC in the bottom right photo, it fell outside the AC foreground reflection which means none of the other reflections will be seen.

What does all the above gibberish mean? It means the 1.25” AC has two limitations:
1- The field of view is significantly limited – this is an intuitive one.
2- It significantly limits the amount of AC tilt before the view goes blank – not an intuitive one.

more reading

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3544267 - 01/06/10 09:11 PM

Quote:

Quote:

...I am still interested to hear your opinion about the following center spot shape.



Seems to combine the benefits of a triangle and a ring--and should work well with a laser diffraction pattern too.




Don't forget about how cool that center spot would look like because of its resemblance to the radioactive symbol. We can always brag about owning a “nuclear” scope.

Ummm, on the other hand I can foresee a troubling headline:
“An amateur astronomer was arrested today at an international airport for transporting what is seems to be a radioactive primary mirror”

Jason

Edited by Don W (01/25/10 07:57 PM)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3544405 - 01/06/10 10:20 PM

Thanks Vic and Jason. Looks like I'll be in the market for a 2" AC (with extra hole and optional attachments!!)

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Re: Concise thread about autocollimators+improveme new [Re: erick]
      #3544435 - 01/06/10 10:39 PM

Erick, make sure to report back

To those who are planning to buy an autocollimator, make sure to have reviewed my earlier post
Your expectation from an autocollimator should be high. After all, the autocollimator is meant to provide that last extra precision above and beyond your other collimation tools. You might not get that last extra precision with an average autocollimator. But you will get that last extra precision with a high quality autocollimator. Of course, your scope mechanics need to be stable enough to take advantage of that extra precision.
Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3544733 - 01/07/10 01:45 AM

Quote:

Erick, make sure to report back

To those who are planning to buy an autocollimator, make sure to have reviewed my earlier post
Your expectation from an autocollimator should be high. After all, the autocollimator is meant to provide that last extra precision above and beyond your other collimation tools. You might not get that last extra precision with an average autocollimator. But you will get that last extra precision with a high quality autocollimator. Of course, your scope mechanics need to be stable enough to take advantage of that extra precision.
Jason



For that last comment:
"Aye, there's the rub!"
If, like me, you find your scope changes collimation with altitude, you will be on a path to perdition! I can't tell you how many modifications I made and how much time I spent tracking down the source of the collimation changes with scope movement.
'Twas the autocollimator that showed them to me.
And now?
90 degrees of movement with no change in the autocollimator.
Learning to use the autocollimator taught me about scope mechanics.
So, you see, the AC has more than one use.

I'm only being a trace facetious. In fact, an AC IS a wonderful device for showing you where your scope needs a little souping up. In elimination of the sources of collimation drift, I ended up with a stiffer and better-built scope. All that from a relatively cheap tool.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3545002 - 01/07/10 09:00 AM

Quote:

...photo (A) is not what you would really see – I lied. What you will see is represented by photo (B)...



Of course, you could use the smaller CatsEye triangle, which should allow the various signatures to be seen with a smaller AC tilt. It's a matter of scale...and readability...


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3545165 - 01/07/10 10:35 AM

Quote:

which should allow the various signatures to be seen with a smaller AC tilt



The "smaller tilt" requirement means that you need to bring collimation close to perfection using other collimation tools before the 1.25" AC can be used. The 2" AC is not as demanding.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3545191 - 01/07/10 10:48 AM

Quote:

Quote:

which should allow the various signatures to be seen with a smaller AC tilt



The "smaller tilt" requirement means that you need to bring collimation close to perfection using other collimation tools before the 1.25" AC can be used. The 2" AC is not as demanding.



That's why I mentioned readability...(and followed with a "thumbs down")


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3550277 - 01/09/10 05:31 PM

Quote:

Quote:

Quote:

which should allow the various signatures to be seen with a smaller AC tilt



The "smaller tilt" requirement means that you need to bring collimation close to perfection using other collimation tools before the 1.25" AC can be used. The 2" AC is not as demanding.



That's why I mentioned readability...(and followed with a "thumbs down")




Just wanted for further clarify to others. With the 2" AC, you can tilt the AC enough to move reflections 1&2 away from reflection P&3 to be able to perform a good CDP focuser axial alignment. With the 1.25" AC, you can't because before reflection 1&2 move completely out of the way reflection 3 will disappear. You can use a smaller center spot to solve this issue, as Vic mentioned, but poorer readability of the smaller center spot and its reflections will compromise accuracy.

Bottom line: If you are planning to invest in an autocollimator, it is highly recommended to get a 2” autocollimator as opposed to 1.25”.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3550281 - 01/09/10 05:33 PM

Vic, with respect to iterations between P+2 and CAM stacks, refer to the following page starting from post #3098491 to post #3098519 (6 posts). I explained how to reduce the number of iterations.

With all this discussion about how the second pupil solves the disappearance act of reflections 2&3, I just wanted to clarify that it is possible to partially solve the disappearance act of these reflections via the central pupil but it would require changing the shape of the central pupil – reference post. Even though the triangular central pupil will solve reflection 2 disappearance act, reflection 2 will dimmer and jumbled with reflections 1&3. The 2nd offset will separate reflections 1&3 from reflection P&2 and show all reflections at their maximum illumination. Therefore, the 2nd offset is a better solution.



Jason


Edited by Jason D (01/09/10 07:37 PM)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3550359 - 01/09/10 06:31 PM

Quote:

Vic, with respect to iterations between P+2 and CAM stacks, if you refer to the following page starting from post #3098491 to post #3098519 (6 posts), I explain how to cut down on the number of iterations.



Jason, thanks for posting the reference page. As soon as I get a chance to set up the 22, I'll give the XLK+CAM another run with special attention to offset lighting and implementing negative CAE. I'll see if the read issues persist, or if my XLK+CAM experience was just one too many reiterations and not enough patience on my part.


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3550549 - 01/09/10 08:19 PM

Wow. I got a headache reading this thread the first time. Information overload but really cool.

SO, I have the Catseye tools and use them for collimation. I actually enjoy collimating my scope (12XTI, F4.9) but the best I've done with the autocollimator is stacking two images. By that time I've gotten bored and just start looking at stuff. Everything looks fine, but I have to ask, if I go the extra mile and do everything in Jason's outstanding thread, would the images be noticeably better?

I'm just curious. I collimate with the Catseye tools while it's still daytime and after the mirror is cooled, then do a brief collimation check at night with my Glatter laser and tBlug.

So, once again, would I tell a noticeable difference following Jason's instructions?

Thanks,
--Walt


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Re: Concise thread about autocollimators+improveme new [Re: BozemanWalt]
      #3550589 - 01/09/10 08:40 PM

Quote:

So, once again, would I tell a noticeable difference following Jason's instructions?




Walt, do you have the XL or the XLK version?

Assuming you have the XL version, the instructions I included are not new. These are the instructions that Vic came up with and these are the instructions that Jim includes with his autocollimators. I just re-presented them with lots of photos and illustrations.

Yes, you need to follow these instructions. I am referring to starting off with stacking P+3 stack (CDP) then stacking P+1 then double check with the cheshire and finally reiterate between the cheshire and autocollimator if you have to. If you do not follow these steps then you are not getting the most out of your autocollimator.

As far as noticing the difference, this is a subjective question. I believe you will at high magnification when sky conditions are stable.

