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Equipment Discussions >> Reflectors

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Nils Olof Carlin
Carpal Tunnel
*****

Reged: 07/26/04

Re: Autocollimator acuracy new [Re: Jason D]
      #5683360 - 02/16/13 12:03 PM

Jason,

I fully agree with your views on the First light optics page.

But a question such as "will such a laser be acceptable to its owner..." is a question about the owner rather than the laser or AC owned. And it is basically of emotions, not logic or even common sense.

I can answer for myself: I like to think in terms of tolerances - in engineering, you (or rather your professional customer) decide the acceptable tolerances, then you see to it that you meet them, but you don't expect your customer to pay extra for much tighter tolerances than needed. I might expect a layman customer, not accustomed to think in terms of engineering, to react differently - how many of us don't go for "perfect collimation"?.

If I had reason to believe that those 3 arcmin (laser or AC) were the maximum errors, and the workmanship in general acceptable, I would accept it if the price were right, knowing that collimation would not introduce significant optical problems. I would mark the laser, or AC, to make sure I always insert it rotated the same way, though.

But I would consider stability as well, perhaps see it as the more important aspect - a knock must nor send the error far outside tolerances.

We know that some people buy inexpensive laser collimators that are known to be easily knocked way out of tolerances, others pay premium price for better stability and workmanship.

But if you suggest that any manufacturer could easily do better than that, I am sure it is so - at a cost depending of time needed. Possibly only marginal, and highly likely a good investment in professional reputation.

If the testing is done at 100 ft or other distance is irrelevant, if the tolerance is given in arcmin (or arcsec).

Nils Olof


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Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: Autocollimator acuracy new [Re: Nils Olof Carlin]
      #5683427 - 02/16/13 12:36 PM

Nils Olof,
I believe there are two different views being discussed. If the collimation goal is to stay within tolerance then reasonable collimation tool errors will be acceptable. But if the collimation goal is to stay within tolerance then why even bother getting an autocollimator?
But there is another view which is what I have been trying to convey. Many, including me, strive for “perfect” collimation and let scope mechanical imperfections use up collimation tolerances. A quality autocollimator with proper use and experience can get me there. That is why I do not evaluate autocollimator errors against known collimation tolerances but I evaluate them against an almost-perfect criteria.
Jason


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precaud
scholastic sledgehammer
*****

Reged: 12/05/12

Loc: north central New Mexico
Re: Autocollimator acuracy new [Re: Howie Glatter]
      #5683511 - 02/16/13 01:13 PM

Thanks for weighing in, Howie.
Quote:

However, it is possible for a drawtube bore to be out of square with its face, and then tightening the clamp screw will force the accessory out of full contact with the drawtube face.



For most of the scopes out there, I think we can say "likely" instead of "possible"...

Quote:

For this and other reasons I have advocated the use of the drawtube bore rather than the front flange face as the reference surface for focuser axis direction.



Makes complete sense to me.

Quote:

But then we still have the problem of inconsistent registration of a smaller cylindrical accessory within a larger cylindrical drawtube due to the ease of slippage between the two clamped surfaces which are almost flat(in the sense of infinitesimals) at their line of contact.



Agreed again. And in addressing this problem, choosing a different surface to collimate to should not be part of the solution.

Quote:

This is the problem that my Parallizer(tm) principle is intended to fix.
<marketing info snipped>
...as it could improve alignment tolerances, as well as help fund my retirement



Gotta respect a man with the courage to put his future security on the line behind his creativity!
If I'm unable to resolve this problem passively, I may end up having to make a contribution to your retirement...
Quote:

(I have a patent on it :- )



Congrats and good luck with the patent. How long did it take them to process? I filed one two years ago this Friday, and still haven't heard a peep...


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Nils Olof Carlin
Carpal Tunnel
*****

Reged: 07/26/04

Re: Autocollimator acuracy new [Re: Jason D]
      #5683527 - 02/16/13 01:25 PM

Jason,
Quote:

But if the collimation goal is to stay within tolerance then why even bother getting an autocollimator?




I don't have an answer to that one.

