> Readout noise can be cancelled by taking so called bias frames. That’s not what bias frames [1] are doing. Bias frames measure a systematic effect in the c

> Readout noise can be cancelled by taking so called bias frames. That’s not what bias frames [1] are doing. Bias frames measure a systematic effect in the conversion of stored charge to a digital number in the CCD device. You can then subtract it out. The calibration offered by bias frames does not fix readout noise, which is a random, not systematic, effect that is inherent in the same conversion. You can use the bias frames to compute the strength of the readout noise (i.e., the rms magnitude of its random part) but you can’t remove the random noise itself. See sec. 1.1 of the linked reference for more. [1] https://users.astro.ufl.edu/~lee/ast325/handouts/ccd.pdf https://users.astro.ufl.edu/~lee/ast325/handouts/ccd.pdf