A GENERAL COMMENT: If anyone has questions about the subjects covered in this thread, please ASK. Please do not be intimidated in case you can’t follow some of the posts in this thread. Please ASK questions to improve your knowledge and to benefit others.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3565536 - 01/17/10 01:59 AM Attachment (149 downloads)

This post is more targeted for those with imaging reflectors.

In multiple posts in this thread, it was clarified that when axial collimation is achieved the final view via the central pupil should include only the reflection of the primary mirror center spot against darkened background.

However, those with imaging reflectors might be unable to get rid of reflection 2 no matter what they do and their final view via the central pupil will include a hexagram.

This can potentially occur with imaging reflectors because:
1- Imaging reflectors tend to have shorter focal length in the neighborhood of 25”
2- The focal plane tends to reside above the focuser.

Combing both reasons above will place the autocollimator mirror significantly below the focal plane – in the neighborhood of >8% focal-lengths below the focal plane. When the autocollimator mirror is located significantly below the focal plane, reflection 2 and the background reflection of the autocollimator including the background reflection of the pupil will enlarge. When axial collimation is met, the foreground pupil reflection can’t fully eclipse the enlarged background pupil reflection which implies reflection 2 can’t be eliminated.

Refer to the attached animation.

The top row is for a normal case where the autocollimator mirror is placed at the focal plane (or within 2% focal-lengths from the focal plane). Foreground pupil reflection fully eclipses the background pupil reflection which is the reason why reflection 2 dimms then disappears.

The bottom row is for the special case where the autocollimator mirror is placed significantly below the focal plane. Foreground pupil reflection partially eclipses the larger background pupil reflection which is the reason why the larger reflection 2 dimms but never disappears.

One solution to the problem is to use a special autocollimator with a extended barrel to place the autocollimator mirror close to the focal plane as Mark has shown in this post

Those with visual reflectors typically will not run into this issue because their reflectors tend to have larger focal lengths and because the focal plane tends to be located within the focuser travel area which implies the autocollimator mirror will be located within 1% of the focal plane.

more reading

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3596928 - 02/01/10 03:38 AM Attachment (218 downloads)

Last year during an email exchange, Nils Olof Carlin referenced the radioactive symbol to clarify a point he was trying to make to me. Though we were not discussing center spot shapes, when the radioactive symbol flashed before my eyes I thought it would make a wonderful center spot. Earlier in this thread, I suggested the new center spot shape which resembles the radioactive symbol – refer to the left diagram.



The new center spot shape includes the best of both the donut and the triangle center spot shapes:
1- It shares the outer circular perimeter with the donut center spot shape for improved alignment with the cheshire. Refer to the middle diagram.
2- It shares the 3 intrusions with the triangle center spot shape for improved P+2 & P+3 reflections stacking and for facilitating cheshire alignment (3 intrusions vs. 3 collimation screws). I believe it is even better than the triangle shape when stacking P+2 & P+3 in the sense it is more precise to visually complete a circle as opposed to forming a hexagram. Refer to the right diagram.

Jim Fly recognized the benefits of my suggested shape. I was pleasantly surprised when Jim contacted me and informed me that he produced prototypes for the newly suggested center spot shape and offered me one for evaluation. Jim Fly called the new shape Hotspot. Few days ago, I received the Hotspot and the accompanied template. With permission from Jim, I am uploading few photos for the Hotspot and the template.

You might have noticed that the color of the Hotspot reflective surface is dark yellow as opposed to the common White and Red colors. When I inquired about the reason from Jim, he explained that as the Hotspot combines the benefits of the donut and triangle center spots, it also combines the benefits of both the White and Red colors, hence, the dark yellow color. Interesting!!

I am planning to replace my triangle center spot with the Hotspot and give it a test drive. I will publish my analysis and photos in a week or so.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3596929 - 02/01/10 03:38 AM Attachment (152 downloads)

Hotspot template

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3596930 - 02/01/10 03:39 AM Attachment (145 downloads)

And both put together

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3597078 - 02/01/10 07:25 AM

This is such fun to watch - the leading edge of new developments.

I wait with interest, Jason.


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Re: Concise thread about autocollimators+improveme new [Re: erick]
      #3597329 - 02/01/10 10:24 AM

Quote:

This is such fun to watch - the leading edge of new developments.

I wait with interest, Jason.




I am looking forward to give Hotspot a test drive as soon as possible; unfortunately, due to a home improvement project in my house, my scope is inaccessible to me at this time -- tucked away safely in the garage behind heavy pieces of furniture. I should be able to give Hotsot a test drive by next week.
Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3597334 - 02/01/10 10:28 AM Attachment (142 downloads)

with permission from Jim Fly, I am uploading several POVRay simulations completed by Jim.

First, here is the expected view via XLK offset pupil


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3597339 - 02/01/10 10:29 AM Attachment (133 downloads)

Via XLK offset pupil but with some P+2 misalignment

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3597340 - 02/01/10 10:30 AM Attachment (140 downloads)

CDP via the central pupil

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3597343 - 02/01/10 10:32 AM Attachment (128 downloads)

Via BlackCat

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3597345 - 02/01/10 10:33 AM

I think I'm liking the clarity of this design!

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Re: Concise thread about autocollimators+improveme new [Re: rockethead26]
      #3598066 - 02/01/10 03:55 PM

The proof will be "in the pudding", as they say.
I look forward to Jason's test.


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Re: Concise thread about autocollimators+improveme new [Re: Starman1]
      #3598255 - 02/01/10 05:05 PM Attachment (175 downloads)

One interesting advantage of Hotspot is assisting the user to optimally place the XLK. In previous posts at CN, I have already established that the optimal placement of the AC mirror is on or close to the focal plane. I also have shown that reflection sizes will change as the AC mirror moves away from the focal plane in either direction. When the AC mirror is below the focal plane, reflection 2 will look larger than reflection P. When the AC mirror is above the focal plane then reflection 2 will look smaller. The Hotspot will make it easier to compare both P&2 relative sizes as shown in the attachment and in the next post.
I need to clarify that both POVRay simulation figures are for cases where the AC mirror is 5% Focal-Length (FL) above and below the Focal-Plane (FP) which is excessive for visual scopes. 5%FL was selected for simulation to magnify the difference in size.

So, if Hotspot reflection 2 is larger than reflection P, then rack-out the AC. If Hotspot reflection 2 is smaller than reflection P, then rack-in the AC. Stop when both reflections are sized about the same.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3598260 - 02/01/10 05:05 PM Attachment (120 downloads)

.

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3603779 - 02/04/10 04:56 AM Attachment (128 downloads)

I decided to pull out my old XT10 stock primary mirror and replace the original stock donut center spot with Hotspot. But first I decided to check how centered is the original stock donut spot. It was 1.5mm off center which is disappointing. That means even if I achieve perfect collimation using whatever method and tool my PAE will be 0.75mm. The 0.75mm might still be within the “coma-free” zone spec for my F4.7 scope but it also means I have lost 0.75mm from my scope’s mechanical tolerance.
I do not understand why the manufacture can’t install a machine to place these center spots with high precision!!!


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3603780 - 02/04/10 04:56 AM Attachment (119 downloads)

Anyway, I went ahead and replaced the stock donut with Hotspot.
I will do my best to set aside time to replace my premium mirror with my stock mirror this weekend and give the Hotspot a test drive.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3603782 - 02/04/10 04:59 AM Attachment (145 downloads)

Behold the first “radioactive” primary mirror

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3603895 - 02/04/10 07:55 AM Attachment (130 downloads)

Quote:

Behold the first “radioactive” primary mirror




... not quite - my 6" has that claim to fame.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3603897 - 02/04/10 08:03 AM

... just a technicality, Jason. You have the template "upside down" (unless you reversed the pic). It certainly doesn't make any difference on centering accuracy but with it right side up (text not reversed) because of the way the holes are punched in the template, the center spot is a bit easier to affix to the exposed "sticky" of the cellophane tape.

