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Subject: Fork clearance / diffraction focusing for CCD use
From: Gary Heath
Reply To: mapug@shore.net
Date: Sun May 14 11:36:35 1995

In reply to a recent post about fork clearance (not enough) when using a ccd camera, I responded with a post on deffraction focusing. I noticed that there were some sugestions of using cardboard to make a "Qwik-Focus" knockoff. I would advise those of you that want to use this type of aid to make your unit out of plastic, or some other material than cardboard, or at least seal the cardboard very well. Cardboard dust is very abrasive, and unless you have a climate or device that negates any build up of dew on the corrector, you are headed in the same direction as using paper towels to clean yor optics. With respect to the clearance problem, University Optics, P.O. box 1205, Ann Abor, Mi. 48106, has about the best price on almost any adaptor you could want. As example, a S.C. thread to "T" adapter cost about $30.00, and is about 1/2" in total length, and maybe 1/8" in effective length. The same type of adapter for my flip mirror cost me more than twice that when purchased as an accessory for the F.M. U.O. will send you a free catalouge if requested, their prices are very reasonable. About the only thing that can be a bit of a problem with these type of adapters is the orientation of the assembled configuration. Unless you slightly "flaten" the last couple of threads, to allow drag that exceeds the weight of the ccd system, you end up with the camera oriented in a random way after tightening everything up. I don't use any draw tubes in my ccd/flip mirror/filter wheel/ off axis guider, and can orient everything as I want. On diffraction focusing, I had mentioned that simply putting a strip of tape at the objective end of the 'scope will work. This is true and will achieve a level of precision focusing that can't be achieved by trying to use the pixel level readout due to the variations in same. Even if atmosperic conditions were absolutely stable, you would see variations in pixel intensity due to the fact that light from stars arives at random intervals and follows Poisson statistics. The variation is about the square of the root of the photons that hit the chip. This leads to inconsistant focusing when using the pixel level method. I'm not sure if the "Qwik-Focus" method will result in the precision required to achive the same level as diffraction focusing will. But it doesn't take long to try the tape method. Set your integration time to a point just before saturation of the star image, and use negative image to resolve the spike. Another reason I like this method is because you can use it to check the optical quality of your system, it is a sort of knife edge test in real time and if one spike on one side comes into focus at a different point than the other, you know the chip is not at 90 deg.'s to the optical axis, (draw tube connections can cause this) or if you use two strips of tape at 90 deg's to each other you can check for astigmatism by comparing their coresponding difraction spikes. For those of you that would rather not use tape, you can use dowels with little blocks attached to the ends for "grip" on the outer tube. This proceedure works even when the sky is not very steady, but still steady enough to do imaging. Once again, there is a complete article on this in the Winter edition of "CCD Astronomy". It works, is fast, and is cheap!

Gary Heath