Bias

ConceptBias⚓

offset

A BIAS image is an image taken in total darkness (cap closed) and at the fastest speed.

In practice, on modern SLR cameras, this corresponds to a speed of 1/4000s.

The BIAS will contain the electronic noise as well as the readout signal from the camera.

Advice

In order to introduce as little noise as possible into the pre-processed image, it is recommended to take a large number of BIAS images (>20), especially as they can be taken quickly and easily.

Warning

Finally, note that the BIAS must be taken at the same ISO as the images to be pre-processed.

Warning

Also make sure you stack your BIAS with a median or mean algorithm with deviant pixel rejection such as the Winsorized Sigma Clipping proposed in Siril.

Make sure that the rejection levels do not exceed 0.5%, and especially choose No Normalization.

Example

Here is a BIAS master calculated from 10 images with Winsorized Sigma Clipping stacking.