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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3604162 - 02/04/10 10:53 AM

Quote:

... not quite - my 6" has that claim to fame.





Quote:

... just a technicality, Jason. You have the template "upside down" (unless you reversed the pic).




Because of the direction the template was rolled-in, I felt I was going to get little more puffiness at the center to keep the sticker away from the mirror while positioning it. So, I did what I did for a reason. However, in retrospect I don't think it really made much of a difference.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3607818 - 02/06/10 01:56 AM Attachment (146 downloads)

First installment of photos.

BLACKCAT Cheshire


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3607820 - 02/06/10 01:57 AM Attachment (137 downloads)

INFINITY XLK offset pupil

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3607825 - 02/06/10 01:58 AM Attachment (127 downloads)

INFINITY XLK central pupil

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3607826 - 02/06/10 01:59 AM Attachment (134 downloads)

CDP via central pupil

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3607830 - 02/06/10 02:01 AM

Great thread but no longer concise.

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3607852 - 02/06/10 02:34 AM

My first impressions:

Hotspot is definitely more accurate when using it with the cheshire. It has the best of both the donut and triangle center spot advantages.

Hotspot is definitely more accurate when using it for stacking P+2 reflections via the XLK offset pupil. Small shifts are easier to detect with the Hotspot shape compared to the triangle/donut shapes.

Hotspot wins again when it gets to stacking P+1 reflections since there is more perimeter to reference compared to both triangles and donuts spots

However, when it gets to CDP the triangle wins because a typical CDP view is already busy with reflections around P+3 and the large size of the Hotspot cluttered the view. But the Hospot wins over donut spots.

I like the yellow color because of its contrast against the cheshire ring but I felt successive reflections of the yellow color in the XLK where not as bright as the white color.

Above are first impressions. I am continuing with my evaluation.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3607875 - 02/06/10 03:04 AM Attachment (127 downloads)

Attachment shows how small and even smaller P+2 shifts can be easily detected with Hotspot

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3607878 - 02/06/10 03:06 AM Attachment (148 downloads)

Attachment shows how little extra effort is needed to avoid pupil reflections and to clear the clutter around P+3 CDP with Hotspot. Right photo is a better CDP setup.


Edited by Jason D (02/06/10 10:53 AM)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3608058 - 02/06/10 08:25 AM

Jason,

As always - GREAT PICS! Once again ... another of your innovative concepts is now a reality!

Were these pics taken with just "room" lighting or did you have any supplemental source directed at the spot?


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3608066 - 02/06/10 08:35 AM

Quote:

... Left photo is a better CDP setup.





Actually, I prefer the "Right" pic queues.


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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3608274 - 02/06/10 10:45 AM

Do I understand correctly, though, that the diameter of the hotspot centermark has to be calibrated to the focal length of the scope to actually correspond well to the inside diameter of the central dark area in the cheshire?
If that's the case, then several different sizes of marker would be necessary.
Though the ability to find the focal point of the scope by matching sizes of reflected spots would seem to be a definite plus of this center mark.
The field could be a little "busy" if this mark were employed with the CAM modification, though.

Edited by Starman1 (02/06/10 10:46 AM)


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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3608305 - 02/06/10 11:00 AM

Quote:

another of your innovative concepts is now a reality!



And thank you for encouraging innovation, Jim.

Quote:

Were these pics taken with just "room" lighting or did you have any supplemental source directed at the spot?



Just room lighting though the OTA was poiting close to a source light but not directly at it. As part of my continued evaluation I will use a diect light source (white and red) in the dark. Stay tuned.


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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3608311 - 02/06/10 11:01 AM

Quote:

Actually, I prefer the "Right" pic queues.



I meant to say "right". I made the edit.


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Re: Concise thread about autocollimators+improveme new [Re: Starman1]
      #3608342 - 02/06/10 11:14 AM

Quote:

Do I understand correctly, though, that the diameter of the hotspot centermark has to be calibrated to the focal length of the scope to actually correspond well to the inside diameter of the central dark area in the cheshire?
If that's the case, then several different sizes of marker would be necessary.



No.
The Cheshire reflection's apparent size is one focal length away but located at infinity.
The Hotspot reflection's apparent size is one focal length away and located one focal length away.
Therefore, one Hotspot size will work for all scopes regardless of their focal lengths.
Think of it this way, if your thoughts are correct then Jim would have been selling different triangle sizes depending on your scope's focal length over the past years.


Quote:

Though the ability to find the focal point of the scope by matching sizes of reflected spots would seem to be a definite plus of this center mark.



I still need to evaluate this benefit. Stay tuned.

Quote:

The field could be a little "busy" if this mark were employed with the CAM modification, though.



Good point. My XLK+CAM is inaccessible to me at the moment. I will evaluate and post photos in few days. Stay tuned.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3608377 - 02/06/10 11:35 AM Attachment (119 downloads)

You can see how my old center spot placement by the factory was off by around 1.5mm. If you purchased quality collimation tools but have no plans to replace the center spot, at least check the placement of the existing spot. In my case, my original stock mirror center spot placement would have introduced 0.75mm PAE.

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3608405 - 02/06/10 11:55 AM

Jason,
Jim HAS offered triangles of different sizes, and does recommend different triangle sizes based on focal length.


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Re: Concise thread about autocollimators+improveme new [Re: Starman1]
      #3608437 - 02/06/10 12:12 PM

Quote:

Jason,
Jim HAS offered triangles of different sizes, and does recommend different triangle sizes based on focal length.




Don,

Half of what you said is true. I do offer a "smaller" triangles for the 1.25" Cheshire (smaller ring ID). The "larger" triangles are for the 2" Cheshire tools (BLACKCAT & TELECAT). In each case, they are all the same triangle size (only 2 sizes offered) with varying hole perforation size (or solid) options.


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Re: Concise thread about autocollimators+improveme new [Re: Starman1]
      #3608450 - 02/06/10 12:17 PM

Quote:

Jason,
Jim HAS offered triangles of different sizes, and does recommend different triangle sizes based on focal length.




I thought Jim offered different perforation sizes which is different from what we are talking about.

The different triangle sizes are for different focuser sizes -- not different focal lengths.

http://www.catseyecollimation.com/spotsbig.jpg

Jim, please clarify.


EDIT: I just noticed that Jim has responded while I was working on this reply

Edited by Jason D (02/06/10 01:01 PM)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3609696 - 02/07/10 12:10 AM Attachment (149 downloads)

Attached is an animation showing how P+2 stacking is more accurate with the HotSpot compared to the triangle.
Note how it is easier to discern the unstacking of P+2 using the Hospot compared to the triangle spot even though both are shifted by the same amount.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3609699 - 02/07/10 12:11 AM Attachment (173 downloads)

Attached is an animation showing how the Hotspot is more accurate than the triangle when used with the cheshire.
The first frame with only the triangle includes a residual PAE error which is hard to see; however, the same amount of residual error is easily discernable with the Hotspot.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3609863 - 02/07/10 03:20 AM Attachment (146 downloads)

Night test passed with flying colors.
The yellow color of the Hotspot did very well under the red light in a dark room.
Note I intentionally unstacked P+2 in the photo.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3609876 - 02/07/10 03:28 AM Attachment (147 downloads)

I also ran the maximum residual error test and again Hotspot passed. Any PAE of 0.5mm or above will be detected.

