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Useful info about secondary mirror alignment

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#401 Jason D

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Posted 26 February 2013 - 11:33 PM

Tilt and rotation (or a combination of the two) often leads to other issues residing outside of the alignment plane...

Vic, in the procedure I outline, I did not specify tilt with the 3 set screws to be one of the possible moves. I listed only center bolt, rotation, and spider vanes thumb knobs. The tilt adjustment is only part of the laser collimation.

This procedure is further complicated when the reference (primary mirror reflection) is suddenly removed from the center of the fov when any secondary mirror "adjustment" is applied (especially when the collimation cap pupil is located near the apex).

I also specified once a secondary movement is determined to go ahead can execute the movement without referencing the primary mirror reflection. The primary mirror reflection can be referenced only after laser collimation is done.

Advantage here goes to the sight tube, with its "fixed" bottom edge defining the alignment cone.

Agreed as long as a quality sight-tube and a focuser are used.

#402 Vic Menard

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Posted 27 February 2013 - 10:10 AM

Tilt and rotation (or a combination of the two) often leads to other issues residing outside of the alignment plane...

Vic, in the procedure I outline, I did not specify tilt with the 3 set screws to be one of the possible moves. I listed only center bolt, rotation, and spider vanes thumb knobs. The tilt adjustment is only part of the laser collimation.

I understand. But tilt will be required "...to bring the whole primary mirror reflection to view." The problem as I see it is that the procedure works better in reverse than forward. This is especially true when there is a misalignment outside of the "alignment plane" (which I think is even better visualized as the plane defined by the axis of the spider hub and the mechanical focal "point"--the center of the focuser at or near the focal plane). This more clearly demonstrates the common issues associated with mechanical focuser "squaring" and decentering of the primary mirror.

This procedure is further complicated when the reference (primary mirror reflection) is suddenly removed from the center of the fov when any secondary mirror "adjustment" is applied (especially when the collimation cap pupil is located near the apex).

I also specified once a secondary movement is determined to go ahead can execute the movement without referencing the primary mirror reflection. The primary mirror reflection can be referenced only after laser collimation is done.

I know that you specified it, but I still see it as a serious complication. Since many spiders tie secondary mirror tilt to the fore and aft center mounting bolt adjustment in an agonist/antagonist arrangement, it's already complicated when trying to separate the tilt/rotation/fore and aft adjustments.

Advantage here goes to the sight tube, with its "fixed" bottom edge defining the alignment cone.

Agreed as long as a quality sight-tube and a focuser are used.

I think regardless of quality, the sight tube has the advantage because it's designed for that application. Obviously, if the sight tube is too long (or too short) or consistent registration is nonexistent, there will be significant read issues. But these are issues that can be, and should be, resolved if the user wants, or needs, to resolve a front end geometry problem impacting the secondary mirror alignment.

I do appreciate your systematic approach--delegating tilt to the focuser axis alignment, and iterating between the two adjustments (focuser axis and secondary mirror placement)--I use the same approach with a sight tube. And for getting the secondary mirror "close enough"--I don't doubt your procedure will get the job done. I guess my problem is that I rarely get asked to fix the "easy" ones--there's always some mechanical issue that simply can't be resolved with the usual adjustments--beyond the common skew and offset errors. It's why I always arrive with my sight tube in hand when I'm asked for an opinion on a particularly troublesome "secondary mirror" alignment problem. It helps me to see the geometry more clearly.

#403 Jason D

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Posted 27 February 2013 - 03:25 PM

Vic, the procedure I described was meant to make the life of beginners easier -- at least that is my theory. I did try the procedure several times to make sure it works as expected and it did but the true judges are the beginners.

What I was trying to solve is what seems to be the complex task of reconciling between the secondary mirror optimal centering and achieving axial collimation. Aligning one without the other seems manageable but aligning both seems overwhelming to many beginners.

I wanted to break down the procedure. Achieve axial collimation first without worrying about the secondary positioning then concentrate on the secondary positioning without worrying about axial collimation – back and forth.

