How many darks required???

[martin_h]

Gina

I've found with temperature controlled CCD darks work very well.

But for DSLR and non-cooled CCD I've had problems, I believe because they simply do not hold the same temperature between the dark and the light frame so the variations in the dark current from pixel to pixel are not correctly moddeled in the dark master frame. In which case there is an argument for just making a hot pixel map and forgetting dark frames.

Derek

Isnt this a case where you would use Bias frames to "scale" the darks?

[dph1nm]

Apart from hot/dead pixels and that sort of very obvious oddities - errors that dithering or software can get rid of - what is the actual gain of darks at all?

Actually this is an excellent question (and I don't know the answer). You might be better off using a bad/hot pixel map. It depends for one thing on whether there is any non-random structure in your dark (e.g. if you have amp glow, you really do need a dark). I certainly don't use them for my Canon 1000D.

NigelM

[rfdesigner]

Sorry - this is incorrect. Dithering on the sky has no effect on the noise due to the dark current

You are correct, but I never said this. Dithering has no effect on the dark current, but it does have a profound effect on noise (errors) due to the "dark master" image, which is why dithered imaging requires vastly fewer dark frames than a non dithered image.

if you don't get it, please just try it, ideally on a narrowband image.

Derek

[rfdesigner]

Apart from hot/dead pixels and that sort of very obvious oddities - errors that dithering or software can get rid of - what is the actual gain of darks at all?

Average dark current varys a little pixel to pixel. A good dark master frame can reduce this variation.

It's best to think of "Hot Pixels" as exhibiting 'temperatures' from Hot to warm, to cool to cold. Most pixels are cold, and hot ones are easy to correct for with sigma clipping etc., but what about cool ones that won't show up on a hot pixel map but add noise?.. dark master frames deal with them.

I suspect what Olly has found is that the Sony sensors have very few 'cool' pixels so dark master frames have little benefit (I'm guessing but it makes sense)

Derek

[Psychobilly]

Actually this is an excellent question (and I don't know the answer). You might be better off using a bad/hot pixel map. It depends for one thing on whether there is any non-random structure in your dark (e.g. if you have amp glow, you really do need a dark). I certainly don't use them for my Canon 1000D.

NigelM

My Observations based on the cameras I have used

The Nikon D200 - Needed well matched Darks as it suffered from Ampglow and was noisy it also needed a gap of roughly half the exposure time between subs to "stabilise" the Ampglow and this took about 10 subs..

The Canon 350D - Needed well matched Darks as it suffered from Ampglow was less noisy than the D200...

The Canon 1000D - Was a lot less noisy and than the 350D and no reall ampglow issues if the darks werent well matched you ended up with predominently red noise streaks in the image.. After a few "experiments" I stopped using darks with the camera...

The Canon 500D - Similar if anything better than to the 1000D again no darks used...

I haven't used the 7D much for astro yet the main camera I used was the Modded 1000D...

It's easy enough to try different calibration/stacking options in DSS if you have a full set of calibration frames (Bias,Darks,Flats and for completeness Dark Flats - yeah I know DSS has wierd terminology for its calibration frames and the way it uses them it has been discussed at lenght in previous threads including input from the "authour") and compare the results.

Peter...

[dph1nm]

if you don't get it, please just try it, ideally on a narrowband image.

OK, I think we had better agree to disagree on this one! If you have truly Poisson noise, then dithering will not improve your image at all - if you are getting an improvement then your noise is not Poisson (which I imagine is always the case).

NigelM