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Re: Tube Size

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Posted by Tom on March 24, 1999 10:02:12 UTC

Yep, leave about an inch all the way around your mirror. You want a space where the rising column of warm(er) air off the mirror can roll out the tube. If you thing about the scope set at a 45 degree angle the warm air will rise up until it stikes the top of the tube and then roll along the upper surface until exiting the top. The 1 inch will give it room to roll along. You usually don't go much more than an inch 'cause it adds to the overall distance between the diagonal and the eyepiece. This distance if too great, forces you to use a larger diagonal to get the whole light cone to the eyepiece. Too large of a diagonal produces extra diffraction, and lowers contrast in the image noticeable mostly in planet viewing (for visual use).

The 1 inch rule is a general guideline that isn't steadfast by any means. Some people keep the tube diameter as close to the mirror as possible to be able to use a smaller diagonal. Optically your tube needs to be a little larger than your mirror's diameter so the tube doesn't block off axis light rays heading to the mirror (wide field viewing) But even in this case you would not notice the light fall of at the edge of the field unless you are doing astrophotography.

Good luck

Tom

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