Originally posted by Scootatheschool1990 What do you mean by dark frames?
A dark frame is exactly that. You take an image in complete darkness (say with the lens cap on the lens), with the exact same exposure time as the real shot and at the same ambient temperature. This dar frame will only contain noise. If you take several dark frames and average them, you get a "mean noise image" of your individual camera. You can substract this dark frame from the real images during post-processing, thus reducing the apparent noise in the image.
You can go one step further, by also taking "flat fields", images of a completely evenly illuminated surface or special light source (available through astronomy dealers). If you use the same lens at the same aperture setting as you use for the real image, the flat field will show the exact vignetting and uneven brightness produced by that very lens (and maybe the sensor). YOu can then also substract that flat field from your real image in post processing, generating a final image with reduced (or eliminated) vignetting.
Both techniques are used routinely by astro imagers and you'll find a lot of info on the usual websites (skyandtel.com, cloudynights etc.)
Ben