master Dark, dark optimization... a couple of questions.

In the Siril preprocessing tutorial at the following statement appears in a WARNING section:

“… However, applying dark optimization makes things different by multiplying masterDark by a coefficient factor not equal to 1. In this case, you must subtract masterBias from each dark frame.”

I understand the initial part of the statement (dark frame had dark signal + bias signal), what I don’t follow is the “… applying dark optimization…”

  1. Where and how is dark optimization done?
  2. How is the coefficient factor arrived at? (calc. automatically, chosen from a range of suitable values, …?
  3. WHY is dark optimization done?

(I’m using a Fuji mirrorless (RAF raw files) and Siril 0.9.12 on a Mac (Catalina))


Dark optimization is done when darks have been shot at different temperature than the lights. It will compensate this difference. No need for you to compute something, Siril will do that. Just check the button.

Thanks for the prompt reply.

So dark optimization will allow me to include darks in my preprocessing that I shot a few months ago when temperatures were lower than last night.

Am I correct then in understanding…

Siril determines the appropriate coefficients to apply to account for the temperature changes and applies them to each of the dark frames. To utilize these optimized dark frames I must subtract my master bias (offset) from each of them before stacking the resulting darks to get an Optimized Master Dark.

I’ve a follow up question: Should dark optimization be avoided, if the darks a shoot at the same temperature? Or is it Ok to apply it always?

There is at least this case in which it’s not useful to enable it, because it requires subtracting the master bias from the darks : with cameras that give the same level of signal for the bias and the dark. It happens with some high quality cooled cameras for which the dark current is very low, and this results in an almost null dark frame when subtracted by the bias, and it’s doesn’t get good results.
In other cases I think it’s quite safe to use it.

Thanks for your fast reply! Its very helpful to shine some light on, when to use what algorithm for processing.

So - if I understand this correctly - for cameras with very low dark current, I would preprocess my lights with:

  • subtract un-preprocessed master-dark (which is in some sens a dark+bias frame)
  • divide by prepocessed master-flat (flat -bias)

In other cases the dark optimised workflow can be used:

  • subtract master-bias
  • subtract preprocessed master-dark (dark - bias) and dark optimisation on
  • divide by preprocesseed master-flat (flat -bias)
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My memory on this subject is quite unreliable, but I think this is the way to go. @lock042 can you confirm please?

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Yes, that looks good.

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