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Old 12-01-2010, 11:34 AM
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alistairsam is offline
Join Date: Nov 2009
Location: Box Hill North, Vic
Posts: 1,820
Talking Moving primary mirror on a Newt for DSLR astrophotography


I have an 8" F6 Newtonian with a 2" Crayford focusser.
I'm very keen on using a DSLR with the T adaptor at prime focus for astrophotography.
I read that the primary mirror has to be moved forward to bring it to focus, my question was if anyone knew how this can be calculated or approximated?
Will the effective F/L reduce from 1200mm if the mirror is moved forward, and thereby reduce overall magnification? Not sure if this is relevant as there is no eyepiece with prime focus.

I presume the same can be achieved by using focal reducers, but i'd rather avoid using additional glass (lenses) if i can.

I also wanted to know what the difference was in the eyepiece projection method, afocal method and prime focus method. I read that in eypiece projection, the greater the distance between the CCD plane and the eyepiece plane, the higher the magnification.
But would'nt that be fixed depending on the focal length of the mirror and eyepiece?

I'm working on a split fork ring mount for the OTA with autoguided tracking, as i'm very interested in long exposure deep space imagery.

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