About debias and darks

Posted by magnusl on Jan 20, 2021; 5:37pm
URL: http://astroimagej.170.s1.nabble.com/About-debias-and-darks-tp1477.html

Hi!

I think I am a bit confused on what the option "deBias" means and does. I am trying to make sense of it but I think I need to at least reality-test my understanding. This is my thinking:

1.
When building a new dark, AIJ subtracts the master bias (assuming it is enabled) from the master dark made, that is, averaging all dark subs, then subtracting the bias from these.

In the subsequent calibration, bias is thus subtracted once (the bias subtraction). Question: why then not just subtract the dark, un-deBiased (that is, including the bias), and not bother with a bias at all?

2.
When using an existing master dark (in my case build by Aij), deBias means that the enabled bias is subtracted. If it was subtracted in the build process (following 1) it will thus be subtracted twice from the master dark... or? Anyway, leaving the resulting calibrated light with an extra pedestal of a bias value, but no other issue (like zeroeing out on pixels). Right or wrong?

3. When using an existing dark, built through the process 1, and NOT enabling deBias - would result in the same result as 1.... righ?

When I experiment, I find that:
a. building a new master dark leaves basically the same background level with or without deBias enabled.
b. using an existing master dark and enabling deBias gives the same result as a.
c. using an existing master dark without enabling deBias makes the flat correction fail - I get very visible remaining vignetting - thus influencing the flat correction process.... why?

I very much appreciate if anyone can help me untangle this or point to a relevant resource to clarify my thinking. Sorry if I am just confused....

Magnus