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Concise thread about autocollimators+improvements

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#76 demiles

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Posted 05 January 2010 - 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?

#77 Jason D

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Posted 05 January 2010 - 01:50 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?


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

#78 Starman1

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Posted 05 January 2010 - 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.

#79 Jason D

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Posted 05 January 2010 - 01:59 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.


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

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  • 3541085-new_center_spot.PNG


#80 Vic Menard

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Posted 05 January 2010 - 02:02 PM

...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.

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.

#81 Jason D

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Posted 05 January 2010 - 02:16 PM

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.

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

#82 Vic Menard

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Posted 05 January 2010 - 02:21 PM

...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...

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?)

#83 demiles

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Posted 05 January 2010 - 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?

#84 Vic Menard

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Posted 05 January 2010 - 02:41 PM

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.

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.

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!

#85 Vic Menard

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Posted 05 January 2010 - 02:55 PM

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. :shrug:

#86 demiles

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Posted 05 January 2010 - 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.

#87 sixela

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Posted 05 January 2010 - 03:04 PM

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.

#88 Vic Menard

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Posted 05 January 2010 - 03:06 PM

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

#89 sixela

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Posted 05 January 2010 - 03:08 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.

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.

#90 Vic Menard

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Posted 05 January 2010 - 03:15 PM

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.)

#91 hudson_yak

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Posted 05 January 2010 - 04:27 PM

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

#92 demiles

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Posted 05 January 2010 - 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.

#93 Vic Menard

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Posted 05 January 2010 - 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.

#94 Jason D

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Posted 05 January 2010 - 05:08 PM

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... :)

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I even built a similar one
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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.
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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

#95 Jason D

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Posted 05 January 2010 - 05:26 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?


- 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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  • 3541588-2_vs_1_25.JPG


#96 Jason D

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Posted 05 January 2010 - 05:48 PM

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. :shrug:


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

#97 demiles

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Posted 05 January 2010 - 05:48 PM

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

#98 Starman1

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Posted 05 January 2010 - 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.

#99 Vic Menard

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Posted 05 January 2010 - 06:33 PM

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.

#100 Jason D

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Posted 05 January 2010 - 07:04 PM

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.

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Jason


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