It is highly dependant on what your imaging system looks like.? You want the guide scope and camera combination ideally to have a 1:3? or worst case 1:5 ratio relative to your main imaging scope and camera image scale.??
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You can play with various scenarios using the astronomy.tools calculator here: remember, the closer you get to 1:3 the better but as low as 1:5 will also give acceptable results.?
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Chip Louie Chief Daydreamer Imagination Hardware?
Astrospheric - South Pasadena, CA?
Chip Louie