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RE: Aid On Telescope Design

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Posted by Jarvis Krumbein, jkrumbein@juno.com on July 31, 2000 22:18:11 UTC

The system you describe would probably not be practical with a negative lense as you described it. The lens would block an appreciable portion of the light cone and have the same effect as a larger secondary. Draw a rough scale picture to see what I mean. If you put the negative lense very close to the flat and let the light go through it both coming from the primary and after being reflected by the secondary you will have a "Mangnin" type secondary and these have been used with success. These are usually designed with the back surface of the negative lense aluminized to form the secondary. Another alternative is to use a positive lense as a transfer lense to obtain access to the primary image. This has also been done successfully and a number of designs have been published although most use a curved secondary, either spherical or a more complex curve to correct some of the more difficult aberations.

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