Why do darks need to be same exposure?

[Heath (Heater)]

Hi all,

I know that you have to, but if anyone else is like me I'm not happy doing something unless I know why.

I understand that darks need to be taken in the same quantity and exposure times as each of the subs you take and using the same ISO settings, but I don't understand why this is? I would have thought that a dark is a dark?

Apologise for my ignorance in advance

[bartman (Bart)]

I believe - and I'm new to this too- that the dark shot accounts for any bad pixels in the camera's ccd/cmos ( defects)and subtracts them from the shots you have taken. Temperature has a lot to do with it as well. If you have been taking 60 sec shots then you should be taking - subtracting- the erroneous pixles of similar shots at the same exposure to cancel out those naughty pixels.
The WHY:
The Dark contains that info to eradicate - subtract ( to some degree) those bad pix at the same time/exposure etc you have made your shots.
At a different time/ exposure/ situation you're camera might not have the same bad pixl's. I have tested this by taking a dark at 60 secs and 600 secs. Totally different.600 gave me a few little red crosses.

I'm sure somebody else will give a more technical explanation, but this is what i have learned .
Bartman

[irwjager (Ivo)]

Hi Heath,

The thing with darks is that you want to get *exactly* the same amount of of noise and warm/hot pixel level readouts as for your light frames; that is, you want to replicate the exact same circumstances that you're taking the light frames in.

Then afterwards, by comparing the two, you can perfectly correct the light frames as you know what part of the signal was 'false' (e.g. it was not part of the actual image, but part of the noise) and which part of the signal was 'real'.

The longer your exposure, the more noise you gather, so the 'lighter' your dark frame.

The higher your ISO the quicker you gather noise, so the 'lighter' your dark frame.

The more frames you take, the smaller the random noise fluctuations, but the more consistent the read out noise and thermal noise becomes.

Strictly, you don't really need to take the same amount of dark frames as your light frames, but you should take at least enough to generate a smooth noise profile. But, just as with light frames, the more the better (but light frames have priority obviously ).

EDIT: Bartman beat me to it

[bartman (Bart)]

Yes what Ivo said!
Thats what I meant to say
Bartman

[Heath (Heater)]

Right, that makes complete sense now. Thank you very much for explaining it clearly.

Cheers Heath

[Octane (Humayun)]

If you end up with a dark frame count which gives you an SNR of say, 4, or so, you'll be doing well. So, aim for somewhere in between 13 and 19 darks, create a master dark, and you will find your images will calibrate beautifully. Same goes for flat lights and flat darks.

H

[Heath (Heater)]

Thanks H,

The only thing I'm unsure of know is flat darks? How does this differ from the master dark?