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Is physical center really optical center for mirror spotting? How do I find out?

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#1 Lighthound

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Posted 21 September 2021 - 11:46 AM

How would we know?

What kind of testing/equipment would I need?

How much does it matter?

Differences for spherical versus parabolic?

Accurate method for finding optical center?

Should I really care?

 



#2 MitchAlsup

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Posted 21 September 2021 - 11:47 AM

Should I really care?

Slower than F/4: probably does not mater very much

Faster than F/4: it maters.



#3 ccaissie

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Posted 21 September 2021 - 11:56 AM

This was discussed by Arthur Leonard in ATMT Vol 1, p. 109.   I attach that citation.

 

When getting a quote for an f/3 from Bob Royce, he did not think that it was a significant issue.  We don't hear of it in general.

 

RE:how to test for it quantitatively...that's a great question for the optical guys.  I would think Bath IF coupled with some rigorous analysis is the key.  

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Edited by ccaissie, 21 September 2021 - 12:05 PM.


#4 Richard O'Neill

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Posted 21 September 2021 - 12:26 PM

"Should I really care?"

 

  The answer is NO and the reason is that for all practical purposes, the physical center is the optical center. Some folks find a center mark helpful with alignment. It serves no other purpose. In practice, I've never used or needed a center dot to accurately collimate any telescope.

 

Richard


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#5 Arjan

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Posted 21 September 2021 - 12:29 PM

Maybe it is less an issue with spinpolished mirrors?
When you find that the parabola vertex is off-center, you can decenter and tilt the mirror to compensate.
I think the vertex can be off several mm, both in slow and fast mirrors, but tolerances are tighter when fast.
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#6 Mike Spooner

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Posted 21 September 2021 - 02:05 PM

Here’s a thread I started with just such a mirror. The scope works great doing final collimating using a star. I expect it would not be a common issue and really insignificant with regards to performance.

 

Mike Spooner
 

https://www.cloudyni...icorn-project/ 


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#7 Lighthound

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Posted 22 September 2021 - 01:41 PM

Thanks mike.

 

Will look at that.



#8 Starman47

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Posted 23 September 2021 - 09:01 AM

For several years I used a store bought mirror with a center spot 5mm off of physical center. I was able to collimate reasonably well with tools or a star test. I discovered the off center center spot when the mirror went out for recoating. The observing is the same as before. but I still recommend getting the center spot in the center.

 

just my 2 cents


Edited by Starman47, 23 September 2021 - 09:56 AM.

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#9 kjkrum

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Posted 23 September 2021 - 08:16 PM

When I first got into the hobby, I wanted to over-engineer everything. I came up with the idea of placing the mirror on a record turntable with the spindle cut off, pointing a laser at it, and adjusting the position of the mirror on the turntable until the reflected laser did not move. There were two major problems with this idea: mirror wedge, and the fact that record turntables are designed to wiggle side to side to dampen vibration.


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