about darks, flats and biases

let us define what are the use of darks, flats and biases.

master dark removes both the thermal signal and the bias of the image. Attention, it does not remove the noise. Indeed, none of the master files remove noise, because the latter, with its random nature, will instead be added to the image.

the master flat flattens the image by removing the vignetting and the possible dusts present on the sensor.

The utility of the bias is more difficult to grasp. However, it is there to remove, as its name indicates, a level of offset that has been added by the manufacturer to avoid having negative values when processing the image by the camera and therefore to avoid losing information. DSLR manufacturers, most of the time, define a value that is a power of 2: 512, 1024, 2048 … while for astronomical cameras, it can be any value.

Taking a bias image must be done at the fastest possible speed, to avoid adding signal and thermal noise that would interfere with the processing. However, a bias will contain, in addition to its offset level, a certain level of noise. This is why we advise you to take as many biases as possible in order to minimize the noise in the final master. Remember that when you subtract two images, their respective noise is added. Therefore, using 20 biases introduces more noise into the final image than using 200 biases.

Synthetic bias 

In the latest versions available, Siril offers the possibility to directly remove a level , without using a noisy image. The advantage is huge and combines simplicity with efficiency: the bias level is removed from the flats and no noise is added to the final image. Moreover, one saves time and disk space (thus processing time) by avoiding having to shoot a hundred of bias images.

-> in SiriL: unter "use BIAS" "=10*$OFFSET" eintragen (Faktor 10 für ZWO 2600MC)

Quelle: https://siril.org/2021/12/enough-with-dark-flats/

Determine the offset level with an imaging camera

Quelle: https://siril.org/tutorials/synthetic-biases/