The top row is for a setup where the focuser axis is parallel to the primary axis at a 0.8mm distance.
1- Note how reflection 2 is visible via the central pupil which is our cue that proper collimation has not been met.
2- Note how reflections P&2 are nicely stacked from both pupils -- that is, there is no P+2 parallax.

The bottom row is for a setup where the focuser axis is parallel to the primary axis at a 0.4mm distance.
1- Note how reflection 2 is mostly invisible which could be missed. In this case, this setup might pass the collimation test with 0.4mm PAE but that is within the XLK spec.
Actually, because of the larger surface area of the Hotspot, reflection 2 ghostly image can be seen with a trained eye even at 0.4mm PAE which means that Hotspot should help attaining more accurate collimation when using the XLK alone.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3635872 - 02/20/10 02:24 AM Attachment (168 downloads)

If you have a BLACKCAT cheshire and an XLK INFINITY autocollimator you might be interested in this post.

Below are alternative steps to collimate with the BLACKCAT and XLK.

In the attachment, the first column photos were taken from the central XLK pupil. The second column photos were taken from the offset XLK pupil. The third column photos were taken from the BLACKCAT pupil.

STEP 1 (row#1): Use whatever means and tools to collimate other than the autocollimator. In the top row photos I intentionally started with a relatively bad collimation setup.

STEP 2 (row#2): Insert the XLK then adjust the secondary mirror to stack P+2 reflections via the offset pupil.

STEP 3 (row#3): Insert the BLACKCAT then adjust the primary mirror to align P+Ring reflections.

STEP 4 (row#4): Re-insert the XLK then adjust the secondary mirror to stack P+2 reflections one more time.

STEP 5 (row#4): Re-insert the BLACKCAT and evaluate. If you are happy with the cheshire view then you are done.

STEP 6 (row#5): Optional: If you are unhappy with the cheshire view then you can re-iterate one more time. Note the differences between rows 5 and 6 photos are very subtle.


In summary:

1 XLK: Stack P+2 via offset pupil by adjusting secondary
2 BLACKCAT: Align P+Ring by adjusting primary
3 XLK: Stack P+2 via offset pupil by adjusting secondary – a repeat


It is a good idea to re-insert the BLACKCAT and evaluate.

Jason


EDIT:
Clarification: The above 3 steps work for me most of the time without additional iterations when the XLK is used to perfect a decently collimated setup. Because starting setups typically have larger FAE than PAE, I start off with the secondary mirror adjustment to expedite collimation convergence.

Edited by Jason D (02/22/10 10:08 AM)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3635898 - 02/20/10 03:17 AM Attachment (123 downloads)

Referring to these earlier posts

Below FP
Above FP

The attachment shows how the Hotspot helps in comparing reflections P & 2 sizes. It is recommended to rack the focuser in or our until both reflections have similar size with no discernable parallax from both of the XLK pupils.

Left photo was taken when the XLK was placed around 3%FL below FP and the right photo was taken when the XLK was placed around 6%FL above the FP. A 2" extension tube was used to elevate the XLK 6% above the FP.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3635902 - 02/20/10 03:22 AM Attachment (126 downloads)

As a testament to JimFly’s POVRay simulation accuracy as depicted in the following post
Above FP

I used a 2” extension tube to elevate the XLK 5%FL above the FP then adjusted the secondary mirror to match Jim’s reflection positions. Finally, I created an animated GIF file. I was amazed how the real photo matched the simulation rendering photo.

Jason

Edited by Jason D (02/22/10 08:22 PM)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3636249 - 02/20/10 10:37 AM



Nils Olof


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3636384 - 02/20/10 11:42 AM Attachment (120 downloads)

Same setup but with the camera focus set at infinity.

Note the offset pupil has two background reflections. The second background reflection (which is larger) can only be seen when the AC mirror is significantly away from the focal plane.


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Re: Concise thread about autocollimators+improveme new [Re: Nils Olof Carlin]
      #3636387 - 02/20/10 11:44 AM

Quote:



Nils Olof




Nils Olof, compliments from you are always special

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3639710 - 02/22/10 12:26 AM Attachment (119 downloads)

I have verified after countless experiments that if the AC mirror is within 1%FL from the focal plane, both P+2 parallax and size difference is indiscernible visually. At 1.5%FL the difference will barely start becoming discernable.
The 1%FL for my 1200mm scope translates roughly to +-0.5” which is a generous range.

The recommendation to position the AC mirror close to the focal plane is not restricted to the XLK P+2 stacking. It is also recommended for the XL since moving the AC mirror significantly away from the focal plane will change the relative sizes and fuzziness of all reflections and will start introducing small residual errors.

It should be relatively easy to position the AC mirror close to the focal plane with the Hotspot as was shown earlier by comparing the relative sizes of reflections P & 2. If you have the XL, decollimate the primary then stack P+2 to compare their relative sizes as you rack the AC up and down. My suggestion is:
1- Rack the AC in until reflection 2 just starts getting larger than reflection P -- then stop.
2- Rack the AC out until refelction 2 just starts getting smaller than reflection P -- then stop.
Position the AC in the middle of the above range. The above is a one time experiment.
You need to decollimate the primary for the XL to keep reflection 2 visible. If you have the XLK then there is no need to decollimate – just use the offset pupil.

As you rack the AC in and out, you might notice that P+2 will start to unstack. This could be due to mechanical imperfection of the focuser but could also be due to residual errors introduced as the AC mirror moves away from the focal plane. The latter is more detectable for scopes with shorter focal lengths since the focuser travel range makes up a larger percentage of the focal lengths compared to scopes with larger focal lengths.

For imaging scopes, the focal plane could reside above a fully-racked focuser which means the AC mirror can’t be brought close to the focal plane. A 2” extension tube might be handy to position the AC mirror close to the focal plane. See attachment.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3639712 - 02/22/10 12:27 AM Attachment (140 downloads)

I finally got to use the XLK+CAM with the Hotspot. Attachment is a starting point for am intentional badly collimated setup. I was concerned the Hotspot reflections in the XLK+CAM will be overwhelming. This was not the case though the view was busier compared to the triangle center spot.

Top row photos were taken from the offset pupil
Bottom row photos were taken from the central pupil XLK+CAM and the BLACKCAT cheshire. The bottom row photos are provided for reference. Neither the central pupil nor the BLACKCAT were used to collimate in the following two posts.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3639714 - 02/22/10 12:28 AM Attachment (113 downloads)

Attached photos where taken after one collimation round of XLK+CAM. The collimation round included:
1- Stacked P+2 pupil by adjusting the secondary mirror. If initial collimation is bad, it is possible to use the central pupil to stack P+2 if it is easier visually.
2- Made a mental note of the CAM unstacked image then adjusted the primary mirror to rotate the CAM unstacked image by 180 degrees (for more info, refer to 6 consecutive posts starting from post#3098491 in the following page)
3- A repeat of step#1 but you have to use the offset pupil to stack P+2

The above 3 steps transferred the badly collimated setup to one with good axial alignment.

Note: You can see CAM crescent which represents as small residual error. If you look hard you should also see the same small residual error in the BLACKCAT view.


