Calibration frames and bias subtraction

Software Siril
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Hello everyone,

I am a beginner in astrophotography and I hope my question is not too basic — please bear with me!

I would like to understand how Siril handles the calibration frames internally when all three checkboxes are enabled in the Calibration panel (“Use Master Bias”, “Use Master Dark”, “Use Master Flat”).

My specific question is: does Siril expect the Master Dark and Master Flat to already be bias-subtracted before being passed to the calibration step? Or does Siril subtract the bias from them internally, meaning I should provide “raw” (non-bias-subtracted) masters?

In other words, when all three masters are provided, does Siril apply something like:

Calibrated Light = (Raw Light − Master Dark − Master Bias) / (Master Flat − Master Bias)

or does it expect the Master Dark and Master Flat to have already had the bias removed?

I have carefully read the Calibration page in the official documentation, but I could not find a clear and general answer to this question. There is a note related to the use of “auto-evaluation” for darks, but my question applies to both darks and flats.

Thank you very much in advance for your patience and help!

Best regards
Claudio

2

Hello,

When you enter a dark or bias master, it will be subtracted. When you enter a flat master, it will be divided. This applies regardless of the state of the images loaded into memory.

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Let’s say your light true signal = 100, dark signal = 5 and you have added a bias value of 10 to each. The pre-calibrated light will = start at 100+5+10 = 115, and the master dark will 5+10 = 15. Subtraction restores the correct value of 100. If you then subtract the bias again, you wind up at the incorrect value of 90.

If you are using the manual GUI pre-processing, you would check only “Use master dark” while pre-processing the lights. You would check only “Use master bias” when pre-processing the flats.

I don’t use the scripts, but I assume you supply both a master dark and a master bias. The script then correctly applies the dark only to the lights and the bias only to the flats.

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