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Test stand question

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4 replies to this topic

#1 jeff.bottman

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Posted 18 September 2020 - 08:47 PM

I'm  getting set up to test a commercial mirror arriving tomorrow.  It's 10 inch, f/4, 35mm thick, GSO manufactured.  So far I've recommissioned my 20 year old Foucault tester from the dusty depths of my garage.  I made an adapter to allow use of my DSLR to take test images.  My question:  how should I support the mirror in the test stand?  would two support points at +/- 45 degrees offset be adequate, or do I need a sling of some sort?  I am looking for reasonably accurate measurements; if the figure is smooth without astigmatism or turned edge or other gross defects, and wavefront error is less than about 1/4 wave, I will be happy.  If it tests well it goes into my astrograph project.  Here is my old tester, which last helped me make a very nice 12 inch f/4 mirror.

 

 

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#2 mark cowan

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Posted 18 September 2020 - 09:08 PM

2 points at 90 degrees are more than sufficient.  Make them so the contact occurs at the COG of the mirror fore-aft.


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#3 jeff.bottman

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Posted 18 September 2020 - 09:40 PM

2 points at 90 degrees are more than sufficient.  Make them so the contact occurs at the COG of the mirror fore-aft.

Thank you.  That's exactly the info I need.



#4 gr5org

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Posted 19 September 2020 - 07:18 AM

Note that if you build it right, the mirror will be just balanced and the slightest tap will make it fall over out of the holder so you need something at the top of the mirror to keep it from falling.


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#5 Arjan

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Posted 19 September 2020 - 08:32 AM

Also good to minimize sideways friction. I rest my mirror on two bearings, so it can rotate freely. This is also nice when you want to test across several diameters.
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