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Someone who can test mirrors near chicago or milwa

Started by astrotoys, 09/19/2016 10:27AM
Posted 09/19/2016 10:27AM Opening Post
anyone know someone near either of these 2 cities who can accurately test/evaluate large telescope mirrors (20-25"). I have one I needed tested and would prefer not to ship it someplace.

thanks

Patrick
Posted 09/19/2016 07:00PM #1
No...but: I did testing all my career and can at least ID the best test configuration for biggish mirrors. If at all possible, the best (most meaningful) test is pointing upward in autocollimation off a downward-pointing precision calibrated optical flat. This is usually called a "test tower." It is of course wonderful for testing complete telescopes (OTAs) but also great for plain vanilla paraboloidal mirrors. For that, a perforated plano with the parab focus in the hole is ideal. That way the interferometer sees the parab twice, in all its magnificence. Nother good (necessary) practice is that the cavity be properly calibrated and backed out, so what you get in the report is your mirror only, without any artifact of the test itself. Ummm... and of course the mirror on its whiffle tree support and well-thermalized to ambient. Under those circumstances, a characterization accurate to 0.005 wave surface vis RMS is likely. I would be a bit skeptical of "Ronchi Only" fringe patterns. Those fringes can look decent because they are most often (even necessarily) highly desensitized. That because the nature of Ronchi is that it shears the wavefront to get fringes. So the fringes are mapping lateral wavefront slope, NOT the raw wavefront itself. You can immediately tell how desensitized it is because the image of the mirror looks sheared like two circles laterally displaced. that displacement relative to the circle diam is also the desensitization. If e.g. 10%, then the fringes you see will look 10X "better" than if regular laser interferometry had been done. This is probably "too much info" sorry for that. Oh yeah, I actually had a 24-inch test tower in my barn years ago! Tom

29-inch Dob in a dome
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Posted 09/19/2016 07:41PM #2
Ed Jones is an excellent resource who will do free testing of mirrors up to 20" and he's located in Ohio, if that helps:
https://opticaleds.com/optical-testing-and-mirror-refiguring-2/
Otherwise, I'd try contacting a local university to see if their physics department has an optics lab and might be able to do it.