From: sc02492 (sc02492_at_antispamdomain.not)
Date: Wed Nov 26 2003 - 16:21:32 EST
Greg, good points- sounds reasonable to me. It will be interesting
to see a well done comparison using real data obtained through the
scope. Keep us posted!
Steve
--- In digital_astro_.at._yahoogroups.com, "Greg Harp" <gregharp_.at._s...>
wrote:
> Steve,
>
> In theory, since it appears that the 10D sensor (at least) is
nearly linear,
> if one had 75 perfectly aligned 60-second exposures and these were
combined
> into 15 sets of 5 by addition, the signal levels should be about
the same as
> 15 300-second exposures. However, I'm not so sure that the noise
levels
> would be. Thus far I haven't seen anything indicating that noise
over time
> is linear with the Canon CMOS sensor. But maybe I've missed it.
>
> However, in reality, if one has 75 60-second exposures, odds are
that they
> need to be aligned before being combined. And before they are
aligned they
> need to be calibrated, removing most of the effects of noise,
vignetting,
> etc., while they are still properly aligned with the calibration
data rather
> than each other. That's going to produce slightly different
results for
> noise, but in theory the resulting signal data should be about the
same.
> According to at least one person who posts here (Roger Clark, if I
recall
> correctly) 60 seconds is actually a noise "sweet spot" for the 10D.
>
> So what I hope to do is take a large number (say 75) of 60-second
exposures,
> combine the using addition into (effectively) a series of 300-second
> exposures, and then average those 300-second exposures. One nice
thing is
> that if, later in the process, I find that I would have preferred
to have
> 240-second exposures, or 600-second exposures, I can make them.
>
> I agree with you that, in my limited experience at least, I have
seen that
> longer exposures averaged together produces much smoother results
than
> shorter exposures simply added or adaptively added. I'm just
trying to
> build up those longer exposures to average by using shorter
exposures that
> are more within the limits of my mount's tracking capability and
more
> tolerant of outside interference (wind, aircraft passing through
the FOV,
> bumping into the mount, etc.).
>
> I could be totally wrong here but I think that this is sound
reasoning...
>
> --Greg
>
> > From: sc02492 [mailto:sc02492_.at._y...]
> >
> >
> > Yes, you would think that it should work, but it doesn't seem to
be
> > equivalent in my hands. I've wondered whether it has something
to do
> > with how the frames are processed when combined (i.e., pure
addition,
> > in which case all of the photons would be represented in the
final
> > product but noise will be amplified, versus averaging, in which
case
> > you may not capture every photon present in each 1 minute frame,
but
> > noise will be cancelled out more effectively).
> >
> > It will be interesting to see your comparison.
Perhaps "addition"
> > might be the best way to combine very short exposures (low
signal,
> > but also low noise), whereas an averaging algorithm might work
best
> > for longer exposures (higher signal, but higher noise that
benefits
> > from stacking).
> >
> > Steve
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