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Then there is a Schupmann medial refractor which is made from two pieces of the inexpensive BK-7 crown glass, and is perfectly color corrected, much better then a APO. It also has a version with all spherical surfaces. Has very close to a perfect MTF curve for ultra high contrast. It can be diffraction limited for all colors over 1/2 degree field of view and of course is unobstructed.

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On my short-list, too. Can be made from refractive quartz if available (as it is now ).

Best,
Mark

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