One advantage of the procedure I outlined is making it easy (at least in my opinion) to figure out the next movement of the secondary mirror without worrying about the 3 set screws. The 3 set screws in my procedure are only and only meant to achieve axial collimation. They will not be used to bring the primary mirror reflection to view. I am assuming the focuser is reasonable squared and the secondary mirror is reasonably centered in the OTA.

Achieving axial collimation with a laser collimator before using the collimation cap will assist in lining up the eye and focuser axes for better evaluation. Referencing the primary mirror reflection will make it easier to figure out the next secondary movement compared to referencing the focuser edge.

#404 Vic Menard

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Posted 27 February 2013 - 07:15 PM

I wanted to break down the procedure...

One advantage of the procedure I outlined is...(using)...the 3 set screws...to achieve axial collimation.

The idea of delegating the tilt adjustment to the focuser axial alignment and rotation/fore and aft adjustment to secondary mirror placement (rinse and repeat) has been discussed at length in these forums (I believe it's pinned to the top of this forum).

I am assuming the focuser is reasonable squared and the secondary mirror is reasonably centered in the OTA.

If you're tweaking the alignments, then the often required step of loosening more than one tilt adjustment screw to effect a rotation or axial alignment adjustment might not be necessary. But if you have to unload the tension by loosening more than one tilt adjustment screw (which is pretty common for most beginners), the starting axial alignment is quickly lost.

Have you tried reasonably "squared" and reasonably "centered" with your scope to see how it affects the procedure? A few degrees off is all it takes...

...Referencing the primary mirror reflection will make it easier to figure out the next secondary movement compared to referencing the focuser edge.

Easier than with a sight tube? (Where the edge of the secondary mirror is bounded by the primary mirror reflection and the bottom edge of the sight tube.)
Or is this procedure intended for users who find the sight tube too confining (because of the narrow field of view)?
Or are you just trying to show that you can get the secondary mirror placement close enough with a collimation cap and a thin beam laser?

I see the thin beam laser as an excellent tool for quick and easy focuser axial alignment.
Barlowed lasers, Cheshires and collimation caps are all excellent tools for quick and easy primary mirror axial alignment.
And the sight tube is my go-to choice for quickly assessing and correcting the secondary mirror placement.

#405 Jason D

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Posted 27 February 2013 - 11:41 PM

The idea of delegating the tilt adjustment to the focuser axial alignment and rotation/fore and aft adjustment to secondary mirror placement (rinse and repeat) has been discussed at length in these forums (I believe it's pinned to the top of this forum).

Typical methods would start off with centering/rounding the secondary using all possible movements including the 3 set screws then concentrate on the 3 set screws to fine tune the focuser axial alignment. All of this might be overwhelming for many beginners. Using the combination of a laser and a collimation cap could make the alignments easier – though it will take more iterations and more effort. There are less confusing reflections to deal with.

Have you tried reasonably "squared" and reasonably "centered" with your scope to see how it affects the procedure? A few degrees off is all it takes...

Steps I found to be helpful when collimating with a laser:
1- Loosen the 3 set screws just enough to rotate the secondary mirror.
2- Rotate the secondary mirror back and forth and watch the arc traced by the laser on the primary mirror as shown below
Posted Image
3- Stop at the closest point to the primary center then start adjusting the 3 set screws to direct the laser to the center.
Following the above steps on a scope with reasonably squared focuser and centered spider vanes will minimize rotation/tilt errors.
Of course, using Milk Jug washers will make the job even easier.
Posted Image

But who am I to judge. The true judges are the beginners.


Easier than with a sight tube? (Where the edge of the secondary mirror is bounded by the primary mirror reflection and the bottom edge of the sight tube.)

I was referring to the collimation cap. Referencing the primary mirror is easier than the bottom of the focuser.

#406 Vic Menard

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Posted 28 February 2013 - 09:21 AM

I had to go back to the beginning of this discussion to make sense of the tangents.

