f/stop doesn't matter in star photography since the light is effectively
point source. It's just the aperture width.
tOM
You are confusing the rule for viewing through a telescope with
photography. Fast lenses do make difference in astrophotography by
reducing exposure times. Else there would be no reason to spend all
that money creating those CT F3 photo only telescopes. Focal reducers
to reduce the F stop of an F10 to F6.3 are standard equipment to not
only flatten the field but to reduce exposure time in
astrophotography.
--
Winsor Crosby
Long Beach, California
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