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Autocollimator acuracy

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#26 precaud

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Posted 16 February 2013 - 02:12 PM

Coming out of the larger theoretical discussion for a moment:

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.

#27 Vic Menard

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Posted 16 February 2013 - 02:20 PM

...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.

#28 Jason D

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Posted 16 February 2013 - 03:58 PM

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

Well said...

#29 Howie Glatter

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Posted 17 February 2013 - 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.

#30 precaud

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Posted 20 February 2013 - 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. :)

#31 Howie Glatter

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Posted 20 February 2013 - 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.

#32 precaud

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Posted 20 February 2013 - 09:58 PM

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.

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.

#33 Howie Glatter

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Posted 20 February 2013 - 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

#34 precaud

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Posted 21 February 2013 - 07:27 AM

A vector has two atributes: magnitude and direction.

Sorry, I should have said "ray".

#35 precaud

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Posted 28 February 2013 - 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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