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Concise thread about autocollimators+improvements

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#201 Jason D

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Posted 29 March 2010 - 10:29 AM

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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#202 Jason D

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Posted 29 March 2010 - 11:06 AM

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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#203 Jason D

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Posted 03 July 2010 - 04:18 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)

Posted Image

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.

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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.

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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.

Posted Image

#204 Vic Menard

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Posted 03 July 2010 - 06:23 PM

I like it! :waytogo:
As usual--the graphic artwork and animations rock! :bow:
The methodology is also clear and concise.
But I'm not sure what you mean here:

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.

#205 Jason D

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Posted 03 July 2010 - 06:37 PM

I like it! :waytogo:
As usual--the graphic artwork and animations rock! :bow:
The methodology is also clear and concise.


Thank you, Vic

But I'm not sure what you mean here:

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.


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.

#206 Vic Menard

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Posted 03 July 2010 - 07:18 PM

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.

#207 Vic Menard

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Posted 03 July 2010 - 07:27 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.)

#208 Jason D

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Posted 03 July 2010 - 09:56 PM

Great suggestions, Vic
I made the updates

#209 Fireball

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Posted 04 July 2010 - 03:40 AM

Amazing explanation! :waytogo:
I got really tempted to acquire those tools in the near future.

#210 CatseyeMan

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Posted 04 July 2010 - 06:28 AM

...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! :waytogo: 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)

#211 jpcannavo

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Posted 04 July 2010 - 07:38 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

#212 CatseyeMan

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Posted 04 July 2010 - 07:55 AM

... 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).

#213 jpcannavo

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Posted 04 July 2010 - 09:30 AM

And just to be priggish with myself..

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

#214 Jason D

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Posted 04 July 2010 - 09:42 AM

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.

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

#215 Jason D

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Posted 04 July 2010 - 09:48 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.

#216 Jason D

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Posted 04 July 2010 - 10:11 AM

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.

#217 CatseyeMan

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Posted 04 July 2010 - 10:15 AM

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". :grin:

#218 Jason D

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Posted 04 July 2010 - 10:28 AM

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". :grin:

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

#219 GeneT

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Posted 05 July 2010 - 01:12 AM

Ye gads! Very interesting. Wouldn't this complicate collimating procedures?

#220 Jason D

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Posted 05 July 2010 - 01:54 AM

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

#221 Vic Menard

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Posted 05 July 2010 - 10:18 AM

...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.

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.

#222 Jason D

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Posted 05 July 2010 - 02:08 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

#223 Vic Menard

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Posted 05 July 2010 - 02:40 PM

...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.

:waytogo:

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.

#224 Jason D

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Posted 19 July 2010 - 03:10 PM

I replaced my Catseye white triangle with the new Catseye white HotSpot.

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#225 Jason D

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Posted 19 July 2010 - 03:12 PM

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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