Edited by Jason D (02/22/10 08:59 PM)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3639716 - 02/22/10 12:28 AM Attachment (106 downloads)

Attached photos were taken after fine tuning collimation by iterating between the CAM and stacking P+2.

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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3639718 - 02/22/10 12:29 AM Attachment (110 downloads)

Now I am illuminating the CAM using a clip-on flex-neck LED provided by Jim Fly. The flex-neck allows me to position the LED in such a way to only illuminate the reflective donut of the CAM by using the bottom edge of the focuser to block LED light from the dark non-reflective donut of the CAM. Doing so also moves the LED direct light out-of-view – will not shine on your eye. Adjusting the flex-neck LED is a one time task. See top row photos of the attachment.

Middle row photos were taken with the flex-neck LED light properly positioned.

Bottom row photos were taken with the flex-neck LED light improperly positioned by illuminating both donuts.

The one thing I did not like about this particular flex-neck LED model is its pocket clip-on. Clipping it to the spider vane is OK but not secured enough for my taste. There is the potential of striking it accidently while my hand is busy adjusting the secondary mirror which means it might fall on the primary mirror.
The flex-neck LED is a good solution for the CAM providing it is held more securely to the spider vanes.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3663929 - 03/06/10 04:19 PM

Hi Jason:
I have been talking to Jim Fly about when he will be offering the XLK + CAM units for sale, and, in his last note to me (dated 2/21) he said you were evaluating a promising illumination hardware set that he thought you would be reporting on "in a few days" on CN.

I am a relative newcomer to this thread and may have missed your post. if I have missed your posts my apologies for asking you to repeat yourself.

Can you give me (us) an update on where you stand on your evaluation.

Thanks in advance,
Jim Miller


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Re: Concise thread about autocollimators+improveme new [Re: jm510227]
      #3664780 - 03/07/10 12:41 AM

Hello Jim,
Jim Fly was referring to the Flex-neck LED light shown right above your post. I liked the Flex-neck feature but I am little concerned about the pocket-clip. The clip is not reliable enough to avoid accidentally dropping the LED light in the OTA.
Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3665520 - 03/07/10 12:20 PM Attachment (108 downloads)

For indoors or outdoors daytime collimation, the CAM produces great results without any special light attachments. By pointing the OTA optimally with respect to any light source, you can use the edge of the OTA opening to reduce illumination on the dark side of the CAM to improve contrast. Just point the OTA around the source light (not directly at it) monitoring illumination at the bottom edge of the AC barrel reflection. Stop when the differential in illumination between any opposite sides of the AC is maximized. Rotate the AC to position the reflective CAM donut on the brightest side and the dark CAM donut on the dimmest side for maximum contrast. I suppose you could also move the light source and leave the OTA stationary -- of course, if that light source is moveable.

See attachment. First column photos are the result of a typical collimation session using the above technique. Note how the reflective donut of the CAM is positioned on the brighter side of the AC. The cheshire confirmed the good results. I used the flex-neck LED to check on the CAM stacking. It was not exact. The contrast I get from the room light will never rival the contrast I get from the flex-neck LED in the dark. Nonetheless, the cheshire view was good.

Then I slightly uncollimated the primary mirror until a CAM crescent became clear as shown in the center column. Then I did the same in the opposite direction as shown in the column. I included the cheshire views at these two opposite points.

Bottom line: Using the XLK+CAM indoors or outdoors at daytime will yield comparable results to using a cheshire without the need for any special light attachment.

Note: Since using the CAM assumes good P+2 stacking, using the "Hotspot" in the above setup was a plus.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3667012 - 03/08/10 04:08 AM Attachment (115 downloads)

My skills in using the XLK+CAM have been improving over the past months. Now I can achieve a high degree of collimation accuracy without the need for any light attachments while collimating indoors or outdoors during daylight. It all comes down to:
1- Positioning the light source properly to increase the CAM contrast.
2- Learning how to best stack P+2 and CAM
3- Understanding how to reduce the number of iterations between stacking P+2 and CAM.

Bottom line: The XLK+CAM in its current form can be used effectively while collimating indoors or outdoors during daylight- there is no need for light attachments.

I am still working on figuring out the best method to illuminate both P+2 and CAM stacks with only one source light.

Attached photos come from an indoors collimation I completed very carefully without using any clip-on light, without a cheshire, and without using the central AC pupil. I took the XLKC cenral pupil and cheshire photos for reference after collimation was completed.

Jason

Edited by Jason D (03/08/10 03:17 PM)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3711884 - 03/29/10 02:59 AM

After spending a significant amount of time analyzing and improving the steps for XLK+CAM, I have reached the end of this journey. Here are my final thoughts about the XLK+CAM versus XLK+BLACKCAT

Positives:
1- XLK+CAM is more accurate than the XLK alone.
2- XLK+CAM is a one-tool-solution capable of achieving accuracies better than 0.25mm for both the focuser and the primary axial alignments. That is, the XLK+CAM+HOTSPOT can match the accuracy of the XLK+BLACKCAT+HOTSPOT.

Check the following photos:

The top row photos illustrate a case with passing XLK visual cues though the CAM and the cheshire visual cues clearly indicate the existence of residual errors – around 0.4mm FAE and PAE in the photos. Because P+2 are stacked with high precision assisted by the HotSpot, the PAE (cheshire) and CAE (CAM) errors will be equal as shown in the following photos which are blown ups from the previous photos. See how both the CAE and PAE have the same direction and the same magnitude.


Negatives:
1- The level of skills needed for the XLK+CAM is more demanding than XLK+BLACKCAT. That is, XLK+CAM is harder to use than XLK+BLACKCAT
2- With the XLK+CAM, both FAE and PAE need to be eliminated to achieve precise axial alignment. With the XLK+BLACKCAT, the user has the option to eliminate PAE without the need to eliminate FAE.

I thought of new ideas and modifications of existing ideas to add a cheshire-like capability to the XLK; unfortunately, none of these “cheshire” ideas can match the accuracy of the BLACKCAT+HOTSPOT.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3711885 - 03/29/10 03:01 AM

One problem I had to deal with is how to eliminate small CAM crescents under typical light illumination (no special light source clipped onto the spider-vanes.) Small CAM crescents are either undetectable or do not have enough clarity to figure out which primary knobs to move to eliminate them.

I did come up with a technique to do it but the technique is harder than using a cheshire – one of the negatives I listed about the CAM.



Photo#1 is the starting point (same as the case shown in the previous post). The goal is to eliminate the crescent as shown. I do assume each HotSpot tip points to one of the 3 primary knobs movements – as it should be.
Here are the steps:
1- I decollimate by twisting only primary knob “A” until the crescent becomes clearly visible as shown in photo#2
2- I make a mental note of knob “A” direction versus the axis of the crescent. My goal is the make both parallel. I can do this by twisting primary knob “C”. Photo#3 shows the result.
3- Since the crescent axis is now parallel to primary knob “A” movement, all I need to do is twist knob “A” until the crescent disappears as shown in photo#4
In summary, I decollimate with “A” then prep with “C” then finally eliminate the crescent with “A” again.

After 1 or 2 iterations between stacking P+2 via the secondary and the CAM using the above technique via the primary, precise collimation should be achieved under typical light illumination. As a final optional check, I decollimate one last time to ensure the final crescent axis is parallel to knob “A” direction and to ensure reflection 2 unstacking is symmetrical as shown within the cyan ovals



Photos shown in this post and the previous one used room light. I did achieve similar success using red light at night. Interestingly, by strategically pointing the red light held close to the edge of the OTA, I can illuminate only the CAM, only P+2, or both.



Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3712341 - 03/29/10 11:29 AM Attachment (139 downloads)

Consider the following tool to locate the focal plane of a scope. It is an interesting tool in theory but I do not know how useful it would in real life. Anyway, it is an autocollimator with 3 offset pupils at 120 degrees – no need for a central pupil. Precision placement of the 3 offset pupils from the AC center and the angles between them is NOT important.

If the mirror is at the focal plane then stacking P+2 from any offset pupil will also stack P+2 from the other two pupils.

If the mirror is below the focal plane, then not only reflection 2 gets larger but the AC background reflection also gets larger. If the mirror is above the focal plane, the opposite effect will be observed. Now stacking P+2 from all pupils simultaneously is impossible. Based on the relative size of reflection 2 and its relative positions from the other two offset pupils, the user can easily discern whether the AC mirror is above or below the focal plane.

I use a similar technique to position the XLK as close as possible to the focal plane. I stack P+2 then rotate the AC 120 degrees. Of course, rotation will pick up minute mechanical imperfections but it brings be close enough to the focal plane.

Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3712414 - 03/29/10 12:06 PM

A follow up to the last post.
Top row represents a case where the XLK mirror is located below the focal plane. P+2 is stacked then the XLK is rotated 180 degrees. Not only reflection 2 is larger but it is pushed out away from the center after rotation.

Bottow row represents a case where teh XLK is above the focal plane. Reflection 2 looks smaller and is pulled in towards the center when the XLK is rotated 180 degrees.




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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3901098 - 07/03/10 05:18 PM


This post is an attempt to explain and simplify how Catseye Blackcat and Infinity XLK can be used to achieve axial collimation.
First, take your time and study the following animation to understand what “axial collimation” means. Axial collimation is achieved when:
1- Eyepiece/primary mirror focal points coincide (More accurately: When the lateral distance between both is zero)
2- Eyepiece/primary mirror focal planes coincide (More accurately: When both planes are parallel)



The Blackcat quantifies the lateral distance between the eyepiece and primary mirror focal points. It is NOT sensitive to the angle between the eyepiece/primary mirror focal planes. The distance between reflection P and ring is TWICE the actual lateral distance between the two focal points. That is, the Blackcat magnifies the lateral focal points distance error by 2X.



The Infinity XLK offset pupil quantifies the angle between the eyepiece and primary mirror focal planes. It is NOT sensitive to the lateral distance between the eyepiece/primary mirror focal planes. The distance between reflection P and reflection 2 correlates to the angle between the two planes. Use the following formula:
Angle_between_eyepiece_and_primary_mirror_focal_planes = arctan(distance_between_P_2 / (4*focal_length))
It is the arctan of P_2 distance divided by four times the primary mirror focal length.



Putting it together
The beauty of the Blackcat and XLK combination is that one tool is ONLY sensitive to the lateral distance between the focal points and the other tool is ONLY sensitive to the angle between the focal planes.
Simply reiterate between the XLK offset pupil and the Blackcat until both tools report perfection. Re-iteration is needed since adjusting the primary mirror or the secondary mirror impacts both the distance and the angle between focal points/planes. Fortunately, re-iteration will always converge. Only very few iterations are required.
Adjust ONLY the primary mirror with the Blackcat
Adjust ONLY the secondary mirror with the XLK offset pupil. TIP: adjust only with the three set screws. Only rotate the secondary mirror when the set screws become tight. In this case, loosen the tight screws a little and rotate the secondary mirror by a tiny amount.



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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3901253 - 07/03/10 07:23 PM

I like it!
As usual--the graphic artwork and animations rock!
The methodology is also clear and concise.
But I'm not sure what you mean here:

Quote:

Adjust ONLY the secondary mirror with the XLK offset pupil. TIP: adjust only with the three set screws. Only rotate the secondary mirror when the set screws become tight. In this case, loosen the tight screws a little and rotate the secondary mirror by a tiny amount.



Assuming the secondary mirror placement (and rudimentary focuser axial alignment) has been corrected prior to these axial tweaks, I don't understand why you're suggesting rotation for the fine secondary mirror adjustment.

I suggest:
"Adjust ONLY the primary mirror tilt with the Blackcat
Adjust ONLY the secondary mirror tilt with the XLK offset pupil."

I think that fits your illustrations better.


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3901272 - 07/03/10 07:37 PM

Quote:

I like it!
As usual--the graphic artwork and animations rock!
The methodology is also clear and concise.




Thank you, Vic

Quote:


But I'm not sure what you mean here:

Quote:

Adjust ONLY the secondary mirror with the XLK offset pupil. TIP: adjust only with the three set screws. Only rotate the secondary mirror when the set screws become tight. In this case, loosen the tight screws a little and rotate the secondary mirror by a tiny amount.



Assuming the secondary mirror placement (and rudimentary focuser axial alignment) has been corrected prior to these axial tweaks, I don't understand why you're suggesting rotation for the fine secondary mirror adjustment.




Vic, I was merely reacting to Sinistar powerful post
The point I was trying to make is to only "tilt" the secondary mirror for "fine" adjustments. "Rotating" is not recommended for "fine" secondary mirror adjustment. In case "tilting" can't go any further due to tightening at least one of the "tilt" set screws then a tiny secondary mirror rotation would be needed to give some breathing room to continue "fine" tuning with the "tilt" set screws.


Quote:


I suggest:
"Adjust ONLY the primary mirror tilt with the Blackcat
Adjust ONLY the secondary mirror tilt with the XLK offset pupil."

I think that fits your illustrations better.




In complete agreement. "tilt" is a better word to use for both adjustments.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3901310 - 07/03/10 08:18 PM

Quote:

Vic, I was merely reacting to Sinistar powerful post
The point I was trying to make is to only "tilt" the secondary mirror for "fine" adjustments. "Rotating" is not recommended for "fine" secondary mirror adjustment. In case "tilting" can't go any further due to tightening at least one of the "tilt" set screws then a tiny secondary mirror rotation would be needed to give some breathing room to continue "fine" tuning with the "tilt" set screws.



I see.
I still would leave rotation out of this precision axial alignment procedure. It opens up a host of other secondary mirror placement/positioning and mechanical focuser alignment parameters that are better assigned to sight tube assessment and correction.

This way, as I've been saying for quite some time now, achieving precise axial alignment with an autocollimator and Cheshire (and/or laser depending on the user's preference) is easy using the distinct signatures to guide the user to the correct adjustment.

Mechanical focuser alignment and secondary mirror placement in the light cone, what I usually refer to as front end geometry, because of its complexity and indirect mechanical adjustment (when adjustment is provided at all!), is going to be the recurring collimation headache for most novices (and some experts who decide to push for perfection). Whether it's a Schmidt- or Mak-Newt or someone trying to correct the orthogonality of the optical axis relative to the mounting axes...the secondary mirror alignment implications can be tricky. Thankfully, for most simple Newtonian applications, getting the secondary mirror close is good enough.


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3901319 - 07/03/10 08:27 PM

One more note on your BlackCat Cheshire animation.
I would suggest showing that tilting the eyepiece when the focal points coincide does not impact the Cheshire read. Your animation currently tilts the eyepiece when the focal points do not coincide, which doesn't demonstrate the Cheshire's relative insensitivity to focuser axial tilt.

(Actually, the animation showing the parallelism signature with the offset pupil should also demonstrate how the signature is not impacted when the focal points do not coincide. This is shown in the final animation demonstrating reiteration--but it seems appropriate to show the signature in the offset pupil animation too.)

