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Equipment Discussions >> Reflectors

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tag1260
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center mark and collimation questions
      #5562160 - 12/09/12 10:36 AM

All the discussion here about collimation and I have a hypothetical question on center marks.

Can your center mark be in perfect center but still not be in the proper location for perfect collimation? If so, how would you achieve collimation? Trial and error?

Also, when using a Chelshire eyepiece you set your secondary then finish collimation with the primary. Is there any harm in fine tuning your collimation with your secondary? Seems as though this is what you do when using an autocollimator, don't you?

Thanks


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Nils Olof Carlin
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Re: center mark and collimation questions new [Re: tag1260]
      #5562187 - 12/09/12 10:53 AM

Quote:

Can your center mark be in perfect center but still not be in the proper location for perfect collimation?




In principle, yes - in practice, it must be rare at least with machine-figured and accurately round blanks. But what you can do is try a precise star collimation when the seeing is good enough, adjusting to make the diffraction rings accurately symmetric near focus (as seen in the exact center of the FOV).
This done, check with a cheshire or equivalent that the spot appears centered - if not, try again and see if the error is consistent - if so, move the spot accordingly. Is there anybody out in the wild who have actually done this?
Anyway, this particular test is the only reason I can think of for doing a star collimation.
Quote:

Is there any harm in fine tuning your collimation with your secondary?


If you fine tune the aim of the primary by adjusting the secondary, you will move the focuser axis (from secondary to primary) by much more than the primary axis (from secondary to focus). The aim of the focuser axis is less critical than that of the primary, and you might have a hard time seeing any ill effect, but it would be prudent to check the focuser axis after doing such a fine tuning.

Nils Olof


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GaryS
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Re: center mark and collimation questions new [Re: tag1260]
      #5562382 - 12/09/12 12:35 PM

Quote:

Can your center mark be in perfect center but still not be in the proper location for perfect collimation?




I've never seen it and I suspect it's highly unlikey, if not impossible.

Quote:

Also, when using a Chelshire eyepiece you set your secondary then finish collimation with the primary. Is there any harm in fine tuning your collimation with your secondary?




That's how I do it. (You can read all about my collimation technieques in the article, "A Beginner's Guide to Collimation" on my web site.) The key is to reserve that adjustment for the final tweak.

Regards,
Gary


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Jason D
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Re: center mark and collimation questions new [Re: tag1260]
      #5562521 - 12/09/12 02:03 PM

Quote:

Also, when using a Chelshire eyepiece you set your secondary then finish collimation with the primary. Is there any harm in fine tuning your collimation with your secondary?




Fine tune with respect to which reflections?
If you are fine tuning by centering the cheshire eyepiece cross-hair with the primary center spot then this might be unacceptable.
If you are fine tuning by aligning the primary center spot with the cheshire eyepiece pupil hole reflection then it is more acceptable.
But if you are interested in an overall accurate collimation then do both alignments.

Quote:

Seems as though this is what you do when using an autocollimator, don't you?




Well, that is not entirely true. It depends on the complete steps you follow.
A) If you use the single hole autocollimator without a cheshire then the recommended method is to start off with CDP by adjusting the secondary mirror then end it with stacking the center spot reflections by adjusting the primary mirror (last step is primary adjustment)
B) If you use the dual hole autocollimator without a cheshire then you stack P+2 by adjusting the secondary via offset pupil then stack reflections by adjusting the reflections via central pupil – re-iterate until both show classic alignments. Of course, you can always start with the above paragraph then fine tune with the steps outlined in this paragraph.
I know one vendor who includes instructions to align with the cheshire then end it with only the autocollimator (secondary adjustment) – no iterations. These are wrong instructions. You need to iterate.
Jason


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Jason D
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Re: center mark and collimation questions new [Re: GaryS]
      #5562526 - 12/09/12 02:07 PM

Quote:

Quote:

Also, when using a Chelshire eyepiece you set your secondary then finish collimation with the primary. Is there any harm in fine tuning your collimation with your secondary?




That's how I do it. (You can read all about my collimation technieques in the article, "A Beginner's Guide to Collimation" on my web site.) The key is to reserve that adjustment for the final tweak.

Regards,
Gary




Hello Gary,
Why not do both?
You never clarified which reflections you align when you fine tune collimation by only adjusting the secondary mirror using the cheshire eyepiece.
Jason


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Vic Menard
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Re: center mark and collimation questions new [Re: tag1260]
      #5562570 - 12/09/12 02:32 PM

Quote:

...Can your center mark be in perfect center but still not be in the proper location for perfect collimation? If so, how would you achieve collimation? Trial and error?



I'm in agreement with Nils Olof's assessment.

Quote:

Also, when using a Chelshire eyepiece you set your secondary then finish collimation with the primary.



By Cheshire eyepiece I assume you mean a combination Cheshire/sight tube collimation tool. If so, then yes--bottom edge of the sight tube to place the secondary mirror, sight tube cross hairs to align the focuser axis, and the final step, bright Cheshire ring to align the primary mirror axis.

