Dark Calibration Frame Scaling

The reasons we need to take and apply dark calibration frames to our image data are (1) to reduce the amount thermal noise in our images and (2) when we take long exposures, we increase the amount of dark current noise.  These are the primary reasons that the dark calibration frames and the light frames need to match in exposure time and temperature for dark frame calibration to be effective.  Temperature matching is particularly important because the rate at which you get dark current noise varies with temperature.  For those of us who use temperature cooled dedicated astro-imaging cameras, the process to take dark calibration frames is very easy.

However, if you're using a DSLR or other type camera that does do not have thermoelectric cooling, taking effective dark calibration frames will be a problem.  Temperatures drop and change during the night and depending on the weather or season and these temperature changes can be considerable.  You can take dark calibration frames throughout the night to minimize the problem and the temperature differences, but you will still have problems getting good consistent dark frames to calibrate your data.

This is where dark frame scaling is used.  Basically, you use a dark frame whose temperature and exposure time are as close to correct as possible and scale it to the temperature and exposure times you need.  Image processing software such as PixInsight have algorithms to do this.  It's also recommended to take and use bias frames since these particular types of calibration frames are constant and do not scale with exposure time.