...Many beginners purchase a laser collimator and follow the proper steps to find out they can’t see all of the primary mirror clips via the secondary mirror. By proper steps I meant aligning the secondary mirror by redirecting the laser beam to the primary center then aligning the primary mirror by redirecting the laser beam back to its source. When beginners run into this issue they wonder if the problem is with the quality of their laser collimators. Then they realign the secondary mirror using the collimation cap to bring all of the primary clips to view. Now the laser beam no longer hits the primary mirror center. At this point frustration builds and no matter what they do, they just can’t reconcile between the collimation cap and the laser collimator.

From this opening salvo I think I now see your vision process.

The goal is to achieve good axial alignment, with the primary mirror reflection more or less centered in the secondary mirror, using a simple thin beam laser and a collimation cap. We'll assume the tools have been checked and are up to the task.

The solution is a systematic approach, delegating focuser axial alignment (secondary mirror tilt) to the laser and secondary mirror placement (rotation and fore and aft movement) to the collimation cap, repeating the two alignments until both are correct. The alignment signatures are the laser beam aligned to the primary mirror center spot (focuser axis) and the primary mirror reflection "centered" in the secondary mirror (secondary mirror placement).
(Final primary mirror alignment I presume will be achieved with the collimation cap.)

Your procedure should resolve (to the limits of the mechanicals) the axial alignments and the secondary mirror placement. You start to address the mechanical alignment issues by including spider adjustments (at the OTA attachments)--but then you stop there (?). If your goal is "close enough", which I think is quite acceptable, I'd leave the spider centered and go with assuming the mechanicals are OK.

But, because the focuser/spider/secondary mirror geometry is basically unknown--there's a good possibility that the user will find himself in an infinite loop attempting to resolve a secondary mirror placement error that can't be resolved because of mechanical issues. This usually plays out something like, either the secondary mirror looks round but the primary mirror reflection isn't centered, or the primary mirror reflection is centered but the secondary mirror looks elliptical, or some variation in between... At this point, the user needs to choose "close enough", or a closer look at the mechanicals (possibly beyond the expertise of a beginner).

And I'm OK with using the "window" analogy (it's synonymous with using the bottom edge of the sight tube--except the window doesn't move with a sight tube--but we'll assume a sight tube is unavailable) for assessing which "direction" the secondary mirror needs to move. But if the secondary mirror isn't already close to where it belongs, the procedure could take many iterations and leave the beginner second guessing how much correction is actually needed (many "tiny" adjustments or one "big one" ;)) and ultimately relying on "hit or miss".

In this sense, starting with the focuser axial alignment and a secondary mirror which could already have a significant tilt/rotation/offset alignment error, or a rudimentary secondary mirror "centering" under the focuser followed by a focuser axial alignment, should have little impact on the beginner's successful resolution of the alignment errors.

I guess what I've been trying to say, is that even with a systematic approach to resolving the alignment errors, even with a sight tube, there's plenty of complexity here to confound the beginner who doesn't fully understand the mechanical geometries and the limitations they impose on the secondary mirror placement.

#407 Jason D

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Posted 01 March 2013 - 01:33 AM

Vic, I agree with the comments you have made.

Few more comments:
1- The combination of a collimation cap and a laser collimator is quite common with beginners. The collimation cap comes with many mass produced scopes and beginners tend to purchase only collimation tool -- a good percentage will purchase a laser collimator. These beginners are the target of the procedure I outlined.
2- True if the scope mechanical alignments are out of whack then that beginners will have hard time collimating with/without the procedure I outlined.
3- Assuming reasonably good mechanical alignment then using the laser collimator method I described in my last post will minimize secondary mirror tilt/rotate errors
4- As I stated, I believe the procedure I outlined will assist beginners who only own a collimation cap and a laser collimator but the true judges are the beginners themselves

Jason

#408 Vic Menard

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Posted 01 March 2013 - 08:45 AM

Keeping with the "for Beginners" simplified method, given the above:
3- Assuming reasonably good mechanical alignment... (which I think most beginners include measuring the spider vanes for "centering")

I would suggest eliminating (in step 4 of your original procedure):
c. You can use the spider vanes thumb knobs to move the secondary across the focuser.