Quote:

Is there any harm in fine tuning your collimation with your secondary?



"Fine tuning" is usually reserved for the axial alignments. Most users adjust the secondary mirror tilt to fine tune the focuser axial alignment, and then finish with the primary mirror collimating screws to fine tune the primary mirror axial alignment. As Nils Olof noted, there's a reason for the alignment sequence. While small adjustments to the secondary mirror tilt impacts both axial alignments, adjustments to the primary mirror tilt have virtually no effect on the focuser axial alignment.

Quote:

Seems as though this is what you do when using an autocollimator, don't you?



No. Assuming the primary mirror alignment has already been fine tuned with a Cheshire eyepiece, any obvious residual error visible in the autocollimator is likely to be focuser axial error. When the error is reduced at the secondary mirror, the earlier Cheshire primary mirror alignment will be changed, which forces the user to repeat the process until both axes are corrected to the resolution of both tools.

A more direct approach with an autocollimator is to carefully decollimate the primary mirror first, then correct the focuser axial alignment by adjusting the secondary mirror tilt, and finally, adjust the primary mirror tilt to correct the primary mirror axis.

Whether you're using a combination Cheshire/sight tube collimation tool, a laser collimator, or an autocollimator, I suggest that you always verify/correct both axes for best image performance.


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tag1260
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Re: center mark and collimation questions new [Re: Vic Menard]
      #5562676 - 12/09/12 03:44 PM

Quote:

By Cheshire eyepiece I assume you mean a combination Cheshire/sight tube collimation tool. If so, then yes--bottom edge of the sight tube to place the secondary mirror, sight tube cross hairs to align the focuser axis, and the final step, bright Cheshire ring to align the primary mirror axis.






Blackcat Chelshire, but a combination tool for preliminary secondary alignment.

If all of your rings appear to be concentric when finished, are you not still achieving the same end results?

Thanks for all the replies.


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Vic Menard
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Re: center mark and collimation questions new [Re: tag1260]
      #5562910 - 12/09/12 06:27 PM

The BlackCat Cheshire can easily read a primary mirror axial error as small as two hundredths of an inch (close enough for most scopes and most applications).

For establishing true centering of the secondary mirror under the focuser, you really need a sight tube. Without the sight tube, the focuser axis could easily be as much as a quarter inch or more out of alignment. For an 8-inch aperture, that's pushing the tolerance, even without coma correction. You might be able to do better than the other readers of this thread by reading the concentricities, but you'll certainly do much better with a sight tube.

Regarding concentricity--the only circles that must be concentric are the bottom edge of the sight tube (or the bottom edge of the focuser drawtube), the reflected edge of the primary mirror, and the reflection of the underside of the focuser (with the primary mirror center spot in the center). The real (viewed directly--not a reflection) edge of the secondary mirror and the edge of the silhouette reflection of the secondary mirror do not have to be concentric, although one can be depending on whether you choose an offset or centered alignment.


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Jason D
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Re: center mark and collimation questions new [Re: tag1260]
      #5562948 - 12/09/12 06:42 PM

Quote:

If all of your rings appear to be concentric when finished, are you not still achieving the same end results?





Not all will look concentic. Just look at my avatar.
Refer to the very recent thread:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5554348/page...
Interestingly, I noticed you have several posts in the above thread.
Jason

Jason


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tag1260
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Re: center mark and collimation questions new [Re: Jason D]
      #5562967 - 12/09/12 06:53 PM

Yes. That thread is what has me thinking about my collimation techniques. I seem to be achieving the same sort of result, as pictured in your picture of your XT10, but I have been tweaking my secondary to fine tune it at the end.

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tag1260
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Re: center mark and collimation questions new [Re: tag1260]
      #5562998 - 12/09/12 07:10 PM

As for the center mark question, it's raining and foggy and I'm just sort of thinking about things.......You know how it goes!!!!!!

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Vic Menard
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Re: center mark and collimation questions new [Re: tag1260]
      #5563147 - 12/09/12 09:19 PM

Quote:

As for the center mark question, it's raining and foggy and I'm just sort of thinking about things.......You know how it goes!!!!!!



I have to admit I've spent some time pondering the "center mark question".

It's remarkable that to date, it seems that tool (Cheshire) alignment always delivers the optimal star pattern when verified with a star test. It's remarkable that so many Newtonian primaries, whether amateur or professionally figured, seem to align properly with a well placed center spot. I mean, even the best available primary mirror isn't perfect!

I think, perhaps, the center spot works so well not because it so precisely represents the center of the primary mirror, but because it precisely represents the edge of the primary mirror. After all, what's the reference for placing the center spot? The edge!

Look at an 8-inch f/6 mirror. The central 4-inches occupies an area about 12 square inches while the outer 2-inches occupies an area of about 36 square inches. And while the outer ring operates at f/6, the central 4-inch diameter circle is operating at f/12 (it's spherical for all practical purposes)! And we haven't even considered that the shadow of the secondary mirror robs even more area from the central circle.

