What Striker said, as well as:
Temperature: Make sure the light frames (e.g. your galaxy images) are taken at the same temperature as your dark frames (your image with the scope cap on).
Flats: Ideally, if you were to take an image of a perfectly smooth, white wall, you should get a nice, white, evenly exposed rectangle on your computer screen. But you won’t! You’ll probably see:
- Vignetting; where the flat field is brighter in the centre and tends to fall off in brightness towards the edges.
- Gradients; where there may be a small but gradual non-even illumination across the frame.
- Blotches; which are dust specks on the optical surfaces near the ccd chip that get magnified into blurry doughnuts.
- Any other imperfections in the optical train, such as finger prints, hairs, etc.
Using the software, you can more or less eliminate all these "problems" and end up with a nice looking final image.
Cheers
Dennis