Edited by Vic Menard (07/03/10 08:33 PM)


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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3901456 - 07/03/10 10:56 PM

Great suggestions, Vic
I made the updates


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3901736 - 07/04/10 04:40 AM

Amazing explanation!
I got really tempted to acquire those tools in the near future.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3901823 - 07/04/10 07:28 AM

Quote:


...TIP: adjust only with the three set screws. Only rotate the secondary mirror when the set screws become tight. In this case, loosen the tight screws a little and rotate the secondary mirror by a tiny amount....





Jason,

Once again, your novel illustrations are powerful with simultaneous clarity and simplicity. Thank You! I've been preaching the "iterative" method of axial alignment by alternating use of the Cheshire and A/C since day one as the "simple" method. It also works with the single-pupil "XL" model until the "2" & "3" images disappear near convergence. This is where the power of the offset pupil kicks in where they DON'T disappear and the need for that final "tweak" can be visualized.

That said, I have to disagree with the recommendation to ONLY effect rotatation of the Secondary mirror via the "tilt" adjustment screws without a qualification that it is Secondary-mount-design dependent. Depending on the design, this advice may or may not be prudent.

My Secondary mount tilt mechanism inherently also "kicks" the tip of the Secondary around as tilt adjustments are made. Rather than risk skewing the Secondary Presentation to the focuser (and undoing what I just did via the sight tube), when a pure "rotation" about the OTA axis is called for to align the A/C images, I gently twist/rotate the entire Secondary assembly a fraction of a degree at the central spider mounting (via smooth movement facilited by a plastic washer placed under the front bushing jam nut)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3901886 - 07/04/10 08:38 AM

Jason
I love your animations!
One comment though. The statement that axial collimation occurs when EP and Primary focal points coincide and focal planes coincide might mislead. Of course what you are emphasizing is lateral distance, and what you are asserting is the fact that in an axially collimated system, when the focal points are coincident then the focal planes are coplaner. But, as you well know, coincidence of focal points itself is not a necc condition for axial collimation but instead describes the state of visual focus for a given EP. Hence, the EP can be racked out of focus - points not coincident - yet axial collimation would still be said to obtain if the optical axis of the EP and primary remain co-linear, since the respective focal points remain coaxial wrt to both optical axes. Once again, I know you are focusing on lateral displacement here, but from a strictly didactic standpoint you might want to consider a change in wording to avoid any confusion.

Once again excellent thread and animation!
Joe

Edited by jpcannavo (07/04/10 08:50 AM)


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Re: Concise thread about autocollimators+improveme new [Re: jpcannavo]
      #3901909 - 07/04/10 08:55 AM

Quote:

... The statement that axial collimation occurs when EP and Primary focal points coincide and focal planes coincide might mislead...




If I might jump in to your point, when axial alignment is achieved, focuser/eyepiece planes are "parallel" AND the focal points are "co-linear", not necessarily "coincidental" (focused achieved).


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Re: Concise thread about autocollimators+improveme new [Re: jpcannavo]
      #3902025 - 07/04/10 10:30 AM

And just to be priggish with myself..

Quote:

the EP can be racked out of focus - points not coincident - yet axial collimation would still be said to obtain if the optical axis of the EP and primary remain co-linear




I assume here that the mechanical and travel axes of the focuser are co-linear with the optical axis of the EP. Hopefully this is typically the case!
And, Jim, in agreement with your comment as well.
Joe


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Re: Concise thread about autocollimators+improveme new [Re: jpcannavo]
      #3902037 - 07/04/10 10:42 AM

Quote:

One comment though. The statement that axial collimation occurs when EP and Primary focal points coincide and focal planes coincide might mislead.




Joe, you must have missed my statements (I highlighted in red) which I included in my first post yesterday.

Quote:

1- Eyepiece/primary mirror focal points coincide (More accurately: When the lateral distance between both is zero)
2- Eyepiece/primary mirror focal planes coincide (More accurately: When both planes are parallel)




I am not trying to mislead but rather to simplify. There are tons of posts in this thread and similar threads about the autocollimator that include tons of details. Unfortunately, these threads have been somewhat intimidating and a turn off to many. That is why I revived this thread with a simpler explanation. The concept of "coincide" is easier to understand and relate to.
Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3902051 - 07/04/10 10:48 AM

The term co-linear makes sense when you have 3 distinct points or more.
The term co-planar makes sense when you have 4 distinct points or more.
Oh, but here is when this thread will yet make another turn to "confusion land" to some readers.


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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3902078 - 07/04/10 11:11 AM

Quote:

when a pure "rotation" about the OTA axis is called for to align the A/C images,



Jim, it is important to clarify that "rotation" is NOT a requirement to stack autocollimator reflections. Referring to a "typical" secondary mirror holder assembly, the 3 set screws have complete freedom to stack all autocollimator reflections without rotation. However, the users might find themselves running into the situation where they start adjusting the correct secondary mirror set screw to stack reflections then all of a sudden the correct set screw gets tightened. This is the only situation were a small rotation is required. In this case, the users will need to loosen the tightened set screws and rotate but a small amount to get the autocollimator reflections as close as possible and revert back to using only the set screws for fine tuning. I just described the method I use. A well squared focuser and well centered secondary mirror assembly in the OTA helps a great deal in the secondary mirror adjustment.
My concerns with using rotation for fine adjustment is that it might not be as easy for some scopes.
Having said all the above, there are many ways to skin a cat. It is possible to use rotation to move the reflections as close as possible to each other before start using the set screws for fine adjustment; however, starting off with rotation will run the risk of ending up with a rotation/tilt error for secondary mirror positioning which is a different subject from the autocollimator.
Jason

EDIT: Let me put it this way: As long as all 3 set screws have freedom to move, then the user has complete freedom to move A/C images without the need for rotation. However, when at least one of the 3 set screws get tightened then the freedom to move A/C images will be impacted and that is when rotation becomes a requiement.

Edited by Jason D (07/04/10 11:19 AM)


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3902087 - 07/04/10 11:15 AM

Quote:

The term co-linear makes sense when you have 3 distinct points or more...





... then we need to add that the line between the focal points must also be orthoganal to the focal planes.

The difficulty in nailing this down in words comes from the fact that you chose to define axial alignment in terms of the relative position of the eyepiece and Primary focal "planes" and "points", rather than the relative position of their "axes" - specifically differentiating "skewed" versus "parallel" versus "coincident".


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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3902101 - 07/04/10 11:28 AM

Quote:

The difficulty in nailing this down in words comes from the fact that you chose to define axial alignment in terms of the relative position of the eyepiece and Primary focal "planes" and "points", rather than the relative position of their "axes" - specifically differentiating "skewed" versus "parallel" versus "coincident".



I am trying to simplify things for those who found the old posts in this thread confusing. I wanted to stick with more understandable terms such as focal points and focal planes. I also felt dealing with the concept of "coincidence" is easier to relate to than co-linear points and parallel planes. I am trying to reach out to a wider audience by keeping things simpler.
Jason


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3903357 - 07/05/10 02:12 AM

Ye gads! Very interesting. Wouldn't this complicate collimating procedures?

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Re: Concise thread about autocollimators+improveme new [Re: GeneT]
      #3903401 - 07/05/10 02:54 AM

Quote:

Ye gads! Very interesting. Wouldn't this complicate collimating procedures?




Can you elaborate?