I think that most of the image information is coming from the outer edge/annulus of the primary mirror, so the critical alignment is the edge alignment, and the reference for that, is the center spot.

OK, let the flames begin!


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Nils Olof Carlin
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Re: center mark and collimation questions new [Re: Vic Menard]
      #5563591 - 12/10/12 06:09 AM

hi Vic,
Quote:

It's remarkable that to date, it seems that tool (Cheshire) alignment always delivers the optimal star pattern when verified with a star test. It's remarkable that so many Newtonian primaries, whether amateur or professionally figured, seem to align properly with a well placed center spot. I mean, even the best available primary mirror isn't perfect!



it is indeed possible to make a paraboloid mirror with its optical center offset (for a Herschelian or as part of a segmented telescope), but it is very much more difficult. With symmetric figuring, the optical center will apparently automatically fall in the center of the blank (even if the bevel is asymmetric?).

I would say most of the information comes from the area between the rim and the center Even if a paraboloid mirror has all sorts of figuring errors (I won't go into Zernikes here), there is still a point in the focal plane where (low-order) astigmatism is zero, and a center mark is a way to make sure this point is centered in the focuser.

There are many posters here (and elsewhere) who claim they finish tool collimation by tweaking with a star collimation. Why? Do they actually see a star image that calls for a tweak?? If so, the conclusion would be that the marker is offset from the optical center - and judging from your experience, also from the geometric center. If it turns out that star tweaking is necessary, the thing to do would be checking the centering of the spot and likely move it.

Nils Olof


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howard929
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Re: center mark and collimation questions new [Re: Nils Olof Carlin]
      #5563882 - 12/10/12 10:14 AM

Quote:


There are many posters here (and elsewhere) who claim they finish tool collimation by tweaking with a star collimation. Why? Do they actually see a star image that calls for a tweak?? If so, the conclusion would be that the marker is offset from the optical center - and judging from your experience, also from the geometric center. If it turns out that star tweaking is necessary, the thing to do would be checking the centering of the spot and likely move it.

Nils Olof




If there are many that do perform a star test and find a tweak is necessary it's because that many have used the most recommended method of placing the center spot that just doesn't work too well. At best, one has to get real lucky to achieve anything close enough to the center with one of those. I bought one, checked it for accuracy and threw it in the garbage.


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Vic Menard
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Re: center mark and collimation questions new [Re: howard929]
      #5564198 - 12/10/12 01:31 PM

Quote:

...If there are many that do perform a star test and find a tweak is necessary it's because that many have used the most recommended method of placing the center spot that just doesn't work too well. At best, one has to get real lucky to achieve anything close enough to the center with one of those. I bought one, checked it for accuracy and threw it in the garbage.



I'm not sure which method you're describing. I find that as long as the primary mirror center spot is placed within about 1/2mm of center (relative to the edge), I get excellent axial alignment with the tools I use. I've used just about every method available, from a full diameter paper disk folded in quarters and then nipping off the vertex, to a marked spot measured with a precision rule, to the acetate overlay provided by CATSEYE. They all give good results if the person using them is diligent. And many economy mirrors that come pre-center spotted from the manufacturer are often corrected to significantly better precision with these methods.


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Vic Menard
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Re: center mark and collimation questions new [Re: Nils Olof Carlin]
      #5564271 - 12/10/12 02:19 PM

Quote:

...Even if a paraboloid mirror has all sorts of figuring errors (I won't go into Zernikes here), there is still a point in the focal plane where (low-order) astigmatism is zero, and a center mark is a way to make sure this point is centered in the focuser.



I guess my point was that the "point in the focal plane" is defined more by the outer zones than the central zones, the center mark is simply a reference. When I used a zonal mask to figure the mirrors I polished, the mask was referenced from the edge. I'm unfamiliar with interferometry (before my time as an ATM), but I'm unaware of any mirror maker using this testing method to define the center of the paraboloid. I understand what you're saying, but I still feel that the central 4-inch diameter has only 1/2 the resolving power of a full aperture 8-inch primary mirror (not considering CO), so the outer annulus is responsible for "the rest of the information". Until the two zones are working together in concert, the central 4-inch diameter's tolerance to decentering is much more relaxed than the full aperture tolerance.

Quote:

There are many posters here (and elsewhere) who claim they finish tool collimation by tweaking with a star collimation. Why?



I think in many cases, it's because they are too quick to settle for "good enough". Then there's sloppy focusers, gravitational flexure, and other inadequate mechanical designs. I also see some posters chasing the wrong alignment signature, often at the expense of the more critical axial alignments.

Quote:

Do they actually see a star image that calls for a tweak?? If so, the conclusion would be that the marker is offset from the optical center - and judging from your experience, also from the geometric center. If it turns out that star tweaking is necessary, the thing to do would be checking the centering of the spot and likely move it.



Until recently I thought most collimation cap pupils were reasonably well centered--but I've been shown that even with a simple collimation cap, you should verify first.