Also, to minimize potential frustration due to "less than perfect" mechanicals, and sticking with "good enough" for most applications, I would suggest changing the signature secondary mirror placement alignment (as viewed in the collimation cap) from "the primary mirror reflection is centered in the secondary mirror" to "all of the primary mirror retaining clips are visible inside the secondary mirror with the focuser racked fully in". Removing "centered" and giving a little more room to get the job done should reduce concerns related to the various mechanical geometries and still deliver "close enough" secondary mirror placement.

This should allow (and encourage) the beginner to pay more attention to the axial alignments and their tolerances--ultimately achieving better image performance.

#409 MDavid

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Posted 01 March 2013 - 01:38 PM

Keeping with the "for Beginners"


Riiight for this is for "beginners"...I'll start with the awesome pics and animation and work backward from there...

But all I gots is my collimation cap (I lost my thinking cap); however, I was considering buying a Cheshire sight tube....Do I really have to have a laser?? sorry if this was already hashed out I skipped 19 or so pages.

#410 Starman1

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Posted 01 March 2013 - 02:03 PM

Keeping with the "for Beginners"


Riiight for this is for "beginners"...I'll start with the awesome pics and animation and work backward from there...

But all I gots is my collimation cap (I lost my thinking cap); however, I was considering buying a Cheshire sight tube....Do I really have to have a laser?? sorry if this was already hashed out I skipped 19 or so pages.


No. In fact, the logical next step from a collimation cap (most of which have off-center holes!) is a combination tool (cheshire + sight tube),
If you go with the lower-priced 1.25" ones ($30-50 from a dozen vendors like Orion, Celestron, SkyWatcher), be sure to measure the centering of the crosshairs. Many are slightly off (but easily correctable by bending).
Some better ones can be had from AstroSystems (both sizes, but ignore his advice on center dotting a secondary mirror), Catseye (2" only).
And here is a tutorial on collimation, with illustrations:
http://www.cloudynig...hp?item_id=2677
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#411 Jason D

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Posted 02 March 2013 - 03:21 PM

Keeping with the "for Beginners" simplified method, given the above:
3- Assuming reasonably good mechanical alignment... (which I think most beginners include measuring the spider vanes for "centering")

I would suggest eliminating (in step 4 of your original procedure):
c. You can use the spider vanes thumb knobs to move the secondary across the focuser.

Also, to minimize potential frustration due to "less than perfect" mechanicals, and sticking with "good enough" for most applications, I would suggest changing the signature secondary mirror placement alignment (as viewed in the collimation cap) from "the primary mirror reflection is centered in the secondary mirror" to "all of the primary mirror retaining clips are visible inside the secondary mirror with the focuser racked fully in". Removing "centered" and giving a little more room to get the job done should reduce concerns related to the various mechanical geometries and still deliver "close enough" secondary mirror placement.

This should allow (and encourage) the beginner to pay more attention to the axial alignments and their tolerances--ultimately achieving better image performance.


Well noted...

Since I can no longer edit the original post, I will repost it later with corrections.

Jason

#412 Jason D

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Posted 03 March 2013 - 02:44 AM



Since I can’t edit the earlier post, I will repost with updates based on Vic’s feedback


First, the following procedure is based on a laser collimator and a collimation cap combination which is common with beginners. If you own a sight-tube/cheshire combo tool then the following procedure will not apply.

In addition to the collimation cap that comes with many mass produced reflectors, many beginners purchase a laser collimator and follow the proper steps to find out they can’t see all the primary mirror clips via the secondary mirror. By proper steps I meant aligning the secondary mirror by redirecting the laser beam to the primary center then aligning the primary mirror by redirecting the laser beam back to its source. When beginners run into this issue they wonder if the problem is with the quality of their laser collimators. Then they realign the secondary mirror using the collimation cap to bring all of the primary clips to view. Now the laser beam no longer hits the primary mirror center. At this point frustration builds and no matter what they do, they just can’t reconcile between the collimation cap and the laser collimator.

