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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
>>
All true, but the problem is the sky. The sky acts as an extended object,
since it is not completely black, but it "shines" a little. The amount of
light coming from the background can strongly depend from the light
pollution of the site. Anyway with a given f stop there is an exposure
limit and when you exceed this limit the backgroung will appear on the
photo no longer black, and the fainter objects will be hidden in the
background. For example, under a good dark sky I'm able to pose up to 10
minutes with an Ektachrome 200 pushed to 800 and a F2.8. Using an F4 I
should use a 20 minutes exposure. Why using the longest exposure possible?
Only because in this case you record the maximum number of objects possible
(stars, nebulae, etc).
Ralf Loi
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