From: sc02492 (sc02492_at_antispamdomain.not)
Date: Wed Nov 26 2003 - 03:09:04 EST
Greg, nice shot of a difficult area. Here's a suggestion that you
may already know about, but I'll mention it for others as well. The
problem with these kind of shots is star bloating when you try to
enhance faint areas using curves. It's sometimes possible to
selectively enhance the nebula, without affecting the stars, by
using a simple Photoshop technique:
1. Use the magic wand to select most stars.
2. Use the lasso tool on "minus" mode to remove any selected areas
that inadvertently involve large sections of the objects of interest
(the nebula in this case).
3. Use "Select", Expand (about 6 to 8 pixels), and then Feather
(about 4-6 pixels). This will make a smooth transition between the
stars and the background.
4. Here's the trick- go to select and choose "inverse". Now you
have isolated only the nebula/sky areas, excluding the stars. You
can make any adjustments you need (curves, selective color, filters
like unsharp mask, etc.) without affecting the stars too much.
Either way, I think you will benefit from a filter as well, as I
think Raoul mentioned. Finally, could you let us know how much you
like the new 16 bit Photoshop?
Steve
--- In digital_astro_.at._yahoogroups.com, <gregharp_.at._s...> wrote:
> I'm processing some shots of the Veil Nebula from last night, but
I'm having
> some trouble. Basically, while I can bring the levels up to the
point where
> I can see the nebulosity, I can't do so without bringing up so
many stars
> that it's like viewing it through a snow storm. I'm using
ImagesPlus but I
> also have Photoshop CS available, and I'm open to any other tools
that can
> help as well.
>
> Here's the relevant data:
>
> Canon 10D DSLR with 70-200mm f/2.8L lens set for 200mm and f/2.8,
ISO 1600,
> 30 individual exposures of 60 seconds, no filter (dark skies!
woohoo!),
> ambient temperature 32F (0C). The images were converted from raw
to 16-bit
> TIFF and calibrated with a dark frame averaged from twelve
originals shot
> shortly after the light frames were taken, a flat frame averaged
from five
> originals shot at twilight metered to produce a medium tone (as an
> experiment to compare results to shooting a gray card), and a bias
frame
> averaged from five shots at ISO 100 and 1/4000. They were then
translated
> and rotated (the latter necessary because I did a poor job of
aligning the
> mount) in ImagesPlus.
>
> I've tried stacking these multiple different ways, but I get
pretty much the
> same results at different levels. I've done min/max excluded
average on all
> 30 frames, I've done adaptive addition of all 30, and I've done
simple
> addition on six sets of five frames (producing, effectively, six
five minute
> exposures) and then min/max excluded averaged those six images.
The only
> processing I've done on the resulting images so far is digital
development,
> and only the values have differed based upon how I combined the
images. So
> far I've just played with the break-point in digital development,
and the
> values for the three aforementioned combinations were about 500,
1500, and
> 3500 for each, basically representing the differences in the
signal levels
> I've managed to produce.
>
> Anyway, I can definitely see the nebulosity but there are just too
many
> stars to see it clearly. I've seen other shots of the Veil that
didn't have
> so many stars in them, and had a much clearer rendition of the
nebulosity.
> Is this just fancy processing, like masking the images so that the
stars
> don't show up so prominently? If there's more to it than that,
what am I
> missing?
>
> Here's a link to a JPEG version of what I have at the moment.
This is about
> 1/4 of the original frame, or a 1536x1024 image if you view it
at "original"
> resolution.
>
> http://www.pbase.com/image/23629356
>
> Any suggestions?
>
> I'm not sure where I could place a full resolution TIFF of the
unprocessed
> version if someone wanted to give it a whack. It's a bit over a
9MB file
> uncompressed, and still about 7MB zipped.
>
> --Greg
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