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Concise thread about autocollimators+improvements

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#326 Jason D

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Posted 26 January 2011 - 03:52 AM

I placed a marker in pencil. I secured the template on the side with the marker, then pressed the center, then I secured the opposite side while still pressing. Finally. I let go of the first side then released the template. The amount of shift is twice the error. Based on the attached photo, I estimate the error to be about 0.1mm.
Bear in mind that the mirror being mimicked in this experiment represents an F2.8 mirror. Typical mirrors will have smaller errors.

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  • 4340784-delta.jpg


#327 michael_m

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Posted 26 January 2011 - 04:52 AM

Jim,

Actually there still is another thing to consider after cutting along one of the dotted lines...the circumference, and therefor the diameter, would change a little. So a person would need to be careful in placing the template over the mirror since the circumference line and the mirror edge would differ slightly. But, once compensated for the rest still works.

It would be interesting if you made a template that was larger than the normal circumference so after cutting, it would match the mirror edge. That's what Don is referring to. It would need to be custom. Probably one for each of the typical focal ratios. However, once the various details were hassled out, this system offers the easiest way to center mark, at least for a template system. No slipping, no estimating errors, no tricks. Just do a good job of cutting and placing the template accurately on the mirror.

I think it's important to remember that this needs to something your average amature can accomplish with reasonable precision. A good system shouldn't require an expert to make work or need all kinds of clever methods. It needs to be easy. You would attract a large user base by keeping it simple while offering superior accuracy-and sell more also.

#328 Jason D

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Posted 26 January 2011 - 11:04 AM

Fascinating.
The center and edges would touch nearly simultaneously, or suspend the center only a tiny bit above the mirror.
It would only touch the mirror at the very edges and the center.
That could work.
But it would have to be "custom", as the sagittae of various mirrors would differ.


Though Jim's idea is interesting, I am afraid it is not practical.
Don, if my math is correct, you have to overlap each of the 3 cuts by 0.06mm for your mirror -- smaller than the width of the "I" in the American Penny "LIBERTY".
My take is that the current method is adequate. The slippage error margin is within the error margin of other human errors.
Jason

#329 CatseyeMan

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Posted 26 January 2011 - 12:16 PM

I placed a marker in pencil. I secured the template on the side with the marker, then pressed the center...


You removed the lid before pressing, yes?

#330 Jason D

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Posted 26 January 2011 - 12:25 PM

I placed the lid to leave around 5mm clearence to mimic an F2.8 8" mirror. The template is far from touching the lid when it is relaxed.
Had I removed the lid, I would be mimicking an F1.0 8" scope.
Anyone can try the same experiment.

#331 CatseyeMan

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Posted 26 January 2011 - 04:53 PM

Fascinating.
The center and edges would touch nearly simultaneously, or suspend the center only a tiny bit above the mirror.
It would only touch the mirror at the very edges and the center.
That could work.
But it would have to be "custom", as the sagittae of various mirrors would differ.


Though Jim's idea is interesting, I am afraid it is not practical.
Don, if my math is correct, you have to overlap each of the 3 cuts by 0.06mm for your mirror -- smaller than the width of the "I" in the American Penny "LIBERTY".
My take is that the current method is adequate. The slippage error margin is within the error margin of other human errors.
Jason


After running the numbers and actually making a beta template, the overlap widths are indeed incredibly small; I agree with Jason about the impracticality of the "cone" acetate template. The theoretical cutting lines were closer than the width of the ink lines themselves making them indiscernable.

#332 Nils Olof Carlin

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Posted 26 January 2011 - 05:10 PM

I also think you need to make sure the spot does not accidentally stick to the mirror before you have actually centered it...
That's one reason I like to use two crossed plastic strips.

Nils Olof

#333 CatseyeMan

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Posted 26 January 2011 - 05:49 PM

I also think you need to make sure the spot does not accidentally stick to the mirror before you have actually centered it...
That's one reason I like to use two crossed plastic strips.

Nils Olof


The inherent stiffness and careful placement of the acetate template facilitates that as well. Once the template is in place resting on the circumference of the mirror with the spot suspended above the mirror center, the operative procedure to prevent significant lateral movement and precision spot placement is to apply "vertical" pressure pressure down with the eraser-end of a pencil.

#334 Nils Olof Carlin

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Posted 27 January 2011 - 05:36 AM

Yes, but if you cut those slits, the stiffness of the acetate will be gone, I suspect. But you will find out if this is a problem or not.

Nils Olof

#335 sixela

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Posted 27 January 2011 - 05:46 AM

After running the numbers and actually making a beta template, the overlap widths are indeed incredibly small; I agree with Jason about the impracticality of the "cone" acetate template.


Just make the slits larger than necessary and leave the edges be; the template will simply sink until it's stopped by the mirror (and form itself into the compliant truncated cone automagically). As long as the centre spot doesn't immediately stick to the mirror you can still adjust the template before pressing the centre spot in place.

#336 Gianluca67

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Posted 27 January 2011 - 10:24 AM

I also think you need to make sure the spot does not accidentally stick to the mirror before you have actually centered it...
That's one reason I like to use two crossed plastic strips.

