Here we go again - dark scaling with bias vs darks

[BrendanC]

Hi all,

Right, so, I've been using darks, flats and flat darks successfully for quite a while with my EOS1000D camera. I use dark flats instead of bias. I have a darks library for 60s and 120s exposures, across all temperatures, at ISO800, which is what I always shoot at. All good.

However, now that I'm guiding, I can start pushing beyond 120s, to 180s, 240s, who knows, even further - I need to test my equipment and see what the limits are. So, as my exposure times start to increase, I'll need to build new dark libraries to match those exposure times. Library building already takes ages and is tedious, and it's going to take even longer and be even more tedious for longer exposures. For example, it'll take three times as long to build a library for 180s as it took for 60s. For 240s, four times as long. And so on.

So, this raises a thorny question: can I, as a workaround to having to create all these dark libraries, use bias frames to scale my darks instead? For example from, say, 120s to 180s? At least, in the short term, until/unless I can be arsed doing it properly?

I'm thoroughly expecting a massive discussion now, in which everyone has a different take on it - because that happens every time calibration frames are mentioned - but if you definitely 100% absolutely know the answer, rather than think you might possibly perhaps maybe have an inkling, then I'm all ears!

Thanks, Brendan

Edited January 6, 2021 by BrendanC

[vlaiv]

I think it is rather simple - use it if you can.

How to tell if you can? That is also easy to check.

Create few darks with say 30s exposures and few darks with 90s exposures. Take set of bias as well.

- stack each set

- take 30s master, subtract bias and multiply with 3

- take 90s master, subtract bias

Compare resulting two - if they are the same - you can comfortably scale your darks.

What does it mean to be the same? Well - subtract the two and examine resulting sub. It should have mean value of 0 and be pure noise without any visible patterns in the image.

Do Fourier transform on it - that also should be pure noise without any distinctive features.

Btw - it is better to scale long darks down then it is to scale short darks "up" - "noise wise".

[BrendanC]

Thank you once again Vlaiv! I think it's fair to say that you definitely 100% absolutely know the answer! 

Edited January 6, 2021 by BrendanC