> Another reason this is often done is to improve signal-to-noise ratio in
the
> final image. A single twenty minute exposure is going to be degraded to
> some extent by twenty minutes of atmospheric turbulence, scattered light
> from cars passing in the distance, etc. By shortening the duration of
each
> exposure, you get less of this noise.
That's not correct. Reciprocity failure is really the only thing that is in
any way affected by using mutiple short-duration exposures. Even then, I'm
not sure that you gain that much by doing it.
You certainly won't reduce atmospheric turbulence unless you are talking
about planetary or lunar photography where the exposures are very short
(less than 1/60th sec to "stop" the atmosphere). As this mail is entitled
"Comet Picture" then I assume that the picture in question was of
multiple-minutes length.
As for passing cars or other light pollution - whether it is 1x20 or 4x5 the
result will be the same.
> The four stacked negatives still have
> the same signal intensity, since the total amount of "signal" from the
comet
> will be the same. However, each of the four negatives will have less
> "noise" than a single 20-minute exposure, and the noise in these four
> shorter exposures will tend to cancel out somewhat.
Actually, I'm not so sure that is right either w.r.t long exposures of
extended objects such as comets, nebulae, etc., but it's been 15 years since
I was doing astrophotography as a hobby. I have a feeling that 4 short
exposures won't "cancel" - you'll get 4x noise. If this technique worked
then astrophotographers wouldn't be using chilled-emulsions, etc. and taking
long exposures.
Well do I remember guiding long exposures. My longest was about 3 hours with
a Celestron 8 inch. You haven't lived until you've done that on a winters
night. The results were well worth the effort. No doubt these days it's all
computer guided.
doctor digi ( *still* waiting for someone - anyone - to answer or comment on
my first email from 4 days ago re Pentax DSLR, the Philips CCD and whether
people on this group are making a camera back.)
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