In a message dated Tue, 13 Aug 2002 12:30:37 PM Eastern Standard Time,
wincros@xxxxxxxxxxxxx writes:
> 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.
This is true for extended objects - galaxies, nebulae, etc... But for stars the
density of the image on the negative will be a function strictly of the
physical aperture (not f/ratio) of the lens, because magnification isn't a
factor.
So a 5 min exposure of a 5th mag star taken with a 100/2 (aperture=50mm) will
have the same density (for the star) as a 5 min exposure with a 200/4
(aperture=50mm). The sky background will be darker with the image taken by the
200/4 though.
Paul Schings
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