Darks, and why I need a CCD or newer model of DSLR

[glennbech]

My 15 minute darks are awfull! A visible pink right side that creeps left when I stretch the image.

I spend the night, clear but very wet, taking 15 minute iso 800 darks. I stacked them with my lates LBN & Caldwell images and they worked reasonably okay.

But! I took notes of the temperature. When I startet taking the darks it was 6c, and when the session ended 5 hours later, it was 0c....

I will never ever get darks at the same temperature as my light frames. And neither darks nor lights will be taken at a constant temperature.

My conclusion is that I need a camera that can give me 15 minute darks with a lot less noise than my canon 10D.

This will get expensive! :-)

[Rossco72]

The easiest way to balance them is to use the onboard NR which takes an auto dark after each exposure. However, this means that you are losing valuable CLEAR night time. Why not use a cloudy night to take darks and keep a note of the temperatures for each, save them with the temp on the file name. Then when you are doing lights, note the temp and just use an appropriate dark from a similar temperature?

[themos]

Take your darks in the fridge!

[George]

A few degree's either side of your lights temperature wont make that much of a difference.

[adamsp123]

Take your darks in the fridge!

Thats how I did my 5C and 0C Canon 1000D darks, main fidge 5C ice box 0C and a set when outside was 10C.

I would worry about exact temp matches 5C steps should do.

Pete

[glennbech]

that accuracy doesnt really matter is great news. thanks for the heads up on that!

[Rossco72]

For the darks as I understand it, its about mapping and removing hot pixels, amp glow etc. inherent in the camera. Flats on the other hand are for correcting vignetting and any dust or errors in the light path and as such need to be taken with the camera attached in the orientation used to generate the image.

[MartinB]

Scale your darks! Take 5 minute darks and a load of bias frames. I use Maxim but I'm sure DSS will scale darks for you. You have to have bias frames though.

With unscaled darks you don't need to use bias frames since the bias is contained within the dark so applying an unscaled dark from subtracts both dark current and bias. However, when scaling a dark you don't want to scale the bias as well (since that is constant) so you subtract the bias first. The dark current is then multiplied by the appropriate factor. You then subtract the bias and the dark seperately.

Scaled darks work very well provided you don't push things too far. Scaling 1 minute up to 15 is likely to be much less satisfactory.

You just need to check how to run this in DSS or whatever software you are using

[dph1nm]

You can subtract off all the darks you like, scaled or otherwise, but you won't get rid of the extra shot noise caused by the amp glow. I vote for the new camera option!

NigelM

[glennbech]

My Canon 400D is getting less and less used for daytime photography. I might modify that, and get a clip-in filter for daytime photography later.

That is probably my best cost/benefit option right now. the 400D has a lot less noise and more pixels than the 10D.