From: sc02492 (sc02492_at_antispamdomain.not)
Date: Wed Nov 26 2003 - 14:12:11 EST
Hi Colin, thanks. You cannot take a 75 minute shot at ISO800 with
these types of cameras without introducing a prohibitive amount of
noise. If this were film, it would be no problem, and would in fact
be necessary in order to overcome the effects of reciprocity failure.
The good news is that with digital cameras, reciprocity failure is
really not an issue anymore. But dark current noise becomes
limiting... many of us take exposures in the 5-6 minute range at ISO
800-1600 by habit, but as the colder temperatures set in, we can go
longer (actually, I've found that 8-10 minutes at ISO800 at 30-35
degrees F can result in acceptable background noise, but beyond that
it can be a problem). The use of dark frames is a must- ImagesPlus
software makes this step quite easy.
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).
Good luck with your new set up!
Steve
--- In digital_astro_.at._yahoogroups.com, "Colin" <guru90a_.at._y...> wrote:
> Hi Steve,
>
> Congradulation on such a nice picture. I just got a 12" LX200 with
> 300D and I am about to shoot some pictures myself. I noticed that
you
> used 15 5-minute shots instead of one 75-minute shot. Is it
> because of the noise, alignment or processing issues? I guess I'll
> find that out once I start shooting pictures. Any advice will be
> appreciated. Thanks.
>
> Colin
>
>
> --- In digital_astro_.at._yahoogroups.com, "sc02492" <sc02492_.at._y...>
wrote:
> > Hi everyone,
> >
> > This is the third photo from my all-nighter session starting on
> > 11/22/03. It's a shot of the Horsehead region, taken with the
Tak
> > Sky90 and Canon 10D:
> >
> > http://www.starrywonders.com/horsehead.jpg
> >
> > Details:
> > http://www.starrywonders.com/horsehead.html
> >
> > I've included a description of the region and provided details
> about
> > the image processing. The use of the background compensation
> > feature in ImagesPlus was essential for removing mild
vignetting.
> > I'm pleased with the amount of faint nebulosity in the shot, but
it
> > needs more work with longer exposure of certain regions (like
> > IC434), and shorter exposures of others (like the region around
> > Alnitak). I like the colors rendered by the 10D- I think that
Doc
> > G. made this point in a previous post as well.
> >
> > Comments welcomed!
> >
> > Steve
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