FWIW, when I observe, every star in every field of view is a litmus test for seeing, equilibration, optical performance, and collimation. If something's amiss, the collimation is the easiest to verify (with the proper tools) and correct if necessary.


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Nils Olof Carlin
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Re: center mark and collimation questions new [Re: Vic Menard]
      #5564314 - 12/10/12 02:56 PM

Quote:

I guess my point was that the "point in the focal plane" is defined more by the outer zones than the central zones,



Of course I agree. Take a f/4.5 mirror as example - the <=1/14 wave (front) RMS zone at the focal plane has a radius of about 1 mm. Say you can tell to within half of this (you may do better, but for the sake of argument)- this means an error of at most 1 mm at the primary.
Stop it down to 0.707 of full diameter - ít is now f/6.36, and the error margin is 2.83 times as large (proportional to the 3rd power of f/).
Thus, if you want to verify collimation with any precision, you need the mirror out to the edge. But I can't say offhand what to expect from the obstructed zone - likely, very little.

Even if it takes a bit of care to place the spot accurately, measuring its centering with a ruler ought to be trivial, I think.

Nils Olof


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howard929
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Re: center mark and collimation questions new [Re: Nils Olof Carlin]
      #5565712 - 12/11/12 10:52 AM

I'm not a fan of referencing the edges of a primary mirror with a circular mask by eye or with placing a new spot on the mask the same way. If memory serves and yes, I did throw out the one I bought, the line thickness on the CatsEye mask was .279mm, half of the .5mm tolerance I wanted to be under. Sure, it needs lines that can be seen but therein lies part of the problem I found with it. IME, using that mask as instructed is too open to placement errors of the new center spot and rotational errors while placing it over the mirror. Then pressing it down by hand even with the block method doesn't ensure accuracy.

It's flat when it needs to be shaped like the mirror is. It was loose and slid around too easily. It wasn't a ridged system that has any aspect built into it that causes or forces accuracy. Not to within .5 mm. IMO.


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Vic Menard
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Re: center mark and collimation questions new [Re: howard929]
      #5565747 - 12/11/12 11:18 AM

Quote:

I'm not a fan of referencing the edges of a primary mirror with a circular mask by eye or with placing a new spot on the mask the same way. If memory serves and yes, I did throw out the one I bought, the line thickness on the CatsEye mask was .279mm, half of the .5mm tolerance I wanted to be under. Sure, it needs lines that can be seen but therein lies part of the problem I found with it. IME, using that mask as instructed is too open to placement errors of the new center spot and rotational errors while placing it over the mirror. Then pressing it down by hand even with the block method doesn't ensure accuracy.

It's flat when it needs to be shaped like the mirror is. It was loose and slid around too easily. It wasn't a ridged system that has any aspect built into it that causes or forces accuracy. Not to within .5 mm. IMO.



Interesting.

I've used the CATSEYE template with good results on numerous occasions and on a variety of large aperture, fast focal ratio mirrors. I didn't have the same problem you had with the line thickness as the various diameters rarely lined up exactly with the mirror blank edge. As long as the line falls inside or outside the edge, it's pretty easy to judge concentricity to less than 1/2mm (probably closer to 1/4mm). When the sagittal depth exceeds more than about 3mm, I usually try to get a handle on the centering first. The CATSEYE template helps here too as the center is accurately perforated. This allows the user to mark the center with a sharpie marker, which provides another opportunity to assess (and correct if needed) the center mark. Once the center has been accurately marked, the template can be used to transfer a perforated CATSEYE triangle or HotSpot with very good accuracy. Even using this procedure, I always verify centering with a precision rule, and when I'm satisfied with the placement of the center spot, then I firmly press it onto the mirror surface.

I can also place the spot without the CATSEYE template, but it's more tedious and requires additional handling of the center spot close to the mirror surface. While an 8-inch aperture isn't too hard, a 16-incher can try your patience, and a 24-incher can try your nerves!

As I noted in my earlier post, regardless of the method, precise centering requires diligence.


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howard929
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Re: center mark and collimation questions new [Re: Vic Menard]
      #5565828 - 12/11/12 12:10 PM

I too placed a sharpie mark where I thought the center was using the thinnest marker I could buy, .5mm. It took 4 tries to get it within 1mm using a photographic method for verification that's exceedingly accurate to within .007mm.

I'm going to go out on a limb here and suggest that one cannot accurately verify the results you mention with any consistency using a rule or standard caliper to within .5mm. The rule relies on hand and eye placement since it's flat and the mirror isn't. A standard caliper slides off the end of the mirror when the other end of it dips down towards the center of the mirror. IMO this entire topic is a disaster. WE shouldn't have to become engineers because THEY don't etch out a spot in the coatings exactly at the center of these damn mirrors during the coating process.

Accuracy to .5mm? Gotta be kidding. I'm used to anything like 1/4" is fine and dandy and I hated the fact that I had to come up with some way to get it done.

