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Posted by Robert May on November 4, 2002 22:05:14 UTC

As you go towards larger diameter refractors, you also run into a problem with getting glass as well as needing longer focal ratios to keep the color problems in control. Remember that as the aperture increases, the Airy Disc size decreases and as a result, the color error grows more obvious.
I'd stick with the 8" size (this from a guy that has a set of glass for a 12" on hand!) as a good max size for the amateur as this size will keep good color in the image.
Before you actually start grinding, use the exact glass indicies and do a good raytracing to get the exact radii that will minimize the color error. The software that comes with the Rutten and Van VenRooij book "Telescope Optics" is a nice cheap raytracer that will work well as well as some of the freeware stuff that is out there. Oslo LT is available but it is a rather complex piece of software to use.

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