How about star-Ronchi testing? Cut out the bottom of the traditional 35mm film can, leaving just a slight flange. Trim a piece of Willmann-Bell plastic Ronchi grating to fit, and carefully glue it to the flange. Find a bright star, remove the eyepiece, install the film can with the grating, and evaluate the Ronchi pattern. This will test the entire system. You will have the same difficulties with turbulance as the star test. But you will be looking for a very simple pattern, straight lines. I know this works, but I am not completely confident about the sensitivity. Most scopes look pretty good when I test them this way. I am concerned that I may be vignetting the full aperture of the mirror, and hence not detecting TDE, for example. And the grating frequency is also an issue. I usually use 85lines/inch and 133 lpi. I acutally installed a piece of grating in an old broken eyepeice shell, sans glass. The grating was located at the field stop. Very convenient to use. Any thoughts from others who have tried this test? BTW, you can easily photograph the patterns created by this test.
PS. Yes, performing the star test is "actually pretty simple." It's just the part where I interpret the test results that I have difficulty with. *>}.
-------------------------------------- Date: 5/1/95 8:59 AM To: Richard Combs From: atm@best.com I disagree for a couple of reasons: 1. Star testing is actually pretty simple, and Dick Suiter's book has made it even simpler. 2. Star testing is the only way for the average amateur to test the ENTIRE TELESCOPE. Ronchi testing at the ROC won't cut it. For newtonians, it doesn't tell you a thing about your diagonal. 3. Sure, autocollimation is good, but even I don't have a good flat, and I am obsessed by telescope making. :) Also for large mirrors this becomes more expensive. That aside, I think the best method is to use ALL methods, and try to figure out what each is telling you about your mirror.
Mark
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Mark T. VandeWettering Telescope Information (and more)
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