Nils Olof


Hi Nils Olof,

can you illustrate your method of using two crossed plastic strips in more detail? How do you attach the hotspot? Can you post a picture?
Gianluca

#337 Jason D

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Posted 27 January 2011 - 10:47 AM

I am perplexed about the problem we are trying to solve. It seems we are trying to get back that 0.1mm (or sub 0.1mm) of potential placement error. But before spending more time in the solution space, we need to get back to the problem space to reassess the issue. I strongly suggest for those who are interested to put together a mockup setup such as a solid plate with diameter and depth similar to your mirror. Go ahead and use the Catseye template. Press down and leave 3 marks through the 3 central holes (save your real spot). Repeat the experiment many times to check for consistency. Assess the locations of the marks against the true center. If the results look good, then there is really no problem to solve.
My take is that the "slippage" error is negligible(hard to measure) and any attempts to get it back might introduce a larger error.
Like the old saying goes "if it ain't broken, don't fix it"
Jason

#338 sixela

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Posted 27 January 2011 - 11:38 AM

I've respotted my mirror repeatedly. Wiggle seems to be around 0.1mm, but it's hard to measure (you need a digital camera).

#339 Jason D

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Posted 27 January 2011 - 11:46 AM

Agreed, Alexis. 0.1mm to 0.3mm seems to be the error margin which will translate to 0.05mm/0.15mm at the EP.
But the error is too small to warrent any drastic measures to fix it.
Anyone can always shift the template by 0.1mm then anchor one side of the template as was explained in a prior post. Still, I do not think it is worth it.

#340 CatseyeMan

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Posted 27 January 2011 - 01:08 PM


After running the numbers and actually making a beta template, the overlap widths are indeed incredibly small; I agree with Jason about the impracticality of the "cone" acetate template.


Just make the slits larger than necessary and leave the edges be; the template will simply sink until it's stopped by the mirror (and form itself into the compliant truncated cone automagically). As long as the centre spot doesn't immediately stick to the mirror you can still adjust the template before pressing the centre spot in place.


I had the same idea - here's the concept: Anyone game to try? Send me a PM.

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  • 4344027-Split Template Concept 16in f4_5.png


#341 Nils Olof Carlin

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Posted 27 January 2011 - 02:12 PM

Gianluca,
try here, 4th picture from top. Here, the spot is centered on the intersection of two edges, leaving one quadrant of it free to tape to the top of the plastic.
Else, I imagine you could glue a piece of post-it, stick the spot to the rather weak sticky layer on the post-it, then measure and mark the radii on the strips (just make them long or stiff enough not to sag all the way until pressed!)

Nils Olof

#342 Jason D

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Posted 27 January 2011 - 02:13 PM

Jim, with that design, the template will definitely sag at the center and keeping the center spot off the mirror while fine adjusting will be a problem.

#343 Jason D

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Posted 27 January 2011 - 02:16 PM

Maybe you can stop the slit cut around 1/2 way along the radius. In this case, the template has room to breathe when the final center press is made -- yet there is enough support to keep the center elevated above the mirror while making adjustments.
That is, the length of the slit needs to be careful selected to control the amount of sag.

#344 CatseyeMan

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Posted 27 January 2011 - 02:34 PM

Jim, what that design, the template will definitely sag at the center and keeping the center spot off the mirror while fine adjusting will be a problem.

Well conceptually, I don't agree because the contiguous (dotted line) triangular area around the center spot is not subject to significant deflection via gravity. This area can be increased in size if necessary to effectively keep the spot off the mirror until pressed down.

#345 CatseyeMan

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Posted 27 January 2011 - 02:36 PM

We're on the same page! :grin:

#346 Jason D

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Posted 27 January 2011 - 03:09 PM

No need for a new template. The three 120 degree lines already exist. Anyone can use a utility knife to make the cuts. The width of the wedge cut is not important but it needs to be small and semi-equal.

EDIT: Do it at your own risk. Personally, I do not believe the benefits are worth the risk or the effort -- but that is my opinion.

#347 CatseyeMan

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Posted 27 January 2011 - 03:25 PM

No need for a new template. The three 120 degree lines already exist. Anyone can use a utility knife to make the cuts. The width of the wedge cut is not important but it needs to be small and semi-equal.

EDIT: Do it at your own risk. Personally, I do not believe the benefits are worth the risk or the effort -- but that is my opinion.


Only problem with implementing on existing templates is the that the radial lines are markers for orienting the spot - It's not adviseable to cut those out with the slit wedges.

If it works and is deemed beneficial to some folks, it's easy enough to add the cut lines to all future templates and let the user decide to cut or not to cut. If someone want to give it a go, let me know via PM.

#348 Jason D

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Posted 27 January 2011 - 03:28 PM

If it works and is deemed beneficial to some folks, it's easy enough to add the cut lines to all future templates and let the user decide to cut or not to cut. If someone want to give it a go, let me know via PM.

Sounds reasonable :waytogo:

#349 Gianluca67

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Posted 28 January 2011 - 06:27 AM

Gianluca,
try here, 4th picture from top. Here, the spot is centered on the intersection of two edges, leaving one quadrant of it free to tape to the top of the plastic.
Else, I imagine you could glue a piece of post-it, stick the spot to the rather weak sticky layer on the post-it, then measure and mark the radii on the strips (just make them long or stiff enough not to sag all the way until pressed!)

Nils Olof


Hi Nils Olof,

what level of accuracy do you expect with your method?

#350 Nils Olof Carlin

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Posted 28 January 2011 - 07:33 AM

what level of accuracy do you expect with your method?



I'd say about the same as with any of the discussed methods - as accurately as you could read the rulers, perhaps to within a 1/4 mm or so. I suspect hoping for better is wishful thinking...


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