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RE: Kind Of Newtonian-Cassegrain Telescope

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Posted by Jarvis Krumbein, on July 3, 2000 19:19:58 UTC

What you propose is not very practical for the following reasons.
1) Use of a flat secondary, since it has no optical power, will have to be larger than 50% of the primaries diameter.
2) The secondary should contribute to the correction of spherical abberation unless a correcting lense of some sort is inserted into the optical path. A spherical primary with no other correction, unless of very long focal length will suffer severe spherical abberation.

If you want a spherical primary and a Cassegrain configuration, the Pressman-Camichel configuration is your best choice but this requires an oblate spheroid secondary which will probably be hard to figure and will have a very small useable field of view before coma becomes objectionable. A better choice would be a Dall-Kirkham which uses an ellipsoidal primary and a spherical secondary. Because of the steep curve of the secondary, it is easy to make without difficult testing.

If you need more info, contact me.

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