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a little math help

Started by Greg Shaffer, 11/14/2009 09:35AM
Posted 11/14/2009 09:35AM Opening Post
In the planning stages of expanding my observatory with the idea of eventually installing an 8" F/18 OTA. Just how does one figure out how long the tube is going to be on an 8" F/18 refractor?

Thanks
Greg

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Posted 11/14/2009 10:20AM | Edited 11/14/2009 10:22AM #1
In the planning stages of expanding my observatory with the idea of eventually installing an 8" F/18 OTA. Just how does one figure out how long the tube is going to be on an 8" F/18 refractor?

Thanks
Greg

A reasonable estimate is to assume that the OTA plus the Dew shield will equal the focal length. This would be 8" x 18 = 144inches = 12 feet. This should be sufficient for observatory planning.

I am curious if you were aware of the length of the tube when you decided to go with an 8 inch F/18? By the time you add the mount height required for such a tube, it makes for a large observatory.

Jon