In the above scenario, the straight laser beam only interacts with a tiny portion of the secondary mirror surface. It does not interact with the secondary mirror perimeter; therefore, how could a laser collimator which does not interact with the secondary mirror perimeter is expected to be capable of aligning the secondary mirror under the focuser!!!

Here is the catch. There are two alignments for the secondary mirror. Let me repeat this important statement: There are two different and independent alignments for the secondary mirror. The first is somewhat “coarse” which positions the secondary mirror under the focuser. The second is more “fine” which redirects the focuser axis (laser beam) to the primary mirror center. The first alignment is responsible for optimizing illumination whereas the second is meant to eliminate focal plane tilt. In layman’s terms, the first is responsible for optimally positioning the secondary mirror with respect to the star light cone reflection off the primary mirror and the second ensures the eyepiece lens and the primary mirror focal plane are parallel. Both alignments are independent. That is, you can align one without the other.

A straight laser beam can be used for the second alignment – not the first. This is the main source of confusion. Following the laser collimator proper steps will ensure the eyepiece lens and the primary mirror focal plane are parallel but it will not ensure the secondary mirror centered under the focuser which means it will not guarantee all primary mirror clips can be seen.

Here is how you can reconcile between a good laser collimator and a collimation cap:

1- Complete the alignment of both the secondary and the primary mirrors using the laser collimator following the proper steps. Completing this step will ensure the primary mirror reflection is centered under the focuser. Let me repeat: Completing this step will ensure the primary mirror reflection is centered under the focuser. Now you can use the primary mirror reflection as your reference to optimally position the secondary mirror under the focuser. You do not need to reference the focuser edge.

2- Insert the collimation cap and ensure the collimation cap pupil’s reflection is aligned with the primary mirror center spot. It should. If it is not then you have a more fundamental problem. Now your eye axis is aligned with the focuser axis. Maintain your eye position and evaluate how much of the primary mirror reflection you can see.

3- Think of the secondary mirror as a window to the primary mirror reflection. As I stated above, the primary mirror reflection is centered under the focuser after completing the proper laser collimation steps. Now figure out how best to move the window “secondary mirror” to bring the whole primary mirror reflection to view. Check the attached animations. In each frame, both secondary/primary mirrors were aligned with a laser collimator following proper steps. See how the primary mirror reflection remains still and centered regardless of the secondary movement.

4- To move the secondary mirror, you can use one or both of the following movements:
a. Use the center bolt to move the secondary up or down the OTA
b. You can rotate the secondary mirror


5- IMPORTANT: As you make the movements as outlined in step 4, ignore the primary mirror reflection. For example, if you decide to move the window “secondary mirror” closer to the primary mirror via the center bolt, make the movement without looking at any reflections. When you are done, go back to step 1. Do not evaluate the secondary position until you have completed the laser collimation steps since only completing the laser collimation steps will ensure the primary mirror reflection is centered under the focuser.

6- Repeat steps 1 to 5 until both the laser collimator and the collimation cap reconcile.

Posted Image


Posted Image
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#413 tomlinp76

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Posted 29 January 2016 - 09:05 AM

This is somewhat a resurrection of this topic but I'm hoping someone can help!

 

Looking at the posts above .....

 

"But, because the focuser/spider/secondary mirror geometry is basically unknown--there's a good possibility that the user will find himself in an infinite loop attempting to resolve a secondary mirror placement error that can't be resolved because of mechanical issues. This usually plays out something like, either the secondary mirror looks round but the primary mirror reflection isn't centered, or the primary mirror reflection is centered but the secondary mirror looks elliptical, or some variation in between... At this point, the user needs to choose "close enough", or a closer look at the mechanicals (possibly beyond the expertise of a beginner)."

 

... this is where I have my problem.

 

I have solved this by using the spider vanes to move the secondary holder LATERALLY 3-4mm (which in the collimating cap means the secondary is now entered top and bottom, as well as left and right).

 

Because the secondary holder is slightly off-centre of the OTA on the spider vanes, can I still achieve decent enough collimation?

 

Thanks!



