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mconnelley
super member
Reged: 03/14/06
Posts: 198
Loc: Fremont, CA
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Hello:
I had this idea while sitting in my office this morning. I've been intrigued by the Schupmann telescope, and have been thinking of ways to improve upon it. One potential weak point I see is the field lens or mirror. Using a field lens results in a longer scope, which already tends to be rather long. Using a field mirror means that you have a reflective coating in focus. Degradation of the coating would quickly affect the image contrast. The idea is to use a plano-convex lens as the field mirror/lens, with the convex side facing the objective lens and giving the flat side a reflective coating. Since the reflection is internal, the coating can be protected from the air. It is also easy to test and figure the flat surface with an interferometer. Since the flat surface is in a focal plane, its optical quality can be fairly poor and the scope would work just fine. A figuring error on the flat reflective surface would be seen as distortion in the eyepiece and wouldn't affect how sharp the image is. A wedge error in making this lens would cause lateral color, which should be able to be tuned out during collimation. In practice, it may not be so bad to use a conventional field mirror with its coated surface in focus. We all use diagonals in our refractors and SCTs, and they are nearly in focus and work just fine. However, I think this idea of a mangin field lens/mirror would make the field element in a Schupmann more robust. Naturally, now I gotta go out and build one of these things to see if it'll work.
Cheers
Mike Connelley
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1084
Loc: Sin-sin-atti
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You also will be introducing two new air glass surfaces near the focus unless the lens is really thick. The coating on this would have to be super good to avoid ghost images.
Ed
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Ed Jones
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1458
Loc: Hockessin, De
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Mike,
Having built a Schupmann and now working on three others I haven't found the coating issue on the field mirror to be a problem. I have a little flip in cover that protects the mirror when not in use, keeping it clean. I've not seen any dust specks or the like in the image.
The active area of the field mirror is small, in my case less then 1" in diameter. My field is about 3" in diameter but mounted off center, with the rest of the mirror being covered all the time. When the coating does go bad I can simply rotate the mirror to expose a section of new surface. Having the field mirror enclosed in a sealed tube and protected when not in use, should give me many years of service and much longer life then a standard newtonian.
My 9.5 f/12and 6" f/10 will be folded via a flat after the objective. The 9.5 will be about 60" and the 6" about 36" long.
I hope you build one of these wonderful 'scope. The images can not be be beat, no color, super high contrast, diffraction limited to around 1/2 of degree and the ablity to easy tune out color dispersion from the atmosphere makes them a top performer
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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