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Mirror Testing

Started by puhtron54, 10/21/2014 09:24PM
Posted 10/21/2014 09:24PM Opening Post
A friend of mine recently acquired a 32" mirror, 2" thick and a FL of 147".
He is wondering if there is a place he can have this mirror tested.
Any suggestions?
Thanks
Tom
Posted 10/24/2014 02:26PM #1
You can try Terry Ostahowski at:

http://www.ostahowskioptics.com/

He did a fine job refiguring a 16" mirror for me. I visited his shop and he is set up with quite a bit of professional equipment.

Mike Ratcliff
Posted 10/29/2014 02:46PM | Edited 10/29/2014 02:54PM #2
Why not do a standard Foucault test yourself or himself? Much more affordable than transport or shipping of such a large mirror to a facility. Such a simple DIY exam will reveal if mounting the mirror is worthwhile and once mounted in a scope the star test is quite sensitive for critical evaluation. Read a mirror-making book and/or Suiter's star testing book for how to do it (public libraries have these books as well as book sellers). BTW, such a relatively thin mirror (1:16 thickness to diameter ratio) will require support when testing similar to that required in the final scope or test results will be in error.
Posted 11/02/2014 01:09PM #3
Thomas Northup said:

A friend of mine recently acquired a 32" mirror, 2" thick and a FL of 147".
He is wondering if there is a place he can have this mirror tested.
Any suggestions?
Thanks
Tom

Depends on the "why". Do you suspect it has a problem? Or do you just want a quick read on its quality? As another poster mentioned, a quick run with a Foucault tester is a lot easier than shipping this big primary off, especially if there is nothing wrong with it. If you can put it in a scope for a star test, even better.

Uncle Rod

Time on your hands?
Waste it with Uncle Rod's Astro Blog!

http://uncle-rods.blogspot.com/
Posted 12/03/2014 09:26AM #4
I'll chime in substantially agreeing with the others. Testing to small fractional wave requires the same mount you will be using AKA the wiffle cell and sling must already be built. Knife edge will reveal overt problems like bad zones, turned edge, etc. Overall correction can be deemed approx correct by zonal scrutiny (KE on a mike slide). Astig (Nemesis of most big thin mirrors) requires an in-plane V-knife, but can be bent out in use with some little springs from the back. Only "proper" test is AC pointing up off a BIG, certified plano. Cary at OWL and Normand Fullum (in Montreal) do testing. A 32 incher in-hand is best kept home and star-tested in the scope...just no getting around it in that size. [I'm retired from optical testing of large optics for Uncle Sam...interferometry pointed up off big flats. The testing is a big part of the expense of making big mirrors!] I have a nice 36" and dread the thought of shipping it out for someone to test it. It was built at a reputable shop so my 1st light will actually be my 1st "test"! Tom Dey

29-inch Dob in a dome
36-inch upgrade soon
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