Shouldn't biases ISO match that of flats?

New to Siril here, many doubts… This is probably a FAQ, but I was unable to find it. Sorry if it is somewhere.

It is regarding the sentence in the Siril’s manual that states:

“Note that BIAS frames MUST be taken at the same ISO than light frames.”

I do not understand why this is. Please allow me to explain myself:

If I take only lights and darks (taken at the same ISO as lights of course), I do not need bias since it is already implicitly included in the darks. So, bias are used to calibrate the flats (this is pretty clear in Siril scripts!). Hence, bias should be taken at the same ISO as the flats, not the lights. And the flats should be taken at lowest ISOs because the nature of its correction (vignetting, sensor and lens dirt, etc).

Hence, in general I would take, say, ISO5000 lights and darks, and ISO100 flat and bias.

What am I missing?


You are right Luis, not missing anything.
I believe this part of the manual was written at a time where flats and lights were recommended to be taken at the same ISO too. Now we recommend just what you said. We are working on completely updating the documentation but it takes time, it will surely be updated for the 1.0 release.


In fact, we need to rewrite doc. But we do not care about bias iso.
Take the bias at the lower Iso. Same for flat. Just because you want minimize the noise.

I think bias and flat iso must match, since bias calibrate flats. Of course, better taken at the lower ISO.

No. We really don’t care. For example we also can use a synthetic master-bias. ISO of flat and biases does not matter.
Taking at low iso is just better to take less picture in the master. That’s all.