- BIAS (OFFSET)
A BIAS frame is an image taken with the shutter closed and the shortest possible exposure time. Basically it corresponds to an exposure of 1/4000s with modern DSLRs. The BIAS shows the electronic noise and readout signal of the camera and the more BIAS exposures are used for the calculation of the master BIAS, the less noise will be introduced into the corrected images. Note that BIAS frames MUST be taken at the same ISO than light frames. Also, you can stack bias frames with the median algorithm (or Winsorized by checking the rejection levels at the end of the process (< 0.500)), but be sure to use No Normalisation !