The reason I took an interest in autocollimators was curiosity - at the time, apparently no one had much of an idea what kinds of reflections (or even how many) there were, and what could be derived in terms of tolerances. Unless you knew, there was no way of knowing even whether "perfection" would meet reasonable tolerances!
It took me a long time to figure out the "anomalous" reflections (1 and 3), but this done, it was straightforward to see how reflections depend on respective axial errors (even to find the unexpected and AFAIK unknown fact that stacking of 3 and P only depends on the focuser axis!)

1 mm at 1250 mm f.l. corresponds to one inch at 100 ft. And to a tilt of
0.04 mm across a 2 in. focuser - much less than a human hair. What does common sense say?

But this is amateur astonomy, done for pleasure, and there is plenty of room for debate and different opinions.

Nils Olof


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Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: Autocollimator acuracy new [Re: Nils Olof Carlin]
      #5683583 - 02/16/13 01:59 PM

Quote:

Quote:

...why even bother getting an autocollimator?



I don't have an answer to that one.



For most users, I think the autocollimator provides a means of improving the axial alignments achieved with their other tools. With practice, I've found that I can routinely achieve a "good" stack simply by being a little more careful with my laser alignment. There is a level of expertise involved, especially when the mechanicals are less cooperative (a condition common to many inexpensive Dobsonians).

With regards to the precision of collimation tools, I have to agree with Jason that properly executed tools should be transparent when making the more "critical" axial alignments. This transparency should extend such that the user's tools provide "agreement" instead of confusion. Finally, concerning tolerances, I believe they should be applied cumulatively to all of the mechanicals (throughout their respective motions), not just the collimating tools.


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precaud
scholastic sledgehammer
*****

Reged: 12/05/12

Loc: north central New Mexico
Re: Autocollimator acuracy new [Re: Vic Menard]
      #5683609 - 02/16/13 02:12 PM

Coming out of the larger theoretical discussion for a moment:

Quote:

If you can get a near perfect stack, when you decollimate the primary mirror reflections 1 and 2 will move away from P (the primary mirror center spot), leaving reflection 3 hiding (mostly or completely) behind P. Then you can tweak the focuser axial alignment by adjusting the secondary mirror tilt to carefully stack P-3.




I borrowed a Farpoint 1.25" Cheshire this morning, and a couple of the red triangular spots they supply with it, and installed one on the XT6 mirror. Recollimated to it and installed the AC. I think the red spot is not a good color for this purpose... I'm not seeing as many reflections as before, and those I can see are less clear. The inverted P3 reflection disappears before I can get it aligned under P. Plus, the larger triangular target occupies a bigger portion of the AC mirror face, leaving not much real estate to move things around and keep them all visible with the CDP technique. So it looks like, for now, I have to be content with pulling P3 under P as best I can and recheck the primary with the Cheshire. Does that make sense?

The white reflective spots won't arrive until mid-week with the replacement AC. I doubt they'll be smaller, though.


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Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: Autocollimator acuracy new [Re: precaud]
      #5683622 - 02/16/13 02:20 PM

Quote:

...The white reflective spots won't arrive until mid-week with the replacement AC. I doubt they'll be smaller, though.




CatsEye Collimation sells small triangles for their 1.25-inch Cheshire and large triangles for their 2-inch tools.


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Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: Autocollimator acuracy new [Re: Nils Olof Carlin]
      #5683784 - 02/16/13 03:58 PM

Quote:

But this is amateur astonomy, done for pleasure, and there is plenty of room for debate and different opinions.



Well said...


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Howie Glatter
Vendor


Reged: 07/04/06

Re: Autocollimator acuracy new [Re: precaud]
      #5685257 - 02/17/13 01:46 PM

Howie - ". . it is possible for a drawtube bore to be out of square with its face. ."
Precaud - "I think we can say "likely" instead of "possible"

I believe that most drawtube bore axis are square with the drawtube face, because both surfaces are usually machined with a single clamping ("chucking") of the part, and machine tool spindles and cross-slides are usually quite accurate. There are exceptions, for example Coulter Oddessy drawtubes were chop-sawed from tubing, and then bored and turned on a lathe (I.D. and O.D.) without having the chop-sawed front flange faced-off.
Drawtube bores may have the axis square with the face, but still cause aligment trouble if the bores have any taper, barrel, or saddle shape.