Edited by howard929 (12/11/12 12:15 PM)


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Starman1
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Re: center mark and collimation questions new [Re: howard929]
      #5565854 - 12/11/12 12:27 PM

a few years ago I posted about an idea I had:
A rigid plexiglass disc the size of the mirror is rested on the mirror's edge, then centered.
In the center of the disc is a spring-loaded plunger with the center marker on the bottom end, below the disc. The plunger is pushed down toward the mirror, sticking the center marker on the mirror.
To keep the plunger from wiggling as it descends toward the mirror, the center of the disc is built up to a thickness of about 1/2" where the plunger passes through it, preventing any lateral wiggle when the plunger is pushed down.
The procedure would take only a second, and noting would sag into the mirror, and the positioning of the marker on the bottom of the plunger would be the only source of inaccuracy.
I could see this resulting in accuracies of small fractions of a millimeter in the positioning of the center marker.

If I have time next year, I may make one of these. If the disc were clear, with diameter markers, it could be made for many sizes of mirror.


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howard929
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Re: center mark and collimation questions new [Re: Starman1]
      #5565887 - 12/11/12 12:46 PM

Yes, that's an excellent idea. A ridged system that forces accuracy is always going to best a hand and eye method.

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tag1260
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Re: center mark and collimation questions new [Re: Starman1]
      #5565918 - 12/11/12 01:14 PM

Sounds like an excellent idea. I think there's a quote somewhere-"If you build it they will come".
You are going to market this when you do, aren't you?


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Vic Menard
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Re: center mark and collimation questions new [Re: howard929]
      #5565942 - 12/11/12 01:28 PM

Quote:

...WE shouldn't have to become engineers...Accuracy to .5mm? Gotta be kidding...



Well, I'm not kidding, and I'm not an engineer.

Final measurement using a millimeter rule from the edge of the mirror to the vertex of the triangle gives a read that's an easy 1/2mm tolerance. I use a block at the edge of the mirror to keep the rule "stop" consistent and I measure all three vertices. But, ymmv...


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Nils Olof Carlin
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Re: center mark and collimation questions new [Re: howard929]
      #5566249 - 12/11/12 04:35 PM

Quote:

I'm going to go out on a limb here and suggest that one cannot accurately verify the results you mention with any consistency using a rule or standard caliper to within .5mm. The rule relies on hand and eye placement since it's flat and the mirror isn't.




You shouldn't have to be an engineer to use a flexible ruler of some kind - I take two plastic rulers taped to a cross, and with the marker taped in the center (the free quadrant). Two strips of cardboard would do as well, no doubt. For verifing, a metal tape measure would do (you could put a few pieces of tape not to risk scratching). web pic


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Jason D
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Re: center mark and collimation questions new [Re: Nils Olof Carlin]
      #5566598 - 12/11/12 09:21 PM

We had a similar lengthy discussion a while back starting with this page
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3532750/page...


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howard929
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Re: center mark and collimation questions new [Re: Starman1]
      #5567272 - 12/12/12 10:15 AM

Quote:

a few years ago I posted about an idea I had:
A rigid plexiglass disc the size of the mirror is rested on the mirror's edge, then centered.




The rest of that idea seems good, though a suggestion. Rather then a plexi disc placed by hand and eye, a piece of square plexi that references both edges of a right angle fixture that the mirror also references. ie: a rigid system that forces accuracy.

Edited by howard929 (12/12/12 10:17 AM)


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howard929
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Re: center mark and collimation questions new [Re: Starman1]
      #5567324 - 12/12/12 10:54 AM

I have a question that's haunting me here. The primary mirror I have is figured and polished only in what I hope is the center with a small "ring" of rough glass. I can easily understand why it's done that way.

Does the 200mm stated size reference the polished section or the overall size of the blank? Further, who could guarantee that a 2 year old 200mm mirror that I have would be the same size as a brand new one or any other 200mm mirror to within "what?" tolerance?

When I verified the placement of the new spot I referenced the polished section but I never thought to measure its size since that didn't matter.

For the prospect of producing a mask, size does matter if forced accuracy is sought. Thoughts or insights?

Edited by howard929 (12/12/12 10:56 AM)


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Starman1
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Re: center mark and collimation questions new [Re: howard929]
      #5567376 - 12/12/12 11:29 AM

When determining the center of a mirror, use the outer edge of the mirror as a reference. The beveled edge is done after the mirror is polished, and it can vary in width.

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howard929
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Re: center mark and collimation questions new [Re: Starman1]
      #5567400 - 12/12/12 11:46 AM

Quote:

When determining the center of a mirror, use the outer edge of the mirror as a reference. The beveled edge is done after the mirror is polished, and it can vary in width.




More to the point. Is the outer edge of any given sized mirror always the stated size of the mirror? And if so, to within what accuracy? I have a feeling that the only one(s) who know these answers are the ones who actually make these mirrors. Does GSO and Synta make for example, primary mirrors that are always a given size to very tight tolerances and would they be he same? I sort of doubt that but it doesn't hurt to ask.


