From: sc02492 (sc02492_at_antispamdomain.not)
Date: Wed Nov 26 2003 - 03:38:25 EST
Greg, ultimately you always end up with the need to adjust the red
curves to achieve a color balance that's pleasing, but the signal
needs to be there in the first place, and that's the problem that
I'm raising. I've found that with longer exposures, although
there's a bit more green and blue, the red "catches up" (and the
green and blue aren't overexposed in the areas of faint red signal,
at least not for the usual exposure times that we use). This is
especially true for the faint regions of emission nebulae. So once
you have the signal to work with, making tweaks in the red curve
becomes a whole lot easier, without boosting the noise too much. I
never adjust the relative contribution of each channel using the
channel mixing function for any of my shots- I just stick mainly to
curves.
The periphery of M42 is a case in point, but even for the central,
brighter portions, most of the time we can go a bit longer, without
fear of exposing the green and blue, in areas that we're interesting
in boosting later. Of course, for the brightest region like the
Trapezium, every channel is overexposed due to the dynamic range of
this object, but that's a different problem.
M42 isn't really representative of most objects due to its broad
dynamic range. But if you look at the Rosette, for instance,
there's no way that blue or green would be overexposed in an attempt
to capture enough red signal to work with. We just need to get
enough of those high ISO, longer exposure frames to combine so that
we have enough of the red signal to work with later.
Steve
--- In digital_astro_.at._yahoogroups.com, <gregharp_.at._s...> wrote:
> Steve,
>
> When you say "I didn't have to coax it out too much at all" are
you saying
> you still resorted to some color balancing or adjustment in the
mix of
> various channels from various different exposures to produce an
image with
> more red signal?
>
> I understand where you're going with longer exposures to capture
more red,
> but don't those exposures also capture more blue and green? So
you still
> have to adjust the balance in the end, right?
>
> I'm not saying this isn't the right approach. I think it probably
is.
>
> --Greg
>
> > From: sc02492 [mailto:sc02492_.at._y...]
> >
> > Hi Forrest, I agree with Greg and others- nice shot. Needs more
> > exposure time to capture the faint stuff though and to enhance
the
> > red signal. Once you've captured as many faint details as
possible,
> > it should be possible to reprocess without clipping too much of
the
> > shadows end.
> >
> > Greg, I think what you are noticing regarding the lack of red in
> > digital M42s is, in the end, all about exposure time. If you
expose
> > long enough, which means cool temperatures and a good filter to
> > avoid sky fog, you can get enough red signal to compose an M42
> > equivalent to that seen with film or CCD work. I think we are
> > seeing many nice shots that are more dominant in blue and green,
but
> > which stop short of acquiring enough red, to be balanced. If
you
> > look at my recent M42, there's plenty of red in there, and I
didn't
> > have to coax it out too much at all. But I used a filter, and
for
> > some of the exposures I went 8 minutes at ISO800....
> >
> > Steve
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