Once you have photographed your target - these photos are also called light frames - you are not yet finished with collecting your images. Besides these light frames, you also need to produce dark frames, bias frames and flat frames as these are also essential for the stacking process. Dark Frames are used to remove the dark signal from the light frames. This step is not necessary if your equipment allows for dithering. With DSLR and CCD cameras, the CMOS or CCD chip generate dark signals depending on exposure time, temperature and ISO speed (DSLR only). The best way to create the dark frames is to shoot pictures in the dark (hence the name) by covering the lens, same as for bias frames. However, the dark frames must be created with exactly the same exposure time, temperature and ISO speed as that of the respective light frames. We usually take 20-30 darks. Since the temperature is important dark frames should be taken at the end or during the imaging session. Bias Frames are used to remove the CCD or CMOS chip readout signal from the light frames. Each CCD or CMOS chip generates a readout signal which is a signal created by the camera's electronics when reading the content of the chip. It's very easy to create bias frames: just take the shortest possible exposure (it may be 1/4000s or 1/8000s depending on the camera) in the dark by covering the lens. The bias frames must be created with the same ISO speed you used for the respective light frames. The temperature is not important. We usually take 40-60 bias frames at the end of each imaging session. Flat Frames are used to correct the vignetting and uneven field illumination created by dust or smudges in the optical train. To create good flat frames it is very important not to remove the camera from the telescope before taking them (including not changing the focus). There are a lot of different methods to take the flats. We use a simple handmade flatbox which we mount in front of the tube and let the camera decide which exposure time to use (Av mode). We set the ISO to 200, exposure time should be between 1 and 2 seconds. We usually take 40-60 flats at the end of each imaging session.
Our first choice of a stacking software is Pixinsight. The stacking batch process provided by Pixinsight delivers very good results. If required Pixinsight allows you to finely adjust every single step of the stacking, but usually the standard configuration is fine.
Pixinsight is also our preferred image processing software. To be honest Pixinsight is not easy to get familiar with and it requires a lot of time to familiaries yourself with the features of the program and then in finding your own way of processing your images. There is of course a recommended way of doing things and we advise to follow these recommendations. Even though there is quite some room for mistakes. Once mastered, however, you have a tremendous toolbox in your hands. We intend to upload our own manual for this program in due course. For more details please go to Pixinsight.