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Starman1
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Re: center mark and collimation questions new [Re: howard929]
      #5567763 - 12/12/12 02:44 PM

I have seen some companies that have changed their sizes to conform with American sizes.
Metric mirror sizes would be logical at 200mm (7.87"), 250mm (9.84") and 300mm (11.81"), and I've seen these sizes.
American sizes are logical at 8" (203mm), 10" (254mm), and 12.5" (317.5mm). There aren't any 12" American mirrors that I know of except in catadioptric scopes.
If a manufacturer were using metric-sized equipment but wanted to conform to American size standards, we might get mirrors of 205mm, 255mm, and 305mm to correspond to 8", 10", and 12".
And, indeed, I've seen that, from more than one company, and varying over the years.
There is usually great consistency in a particular size of particular scope over a couple years. But in the long run? I wouldn't count on it.

Yes, the size of the mirror is measured on the outside edge of the glass itself. The reflective surface is always a little smaller than that, but the width of the bevel varies greatly. I've seen zero bevel (on a refigured mirror) and I've seen around 1/8" (!). The mirror with the large bevel had a bevel of varying width, to boot. The center of the mirror wasn't the center of the reflective surface!. The mirror performed fine in the field, though. I wondered if the mirror maker increased the bevel size to help take care of a Turned Down Edge or some other issue.


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howard929
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Re: center mark and collimation questions new [Re: Starman1]
      #5569170 - 12/13/12 12:32 PM

That's bad news for pre-made masks or right angle fixtures. Makes me wonder how many mirrors aren't polished in the exact center of the blank and it's no wonder that so many find star testing reveals the need for tweaks.

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Starman1
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Re: center mark and collimation questions new [Re: howard929]
      #5569397 - 12/13/12 02:42 PM

Quote:

That's bad news for pre-made masks or right angle fixtures. Makes me wonder how many mirrors aren't polished in the exact center of the blank and it's no wonder that so many find star testing reveals the need for tweaks.




You misinterpreted what I said.
When the mirror is made, the center of the mirror is the glass blank's center. The anti-chip bevel is put on the mirror AFTER the mirror is finished.
If the bevel is exactly equal all the way around, the center of the mirror blank and the center of the reflective surface coincide. The two circles are concentric.

I have seen one 16" mirror with a bevel that varied in width all the way around, and was quite large (looked like it had been done with a file). The center of that mirror was the center of the glass blank, as you would expect. But it wasn't exactly in the center of the aluminized reflective surface because the outside edge of the reflective surface and the glass blank were not concentric!

I did not measure to see if the center marker was positioned in the center of the aluminized surface or the center of the glass blank, but given the precision by which center markers are applied, I don't think it would have mattered. The maximum error caused by the exaggerated bevel would have been less than the likely error in application anyway.

Your other point, about the center of the mirror's figure of revolution not being the center of the blank? Unlikely. Machine-made mirrors would have to be eccentric (i.e. not round) for this to occur.

The point to take from this is: When positioning a center marker on your mirror, use the outside edges of the glass as the place you determine the center from. IF the bevel is perfectly even, you can use the edge of the reflective surface.

I do it a very easy way: place the acetate template down on the mirror, and place pins through the acetate at 4 places to hold the acetate template in place over the mirror (the pins press against the side of the glass mirror). Then, I use a pencil with an eraser tip to press down the center of the template until the center marker is stuck on the mirror. The pins prevent a lateral movement of the acetate when I'm pushing down, and even if there is differential stretch in the acetate as I press down, the lateral movement of the center marker is a small fraction of a millimeter, and I can be off a LOT more than that before the error makes collimation iffy.


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tag1260
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Re: center mark and collimation questions new [Re: Starman1]
      #5569444 - 12/13/12 03:11 PM

Boy!! Ask a couple questions around here and you get a heck of an education!!!

I guess , not being around ATM'ers, I still can't actually see how your mirror CAN'T be made off center.

I know I see .5 mm out of place for your center mark but in reality, how far off can you be before it REALLY affects your collimation in that we can see it?

Thanks again for all the discussion. (Even those of you who are talking WAY over my head!!! )

Tag


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Starman1
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Re: center mark and collimation questions new [Re: tag1260]
      #5569501 - 12/13/12 04:00 PM

Usual rule of thumb is that the center marker should be within 0.005x the diameter of the mirror (on a 10", that's 1.27mm).
Since the error, IIRC, in a cheshire, due to the double bounce, is doubled, that means within 0.635mm from exact center (someone correct me if I don't remember correctly).
0.635mm is 0.025", or 1/40 of an inch.


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Mike Lockwood
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Re: center mark and collimation questions new [Re: Starman1]
      #5569508 - 12/13/12 04:03 PM

Quote:

When the mirror is made, the center of the mirror is the glass blank's center.



Not necessarily. More below.

Quote:

The anti-chip bevel is put on the mirror AFTER the mirror is finished.



That is completely wrong.

