Stacking Images with Photoshop  
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This was "purloined" this from another site (you might want to take a peek 
at astro-imaging on the Yahoo forums):

Stacking in Photoshop
This is how to do it:

1) First subtract the dark frame from each picture (use Difference
in the Layer Window), then flatten,

2) With these preprocessed frames, copy and paste all of them into
separate layers of one picture,

3) Align the layers (use opacity 50% for the layer to be aligned,
make others invisible, except the "reference" layer, return
opacity back to 100% and blink to check the alignment, cursor
buttons are great for fine tuning),

4a) Averaging: set the opacity of the first layer to 100%, the next
to 50% (1/2), then to 33% (1/3), 25% (1/4), 20% (1/5) etc., *OR*
4b) Adding: invert each layer separately (Image->Adjust->Invert),
then add them by selecting "Multiply" in the layer menu (except for
the bottom layer)

Experiment with these two techniques, they seem to have different
pros and cons and work with different kind of images.

5) Save the image here as a Photoshop file (huge file!), so that 
you can come back if you find everything was not right in the 
next step,

6) Flatten the image and save it as a JPG file (so that you don't
accidentally press ctrl-S by instinct and lose the layered PSD
file - been there myself...),

7) Now is the time to play around with levels and other adjustments
to make it look pretty, and save the final version. Or many
versions, as I've found that some setting show faint stars the
best while others show DSOs better.