From: sc02492 (sc02492_at_antispamdomain.not)
Date: Wed Nov 26 2003 - 15:29:15 EST
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
--- In digital_astro_.at._yahoogroups.com, <gregharp_.at._s...> wrote:
> Steve,
>
> I'm still hoping to prove to myself that due to the linear response
of the
> sensor, taking (for example) 75 x 1 minute exposures and adding
them up
> (simple addition) in sets of five (after calibration and alignment)
will
> produce effectively the same signal information as 15 x 5 minutes.
The 15
> resulting images could then be processed in whatever way 15 x 5
minute
> exposure of that region would need to be processed (presumably one
of the
> averaging algorithms such as min/max excluded).
>
> Unfortunately high, thin clouds cut short my imaging session of
that part of
> the sky on Monday night so I only got 15 usable 60-second
exposures. I got
> 16 more with halos around the brighter stars, which is a real
problem since
> Alnitak producing a large halo pretty much blows away a good part
of IC 434
> and NGC 2024.
>
> Of course, I never delete anything so I'll keep those around for
whenever I
> can get more exposures of the area under dark skies or through a
filter.
> When I get some time I still need to try and process those 15 into
something
> I can share as a work in progress.
>
> --Greg
>
> > From: sc02492 [mailto:sc02492_.at._y...]
> [...]
> >
> > So the strategy is to take many frames at relatively short
exposure,
> > with subsequent dark frame subtraction (calibration), alignment,
and
> > stacking to increase the signal to noise ratio. Some argue about
how
> > short is short, but in general I find that it's better to stack
15 x
> > 5 minute, guided exposures compared to 75 x 1 minute, unguided
> > exposures (better as defined by capturing the faint stuff and
still
> > maintaining an acceptable background).
> [...]
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