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RE: RE: RE: Off-axis Parabolic Mirrors

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Posted by Robert May on July 19, 2000 22:56:19 UTC

If you want high contrast for looking at the planets, etc. at high power, get a long focus scope as it will do several things at once.
First, you get a smaller secondary with the longer focal ratio as the distance from the center to the focuser is smaller relative to the focal length.
Second, you want to use higher powers (300+) to do a decent job of seeing the planets and a 12mm EP is a lot easier to look into than a 4mm EP for the same diameter mirror. 3600mm FL vs 1200mm FL is what that is all about. If you`re into length problems, the longer FL will also reduce the amount of coma that you will see in the image and that will mean it`s a lot easier to fold up the scope as you can get the secondary mirror off of the centerline and get it back towards a usefull position.
For a general purpose telescope tho, you do better to make the focuser as close to the light path as possible and reduce the size of the secondary so that it reduces the problems from the obstruction.

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