Howdy.
1ponders, I use IRIS to do the dark subtraction, I generally try to take at least 2 darks just after my imaging run (so is at similar temperature) as the camera isnt temp controlled theres always a slight variation, so the darks never 100% match the light frames, Iris fixes that by a dark optomization proceedure where it askes me to draw a small box on one of the light images in a part free of stars or nebulae, but encompassing some hot pixels, it then matches the dark for a close to perfect subtraction. In theory, a 5 minute dark could be used with 10 minute light images and so on, though for best results I try to keep same dark times as light exposure times. Ive actually used darks taken on different nights to reasonable effect. I believe the next update of Images Plus will have this feature also.
Info on the dark optomization here
http://www.astrosurf.com/buil/iris/t...3/doc13_us.htm
scroll down to Preprocessing where it will say
"Note: you can optimize the dark map during the preprocessing phase, or in a separate step (for this run Dark optimization command of Digital Photo menu). Here, for simplify, we consider similar conditions between the science images and the dark images."
Scott