What is so complicated about iterating between aligning the primary mirror with the Blackcat and aligning the secondary mirror with the XLK offset pupil?

Jason


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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3903785 - 07/05/10 11:18 AM

Quote:

...My Secondary mount tilt mechanism inherently also "kicks" the tip of the Secondary around as tilt adjustments are made.



I'm not aware of any secondary mounts that don't.

Quote:

Rather than risk skewing the Secondary Presentation to the focuser (and undoing what I just did via the sight tube), when a pure "rotation" about the OTA axis is called for to align the A/C images, I gently twist/rotate the entire Secondary assembly a fraction of a degree at the central spider mounting (via smooth movement facilited by a plastic washer placed under the front bushing jam nut)



I do the same thing--when I'm aligning the secondary mirror geometry (which includes coarse focuser axial alignment). But I do it with a sight tube that more clearly shows skew, rotation, offset and mechanical focuser axis misalignment errors, balanced against the coarse focuser axial alignment.

But when I'm tweaking the fine axial alignment (which I believe is what Jason is illustrating), I don't worry too much about the secondary geometry. Since I'm already quite close after I've completed the corrections I can with the sight tube, any minor skewing or offset errors induced by the very small secondary mirror tilt adjustments are negligible.

This doesn't discount that some secondary mounting systems are prone to rotation errors, and that routine correction by tweaking the secondary mirror tilt can eventually cause a significant skew error. It also shouldn't discount the fact that an uncorrected mechanically misaligned focuser may require a slight secondary mirror skew to achieve the optimal focal plane illumination (much more common now that we have the tools to reveal it).

I've simply made the assumption that the preliminary front end geometry has already been assessed and corrected (usually with a sight tube) and the critical axial alignments are then being tweaked with Jason's procedure. In this case, it's unlikely that the secondary mirror is so misaligned as to significantly skew the secondary mirror alignment or cause one of the tilt adjustment screws to become too tight or otherwise fail to function. If the secondary alignment is compromised to this extent, I would suggest more sight tube work before attempting to correct the misalignment with these fine tolerance axial alignment tools. But that's another procedure--one that doesn't need to complicate Jason's simplified axial correction explanation.

Edited by Vic Menard (07/05/10 02:21 PM)


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3904207 - 07/05/10 03:08 PM

Good post, Vic
Yep, if someone follows the proper collimation procedure starting with the "one" time procedure of squaring the focuser and centering the secondary assembly in the OTA followed by the procedure to round/center the secondary mirror via a quality sight-tube then it is HIGHLY unlikely a small rotation would be required at all with the autocollimator procedure. In this case, "tilting" the secondary mirror would be enough to stack the autocollimator reflections.
If the user gets to a point where a rotation is required because one of the "tilt" set screws can't be tightened any further then it might be a good idea to re-examine the secondary mirror positioning with a quality sight-tube.
In my case, I never need to use small rotations when I am at the stage to fine tune my collimation with the autocollimator. That is, I only use “tilting” to fine tune my collimation with an autocollimator.
Jason


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Vic Menard
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3904256 - 07/05/10 03:40 PM

Quote:

...If the user gets to a point where a rotation is required because one of the "tilt" set screws can't be tightened any further then it might be a good idea to re-examine the secondary mirror positioning with a quality sight-tube.





Quote:

In my case, I never need to use small rotations when I am at the stage to fine tune my collimation with the autocollimator. That is, I only use “tilting” to fine tune my collimation with an autocollimator.
Jason



My heavy 4-inch minor axis secondary mirror mounting is configured with plastic washers (and a 1/2-inch stainless steel mounting shaft), so I watch for skew signatures like a hawk! And I'm not adverse to pulling, pushing, twisting and/or torquing my secondary mirror to see which adjustment might be more expedient. Still, for the vast majority of secondary mountings out there, you'll find three tilt screws and no plastic washers--and the most "precise" solution to an axial "tweak" at the secondary can be made even easier with the addition of no-tool knobs.


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: Vic Menard]
      #3931643 - 07/19/10 04:10 PM Attachment (114 downloads)

I replaced my Catseye white triangle with the new Catseye white HotSpot.

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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3931648 - 07/19/10 04:12 PM Attachment (124 downloads)

Below are XLK and Blackcat photos with the new color. In general, I found both the current yellowish and the new white HotSpots are comparable. Under red light at night, both performed the same. Under white light at home, the white HotSpot has an edge in terms of contrast. If you are planning to get a HotSpot for your scope, do consider the white HotSpot. If you already have the current yellowish HotSpot installed, I would NOT recommend replacing it with the white one – it is not worth the effort.

Jim, you might want to consider placing the white HotSpot on a darker paper. Positioning the white HotSpot under the template could have been easier because it was hard to discern the white HotSpot edge against the background white paper. I had to hold up the template plus taped HotSpot against a bright light to confirm I got it right. Just a thought.


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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3931652 - 07/19/10 04:13 PM Attachment (110 downloads)

XLK offset pupil

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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3931653 - 07/19/10 04:14 PM Attachment (113 downloads)

XLK central pupil

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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3931655 - 07/19/10 04:14 PM Attachment (107 downloads)

Below are photos for a slightly miscollimated scope.

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bsim
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3931729 - 07/19/10 05:07 PM

Quote:


What is so complicated about iterating between aligning the primary mirror with the Blackcat and aligning the secondary mirror with the XLK offset pupil?





Alternating between the Blackcat and the XLK offset pupil is simplicity. I have to thank you for those "simple" instructions earlier in the thread. I do the initial collimation with my Glatter laser and TuBlug and then fine tune using your method.


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cdndob
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Re: Concise thread about autocollimators+improveme new [Re: bsim]
      #3932214 - 07/19/10 10:15 PM

FYI, for using the hotspot in the field I mounted a switch and 12v car brake light LED bulb to a paper clamp. I just coonect the power to my battery box and attach the paper clamp to the outside of the tube then flip the switch to illuminate the hotspot when using the XLK, works great indoors or outside.

Steve


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CatseyeMan
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #3932363 - 07/19/10 11:14 PM

Quote:

Jim, you might want to consider placing the white HotSpot on a darker paper.



I've got no choice on this one. The reflective media the spots are cut from comes with the "white" backing paper.


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: CatseyeMan]
      #3932478 - 07/20/10 12:24 AM

Quote:

Quote:

Jim, you might want to consider placing the white HotSpot on a darker paper.



I've got no choice on this one. The reflective media the spots are cut from comes with the "white" backing paper.




No problem. It would have been nice to have the darker paper but I was still able to place the white HotSpot with high precision on the template and mirror.

Jason


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: bsim]
      #3932491 - 07/20/10 12:39 AM

Quote:

Alternating between the Blackcat and the XLK offset pupil is simplicity. I have to thank you for those "simple" instructions earlier in the thread. I do the initial collimation with my Glatter laser and TuBlug and then fine tune using your method.



Glad to hear the steps are working for you.
By the way, how easy is it to fine tune adjusting the secondary mirror to stack P+2 from the offset pupil on your Teeter? Do you have to deal with backlash movement when using the secondary mirror tilt set screws?
Jason


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: cdndob]
      #3932514 - 07/20/10 12:52 AM

Quote:

FYI, for using the hotspot in the field I mounted a switch and 12v car brake light LED bulb to a paper clamp. I just coonect the power to my battery box and attach the paper clamp to the outside of the tube then flip the switch to illuminate the hotspot when using the XLK, works great indoors or out