Question about averaging Dark Frames

[[1ponders] (Paul)]

I've heard Robby and Eddie mention it and I'd like to know a bit more about it. I think I know how to use darks , maybe, but I'd like to know how to do the process of averaging them. I get the idea of taking 10 or so dark images at the same ISO and the same length of time. Than what??? Do you stack them as in PS or combine then as in Registax???

All info greatfully received.

[acropolite (Phil)]

I can't see why you would need any more than 1 dark frame for a given exposure. Surely you're subtracting noise, which would be relatively constant for a given exposure time. I know my 20D has an inbuilt dark frame subtraction when you turn on long time exposure noise reduction and it takes a dark frame of an equivalent time to the exposure time used. I can see it may be useful to build a library of dark frames for varying exposures and ISO's though, that way you could save taking a dark frame every time, but of course temperature would also need to be factored in.

[EddieT (Eddie)]

Paul,

You need to average combine them without any registration or alignment. If you can do that with Registax, great!

The reason for using more than one, is because there may be random variations from one dark to another like cosmic ray hits or migrating hot pixels.

If you can animate your ten darks somehow, you may be surprised at the variations that appear from dark-to-dark.

[[1ponders] (Paul)]

What process are you using Eddie? Something nice and simple I hope. Actually while your explaining that, can you explain a bit about registration. I hear it mentioned quite a bit but as yet I don't fully understand it

[EddieT (Eddie)]

I use Mira for this but any image processing application should allow you to average combine.

Registration is the processing of determining the alignment of one image to another. It's the step before alignment.

With darks you don't need to register or align them. Just combine them using the average method.

[Robby]

You can stack in Photoshop easily enough. I use 9 darks and average them.
Just put each dark on a seperate layer and adjust the opacity by 1/x for each layer.
Eg. Layer1=100%, Layer2=50%, Layer3=33% ... Layer=9 11%
Cheers

[trufflehunter (Wayne Robinson)]

Robby, which layer blending mode do you use in Photoshop for the process you just described? I would have thought the 'screen' mode would be the way to go, with no opacity adjustments. But that would result in a lighter end result if the individual layers were not 100% black (and they wouldn't be!)

[[1ponders] (Paul)]

That wouuld be the normal mode Wayne. I've just done a few and compared the PS method, ImageStacker and a couple of other progs and I've not seen alot of difference between them