#414 Jason D

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Posted 29 January 2016 - 10:04 AM

 

I have solved this by using the spider vanes to move the secondary holder LATERALLY 3-4mm (which in the collimating cap means the secondary is now entered top and bottom, as well as left and right).

 

Because the secondary holder is slightly off-centre of the OTA on the spider vanes, can I still achieve decent enough collimation?

 

Thanks!

 

Collimation is about aligning optics (Primary mirror, secondary mirror, eyepiece) with respect to one another. Scopes' structural components mechanical alignments are meant to facilitate collimation. There is nothing magical about mechanical alignment of structural components as long as they stay out of the light path. For example, whether the spider vanes are perfectly centered in the OTA or not is irrelevant to collimation as long as this misalignment does not cause any other structural components such as the OTA opening edge to get in the light path.

 

Jason


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#415 tomlinp76

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Posted 29 January 2016 - 10:13 AM

Thank you very much, that has clarified the issue, although I have read I need to make sure the vanes are still as straight as possible to avoid diffraction spike problems. I really don't want to start bringing in focuser alignment to the whole process! This scope is purely for observing, not imaging.

 

I am however making some assumptions which I hope are correct:

 

I am ready to proceed to the final primary mirror alignment because the secondary is circular, and is centred, and shows the whole area of the primary mirror including clips, and a laser collimator dot hits the centre of the primary mirror.

 

I believe it is now fine adjustments of the primary to align the red dot on the laser collimator.

 

Thanks again, 3 years after the last post!



#416 Kipper-Feet

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Posted 29 January 2016 - 10:38 AM

Because the secondary holder is slightly off-centre of the OTA on the spider vanes, can I still achieve decent enough collimation?

 

I see that your post #1 above was your first post here on the CN forums and so, Welcome!

 

As you move on to collimate your primary mirror please bare the following in mind.  

 

There are many Newtonian reflector scopes in which the secondary mirror is intentionally mounted off-center relative to the spider-hub in the center of the spider-vanes.  And as such, even when the spider-hub is well centered in the cross section of the OTA, the secondary mirror is, itself, not at all centered in the cross section of the OTA.  This is by design and does not change the way in which those owners collimate their scopes.  

 

You must always ignore the position of the spider-vanes, and particularly the reflections of the spider-vanes as seen through a collimation cap or sight-tube, when collimating.  The spider-vanes are never to be considered a reference against which anything else should be aligned.  

 

If you're ever using a sight-tube equipped with cross hairs, it makes good sense to rotate the tool such that the actual cross hairs are positioned in between the reflections of the spider-vanes.  As such, you'll be less tempted to align each with the other.

 

Please sure to let us know how your collimation turns out.  Some photos perhaps?


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#417 tomlinp76

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Posted 29 January 2016 - 10:53 AM

Hi

Thanks for replying. 

 

As part of the collimation process I am definitely ignoring the reflections of the spider vanes, my only query above was being able to complete the first two steps only by having a slightly out of centre secondary mirror. And slightly out of centre left to right, rather than towards/away from the focuser.

 

I was also double checking that with everything centred and primary all in view on the secondary mirror, that I was ready to proceed to the final steps, which I believe to be very minor adjustments which won't affect the current view too much.

 

By the way, it is an Orion XT8 1200mm dob scope.

 

When I first checked collimation (I bought used) the red dot was off as much as 10cms from the centre of the primary mirror. I don't want to get caught up too much in this stage of collimating because surely anything I do should be better than how it was in the first place.

 

Thanks Richard.



#418 Jason D

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Posted 29 January 2016 - 12:53 PM



although I have read I need to make sure the vanes are still as straight as possible to avoid diffraction spike problems.

 

 

Opposite spider vanes need to be parallel -- they need not be inlined.

 

 

 

Thanks again, 3 years after the last post!

 

 

 

This thread never died ;)

It has been referenced periodically over the past 3 years.

 

Jason


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#419 tomlinp76

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Posted 29 January 2016 - 02:42 PM

Thanks for all your help.

