dslr imaging advice

[Astroman (Andrew Wall)]

Got me thinking now so I did a comparison on a 5minute shot in RAW @ ISO1600 on my 400D. One with in camera noise reduction, one without. 100% crop. no Processing

http://southcelestialpole.org.au/for...ach=5965;image

[RB (Andrew)]

Originally Posted by luka

If you take multiple exposures that you stack later, it would make sense to take several darks and subtract them in software later and not use camera's internal dark subtraction.

Several darks averaged together in software will be a much better noise representation than only one dark image which is used by the camera. In other words your software will be doing background subtraction of averaged, more accurate dark from each of the images than if you let the camera do the job.
In theory even with two darks you should get a better result than with camera's internal dark subtraction.

Of course this only applies if the temperature/humidity conditions do not change a lot during the exposures.

I'm not sure I follow your explanation.
The camera applies a new dark frame directly after each exposure equal to the length of that exposure at the current temp.
Hence it takes twice as long to save it to the memory card but each exposure ends up with a unique dark frame subtracted from it.

[[1ponders] (Paul)]

As I understand the process, the subtraction of the single dark removes the regular and repeatable noise for may actually add noise from the irregular and non repeatable noise, hence the need for an averaged master. This will reduce the irregular and non-repeatable. I'll try to dig up the reference.

[luka]

As 1ponders said, your dark has not only hot pixels and repeatable noise but also other irregular "noise".
By averaging several darks you will be improving regular (true) noise to irregular (random) noise ratio so that you dark will be a more accurate representation of the real noise.

[little col]

, i wonder is it possible to take a dark frame and save it on file for future images as long as the imaging setup is the same?

[luka]

The noise and hence the dark frames will change, for example, with ambient temperature. Therefore it is the best to make new dark frames every time you make images.

[xelasnave]

Originally Posted by ballaratdragons

Alex, yourself and some others keep recommending Deep Sky Stacker.

I have not been able to get a good result from it once! All I ever get is a totally over-exposed images and a big mess. I gave up trying it after about the 20th attempt.

Have you played with the result using the controls in the program? group the colours and move them one way or the other to lighten or darken get something and take it elsewhere like photo shop to play with it...

May be the program make not like the cam ..

anyways you are doing great..its like registax I cant get it to work for me and it is a great program given the results posted by folk using it.

I just go with what works for me as should you..er works for you that is not er me... you go for it your way .
alex

[g__day (Matthew)]

Very interesting reading, especially in that two or more averaged dark frames out performs one real time one in most conditions. The mathematican in me will be thinking that over (fun with standard deviations and anovas).

What is amp glow by the way - I've seen that a few times on 15 minute shots sometimes.