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Hi Telis,
in order to get sufficient detail you need a large aperture (Ap); in order to get the required magnification (mag) you need a large focal length (fl). The relation between fl and Ap is the F-factor (fl/Ap=F). The F-factor tells you something about the brightness of your image and therefore about your required exposure time. A scope with F3 is called "fast" as it allows for short exposures (and gives bright images) a scope F15 is called slow for the opostite effect. If your target is very bright and very small, like a planet, you want a long fl scope and don`t worry about F15 or F25, because a scope F5 with 7m fl would be way to big and expensive.
Angular resolution (amount fine detail contained in image) is a function of Ap - the rule of thumb is: 2xAp=limiting magnification: 800mag/2=400mm (so you are probably looking at a 16" Cassegrain).
If you consider, just for the sake of argument and without thinking about the real math behind it, that a webcam sized CMOS/CCD sensor, with its crop factor and what have you not, behaves visually almost like a 4mm eyepiece than you come to the following equation:
Mag=fl (telescope)/fl (eyepiece)
or 800mag x 4mm fl (cam) =3200mm fl (telescope)
[the real calculation is done by incorporating the angular diameter of the object, the fl of the scope and the size of the pixels in the cam but as I said I don`t go there now.]
So a scope that does your job would be something like a Celestron CPC 11 or Meades 12" equivalent - both are F10
The biggets problem you are getting with 800xmag is the atmosphere. Bear in mind that you magnify every turbulance in the coloumn of air between you and the ISS. This infact the reason why people stack hundreds of stills out of a few thousand.
Good Luck with your project and if you succeed please post it here!!!
Cheers
Max
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