Tonight in a more relaxed state I started again. I centred the secondary holder in the vanes/OTA. I set the secondary mirror back level on its holder. Got that centred, adjusted the three screws to get it aligned. Guess what? Perfect view of the primary through the eyepiece. Slightly off-set as it should naturally be. 10 minutes later had the primary collimated back to the secondary.

 

Amazing that last night it was an impossibility, start again with a fresh mind (and lots of research) and it's as simple as anything.

 

Couldn't have done it without reading through this entire thread though!

 

Cheers.


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#420 Vic Menard

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Posted 29 January 2016 - 02:46 PM

Looking at the posts above .....

 

"But, because the focuser/spider/secondary mirror geometry is basically unknown--there's a good possibility that the user will find himself in an infinite loop attempting to resolve a secondary mirror placement error that can't be resolved because of mechanical issues. This usually plays out something like, either the secondary mirror looks round but the primary mirror reflection isn't centered, or the primary mirror reflection is centered but the secondary mirror looks elliptical, or some variation in between... At this point, the user needs to choose "close enough", or a closer look at the mechanicals (possibly beyond the expertise of a beginner)."

 

... this is where I have my problem.

 

I have solved this by using the spider vanes to move the secondary holder LATERALLY 3-4mm (which in the collimating cap means the secondary is now entered top and bottom, as well as left and right).

 

Because the secondary holder is slightly off-centre of the OTA on the spider vanes, can I still achieve decent enough collimation?

 

As I stated in the paragraph you're quoting, "you can choose close enough". By moving the secondary mirror "laterally" (top to bottom) away from center, you end up moving the optical axis in the OTA. As long as you didn't need to move it too far, close enough is good enough. How far is too far? Well, if you can see the edge of the front aperture in the primary mirror reflection, that would be too far, and if the reflection of the spider vanes no longer make a perpendicular X but are beginning to look like a bird's foot or a peace sign, that would also be too far.

 

(Just noticed while I was finishing this post you've resolved your alignment problem with a (mechanically) centered secondary mirror. That's even better than "close enough"!  :waytogo: )


Edited by Vic Menard, 29 January 2016 - 02:48 PM.


#421 tomlinp76

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Posted 29 January 2016 - 02:56 PM

Thanks. Relieved and happy, even the final view through the sight tube looks like a classic offset pattern. I may have been tired/rushing it last night.


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#422 tomlinp76

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Posted 29 January 2016 - 03:01 PM

An addition is that I've also learned about focused slop prevalent in many consumer scopes so I did all the above when I had the focuser at a mid way point with very slight wobble. I've had this before but In real time viewing you just don't notice anything....


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#423 juram

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Posted 15 February 2016 - 09:06 PM

I was looking through this thread trying to determine why my collimation seemed slightly "off."  The views are sharp and clear when I use a laser to get tilt of secondary to center the beam on the primary center spot triangle, use a sight tube to ensure the secondary is round/centered in sight tube and use a Cheshire to adjust the tilt of the primary.  The primary reflection is perfectly centered in secondary and everything seems concentric except for the silhouette of the secondary/spider vanes.  I was beginning to think I needed to shim the focuser. I was struggling to determine why the silhouette from the secondary holder/spider vanes was slightly off axis from everything else.  After reading through most of the 17 pages of this thread, it turns out that's what I'm supposed to see!  When I've read simplified methods for collimation with illustrations, they always seem to imply that everything, including the silhouette of the secondary/spider vanes are supposed to be perfectly in line; that apparently caused my confusion in my quest to get my collimation as good as I can.  This thread was immensely helpful.  Several of the discussions and explanations, particularly from Jason and Vic, as well as the illustrations really clarified what I need to do and what it needs to look like when I'm done.  Thanks!  


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#424 Jason D

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Posted 16 February 2016 - 01:27 AM

:waytogo:

 

It is beyond my comprehension why many telescope vendors do not update their manuals with the proper collimation illustration.

 

Jason


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#425 Starman1

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Posted 22 February 2016 - 05:54 PM

+1 for Jason's comment.

And the number of posts and threads about that here on CN is mind boggling.

I've communicated with some of those people and they seem unwilling to change their manuals and illustrations.

Inscrutable.




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