|
I generally have not used the number of light frames to determine the number of darks and flats to use. For both I would normally consider 20 to be a bare minimum. Having a cooled cam I have the advantage of being able to control the sensor temperature where you can't with a DSLR, so I pick my temperature and shoot a dark library of the exposure times I expect to use, 100 shot sat each exposure time, then I generate a master dark from those. I do the same for flat frames, I use a flat panel so it is easy to control the light input, so I shoot 100 flats to make a master flat, and dark-flats to go with them.
Per the information above, darks characterise your sensors dark current (The electrons that each pixel well will accumulate in a given exposure time) including hot pixels (Pixels which accumulate electrons faster than the rest of the sensor) so that the repeatable unwanted signal can be subtracted from your light frames. Shooting at least 20 averages out the readout noise, which is more or less random.
Flats characterise the whole system for vignetting or dust motes, or cold pixels, which react less to incoming light than the rest of the sensor. 20 at least means that the readout noise that is random in nature is averaged out of the master flat. If you use flats of more than a second or so you want to shoot dark flats (Which are just dark frames at the same exposure time as your flats and used to create the master flat) to eliminate dark current and hot pixels from the flats.
|