The bevel is put on BEFORE generating or grinding is done to prevent chipping of the edge of the blank during those aggressive operations. (The bevel may need to be enlarged again during grinding if a lot of glass is removed.)

After the mirror is done, the bevel provides some protection from chipping should the mirror be banged into something hard. (I often ease the sharp edges of the bevel, front and back, to further reduce the chance of future damage from handling mishaps.)

Quote:

If the bevel is exactly equal all the way around, the center of the mirror blank and the center of the reflective surface coincide. The two circles are concentric.....



No, that is not enough to guarantee that the optical center and mechanical center are coincident.

If the blank has WEDGE, then it is thicker on one side than another. In that case, all bets are off - even if the bevel is uniform the center of the optical figure may not be the mechanical center of the blank. Finding the true optical center can then be very difficult.

Quote:

Your other point, about the center of the mirror's figure of revolution not being the center of the blank? Unlikely. Machine-made mirrors would have to be eccentric (i.e. not round) for this to occur.



Sorry, but that is also wrong. If they have wedge this could happen too.

Thankfully most blanks are machined with good equipment, and therefore have uniform thickness.

I have seen mirrors that are non-round, have non-uniform bevels, and have significant amounts of wedge (up to 3/16"). However, none of those were from major glass suppliers, who will generally provide nicely round blanks of uniform thickness.

All that I can say is that if a mirror has a uniform edge thickness (which can be measured carefully with calipers), it is VERY LIKELY that the optical center coincides with the mechanical center. It could even have a non-uniform bevel and still be fine, though that would throw off edge thickness measurements.

However, for a mirror that is non-round or has wedge, all bets are off, and center spot at your own risk.


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Re: center mark and collimation questions new [Re: tag1260]
      #5569509 - 12/13/12 04:03 PM

Quote:

...I see .5 mm out of place for your center mark...how far off can you be before it REALLY affects your collimation in that we can see it?



It depends...

For your f/5 Dob, the high magnification (300X to 600X for your 12-inch aperture) primary mirror axial tolerance is a bit less than 1mm. If your primary mirror center spot is offset 1mm, you'll need to add 1/2 of that offset to your final primary mirror alignment result, which means you'll need to keep your Cheshire alignment within 1mm of "perfect" (the Cheshire magnifies the primary mirror axial error read 2X). This is the "worst case scenario", and is the one I use since I rely on tool alignment to get the job done (I like to know that my alignment is up to the task when my seeing gets really good).

Of course, both the center spot placement error and the Cheshire error have direction and should be considered as vector quantities. This basically means, in the "best case scenario", a residual Cheshire error could cancel out a center spot placement error. Unfortunately, the only way to be sure is to verify the alignment on a star (when I would rather be observing Jupiter).

Tolerances aside, considering the effects of gravitational flexure on truss OTAs and torque forces on the UTA, the closer you can keep the primary mirror axis to "perfect" alignment, the more likely you'll stay within tolerance throughout an observing session--which means you won't miss out when those moments of extraordinary seeing unexpectedly arrive.


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Re: center mark and collimation questions new [Re: Mike Lockwood]
      #5569531 - 12/13/12 04:22 PM

Mike's post is illuminating.
I guess I was assuming a round blank with no wedge, but home-made mirrors can sometimes have wedge generated by the grinder, if I recall my mirror-making class back in the '70s.
Mike, thanks for correcting me on the bevel. In our class, most chose to ignore a bevel, and those that did one did it after the mirror was made, and solely for edge protection. I can see how having the bevel done first would help avoid chips.

But, though you have seen some horror stories in mirror blanks, how many commercial mirrors are likely to be delivered to customers that aren't round and have significant wedge or which have radically non-uniform bevels? I would think a vanishingly low number. I would certainly counsel an amateur to assume the center is the center unless placing a center marker there results in visible aberrations in the star images.


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Re: center mark and collimation questions new [Re: Mike Lockwood]
      #5569533 - 12/13/12 04:27 PM

Quote:

...I have seen mirrors that are non-round, have non-uniform bevels, and have significant amounts of wedge (up to 3/16"). However, none of those were from major glass suppliers, who will generally provide nicely round blanks of uniform thickness.



I tested a 13-inch (early) Coulter primary that had a "wrinkle" on the back of the mirror from about 10 o'clock to 2 o'clock. It looked ugly, but the mirror gave a pretty good image. I haven't seen one with 3/16-inch wedge though! I'm guessing if you had to refigure such a piece of glass, you would start by removing the wedge (assuming there's enough glass), yes?

Quote:

All that I can say is that if a mirror has a uniform edge thickness (which can be measured carefully with calipers), it is VERY LIKELY that the optical center coincides with the mechanical center. It could even have a non-uniform bevel and still be fine, though that would throw off edge thickness measurements.



I recall chamfering my first ATM project, a 10-inch mirror, after it was parabolized. Back then, Edmund was selling blanks with 1/4-inch chamfers, but University Optics blanks had chamfers that were barely 1/16-inch. I wanted all the aperture I could get, but I got careless and ended up with a small chip on the edge, removable, with about an 1/8-inch chamfer...


