Re: [APML] Ash Lurie Schmidt alternative

[Aplanatic@aol.com]

Kent,

<<<It's amazing they are getting a ten degree field with a 500mm fl and f2.5.
I can get a 10 degree with my 8inch f1.5 Schmidt Camera and Vehrenberg film 
holder, with some vignetting in a 54mm diameter image.>>>

Vignetting in a Schmidt camera (and in the Lurie) is due to an inadequate primary mirror size.  For the Schmidt the primary diameter needs to be A+2D for no vignetting, where A is the aperture and D is the diameter of the film.  In other words, for an 8" Schmidt and a 50 mm film circle, the primary should be 12" in diameter for full edge illumination.  To save size, weight and cost, Celestron made the primary 10" in diameter for the 8" Schmidt.

Since the corrector in the Lurie design is closer to the primary, the primary need not be quite so large as the Schmidt for the same unvignetted field.  The ASH Lurie camera has about 15% vignetting at the edge of the 90 mm diameter field.  Very nice.  The Lurie camera has an advantage in this respect.  The corrector lens pair is not too hard to make, at least up to 8" apertures.  Larger than that gets to be a problem.

I wonder what the darned thing costs?

Dave Rowe

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