>good luck with the patent.
>How long did it take them to process?

Thanks. I filed a provisional application in November 2008 and followed it up with the full application in November 2009. The examiner rejected it the first time around (as almost always happens) but I was able to overturn the rejection, and the Patent was issued in September 2011. So it took about two years. I think two to three years is about average now.


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precaud
scholastic sledgehammer
*****

Reged: 12/05/12

Loc: north central New Mexico
Re: Autocollimator acuracy new [Re: Howie Glatter]
      #5691626 - 02/20/13 07:51 PM

Well after further thought, I measured the cylinder diameters of my eyepieces/collimation tools. They were amazingly consistent. The majority were 1.247", with one at 1.246" and one at 1.248". Worst case, that's a maximum deviation of eyepiece centering of 0.025mm.

Compared to the vector errors that can be caused by registration and bore angle inaccuracy, a 0.025mm linear offset is small potatoes, and the need for more accurate "centering" is almost nil. The Parallizer addresses the more meaningful potential adapter vector inaccuracies, and so today I made my contribution to Howie's retirement security.


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Howie Glatter
Vendor


Reged: 07/04/06

Re: Autocollimator acuracy new [Re: precaud]
      #5691773 - 02/20/13 09:21 PM

"The majority were 1.247", with one at 1.246" and one at 1.248". Worst case, that's a maximum deviation of eyepiece centering of 0.025mm."

Assuming the drawtube or adapter inside diameter is 1.250" (in reality it will be larger), the de-centering of a clamped 1.246" diameter eyepiece will be 0.002", which is 0.050 mm. If the de-centering is the same amount and in the same direction at both ends of the accessory, no problem (axis remain parallel). When the de-centering at both ends is different, that's a problem.


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precaud
scholastic sledgehammer
*****

Reged: 12/05/12

Loc: north central New Mexico
Re: Autocollimator acuracy new [Re: Howie Glatter]
      #5691824 - 02/20/13 09:58 PM

Quote:

Assuming the drawtube or adapter inside diameter is 1.250" (in reality it will be larger), the de-centering of a clamped 1.246" diameter eyepiece will be 0.002", which is 0.050 mm.




OK, but my view of that "actual" center point is: it is nice to know that it exists, but everything I have to put in the drawtube will be offset from it... slightly... and the closer my 2ndary collimation tool's cylinder diameter is to the eyepieces, the better.

Quote:

When the de-centering at both ends is different, that's a problem.




Yes, we're saying the same thing in different words. I prefer to preserve the notion that the focuser axis is a 3-D vector; parallel lines is a 2-D concept.

Regardless, your device appears to have conceived and solved the problem nicely. Congrats, and thanks.


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Howie Glatter
Vendor


Reged: 07/04/06

Re: Autocollimator acuracy [Re: precaud]
      #5691950 - 02/20/13 11:17 PM

"we're saying the same thing in different words."

Yes. That has a nice ring to it. One nit-pick though:

". .the focuser axis is a 3-D vector. ."

A vector has two atributes: magnitude and direction.
The axis is scalar: direction only


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precaud
scholastic sledgehammer
*****

Reged: 12/05/12

Loc: north central New Mexico
Re: Autocollimator acuracy new [Re: Howie Glatter]
      #5692283 - 02/21/13 07:27 AM

Quote:

A vector has two atributes: magnitude and direction.



Sorry, I should have said "ray".


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precaud
scholastic sledgehammer
*****

Reged: 12/05/12

Loc: north central New Mexico
Re: Autocollimator acuracy new [Re: precaud]
      #5706286 - 02/28/13 10:01 PM

I got the replacement AC today, and I'm happy to report, it is good. The reflections show minimal movement when it is rotated in the drawtube.

What a clever and useful tool you came up with, Nils Olof!


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