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Mike Lockwood
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Re: center mark and collimation questions new [Re: Starman1]
      #5570119 - 12/13/12 10:59 PM

Quote:

Mike's post is illuminating.
I guess I was assuming a round blank with no wedge, but home-made mirrors can sometimes have wedge generated by the grinder, if I recall my mirror-making class back in the '70s.



Yes, that's a good recollection - non-machined, hand ground mirrors are at greatest risk of having wedge since they have never been trued up by a machining operation.

Quote:

Mike, thanks for correcting me on the bevel. In our class, most chose to ignore a bevel, and those that did one did it after the mirror was made, and solely for edge protection. I can see how having the bevel done first would help avoid chips.



If a bevel gets to less than 1/16" wide, during grinding, then I widen it. If the bevel goes away and one so much as touches it against something hard, there is a risk of a chip coming off.

Quote:

But, though you have seen some horror stories in mirror blanks, how many commercial mirrors are likely to be delivered to customers that aren't round and have significant wedge or which have radically non-uniform bevels?



Not very many, thankfully, but any mirror that is not machined front and back could have an issue.

Molded blanks are at risk if they are not machined on the back, as are those that are simply cut from sheet glass that is not flat like old sheet Pyrex, which had an uneven surface.

Also, don't forget about non-monolithic blanks (fused, etc.) that the maker may not wish to subject to certain stressful machining operations. If these are not machined front and back, they should be checked.

So, if you see the back of a mirror and it is not machined, just be aware of the possible issue.

The same is true for the SIDE of the blank - a molded blank is often not perfectly round, and it is good to be aware of that too, especially if you are setting the cell to have little clearance around the mirror. The mirror may eventually rotate due to vibration in transport, etc. and may become wedged in the cell. This condition will show up as triangular or astigmatic star images.

Quote:

I would certainly counsel an amateur to assume the center is the center unless placing a center marker there results in visible aberrations in the star images.



That's good advice. Just remember that a non-machined back or side of a mirror means it should be looked at carefully.

Quote:

I haven't seen one with 3/16-inch wedge though! I'm guessing if you had to refigure such a piece of glass, you would start by removing the wedge (assuming there's enough glass), yes?



Yes, that was a large BVC blank and it was machined away.

The most maddening thing I have to deal with are smaller non-round blanks, often cut from sheet glass. These pop up from time to time. As polishing is done, the mirror may rotate slowly on what it is placed on (carpet, etc.). This is normal. However, if the retaining "bumpers" on the turntable are not set with sufficient clearance, the mirror will eventually rotate far enough that the wider parts of the mirror become wedged between the bumpers and this inevitably will cause nasty astigmatism or other distortions.

The moral of the story is - a nicely machined blank is a wonderful thing, and it is something that I insist upon starting from. (Refigures must be treated carefully since dimensional uniformity and anneal are not guaranteed to be ideal.)

By the way, it is standard practice for me to center a finished (but uncoated) mirror on a turntable and scribe a series of concentric, precisely centered circles into the glass to mark the exact center of the mirror. This makes center spotting much easier, and my clients love it.


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Chucky
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Re: center mark and collimation questions new [Re: Mike Lockwood]
      #5570370 - 12/14/12 06:47 AM

<< By the way, it is standard practice for me to center a finished (but uncoated) mirror on a turntable and scribe a series of concentric, precisely centered circles into the glass to mark the exact center of the mirror. This makes center spotting much easier, and my clients love it. >>

Now this is nice! I've never heard of such a practice.


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Re: center mark and collimation questions new [Re: Mike Lockwood]
      #5570374 - 12/14/12 06:51 AM

Mike, what can ATM'ers do to avoid getting improperly made or machined blanks? Who to buy from? Who to avoid? I've been wanting to try making my own 6" or 8" mirror and build a telescope around it for quite a while.

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Mike Lockwood
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Re: center mark and collimation questions new [Re: Chucky]
      #5570899 - 12/14/12 02:00 PM

Quote:

Now this is nice! I've never heard of such a practice.



A commercial client requested it a while back, and after I figured out how to do it I decided to offer it as a standard feature. Since I mostly make fast mirrors, accurate center spotting is even more important.

Quote:

Mike, what can ATM'ers do to avoid getting improperly made or machined blanks? Who to buy from? Who to avoid? I've been wanting to try making my own 6" or 8" mirror and build a telescope around it for quite a while.



I can't recommend or warn against specific vendors because that would violate the TOS.

Machined blanks will probably be more expensive, and whether you hog out by hand or use machined blanks to save time is up to you. In either event, check the anneal of the blank before devoting too much time to it.

There is nothing wrong with molded blanks or those cut from sheet. Just make sure they're reasonably round and check the edge thickness after you start grinding. If one side is significantly thicker, grind on that side until it is not. (Wedge in refractor lenses effectively makes them a prism, so lens makers check for this and grind it away.)

With hand grinding, wedge is less likely to cause problems than with machine work.


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