Hi, James.
Officially, the best screen for Astrophotography is the 1-8. However, any
all-matte screen would be OK -- the 2-4 is bright and sharp, so it has to
be great.
The 1-7 screen is intended for long telephoto lenses, like the 1000/11, but
these have fully *refractive* optics. Your telescope is of the *reflector*
type, and they have a central obstruction which *may* affect the operation
of the miscroprism. Maybe it depends on the relative size of the
obstruction -- Moose reports no problems with the 1-7 and his Celestron
'scope.
I don't know if your Orion camera adapter *replaces* the eyepiece of the
telescope, or if it has room to put the eyepiece inside -- the first setup
is called 'prime focus', and the 'scope behaves like a long telephoto lens
(maybe about 1000mm); the second option is the 'eyepiece projection'
method, and allows much higher magnifications (about f=13000mm at f/90 or
so!!!)
If your telescope's mount has no tracking device, a 10 second exposure will
show clear trails on the film, even with the prime-focus system. With
eyepiece projection, tracking is *a must* -- Mars is a very bright object,
it doesn't need long exposure times: around 1 sec. (with ASA 100 and f/90).
Exposure with prime-focus should be quite shorter, but due to the small
size on the film, it must a few stops longer than the theoretical value of
f/8 @ 1/125 for ASA 100 (like the sunny-16 rule, but -8 instead).
Hope this helps,
...
Carlos J. Santisteban
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