Note the little dark spot -- turns out that if you use autolevels from Photoshop 5, that shows up. In the next image I took the stacked image and did my own levels adjustments from the get-go and this little artifact didn't show up. :-)

I redid the processing on this one because I decided I hated having a really dark background -- it made the image look overprocessed, forced, fake, blotchy and just plain yucky. I'm much happier with this one and I'll refrain in future, from trying to get a background to be dark at the expense of the rest of the image.

I also decided not to crop the image so you see the overlapping from the different positions M51 took on the screen. You also get to see the satellite that went through one frame.

BTW, I while I took several hours of 90s images, I tried very hard to be discriminating in my selection of "keepers" for stacking. It is very easy to accept a little smearing in a star in order to get the maximum number of frames possible for stacking, and that's cool because it reduces noise further, but nice tight stars are better, IMO. We're looking at about 30% of the night's haul of images in the final stack.

Now, let's talk about color! Here is 14 - 60s (14 minutes) colors stacked -- that's from FITS so there are 14 red, 14 green and 14 blues. They're dark subtracted, but I didn't do any further processing other than a histogram stretch so you can see the image at all. FITS are weird because they almost look totally black when you open them in Photoshop. You have to stretch to see that there is anything there at all.

I tried to process this, but the colors balance is off. I need to do some reading on recombining color images. They're a lot more work unless there is something I'm missing in the automation realm. Once you've stacked each color, they're not 100% the same so you have to line them up and then crop them to overlap properly. Then you can recombine them.

Lots left to learn here. But I'm having fun.