A question on dark frames

[aarong]

Hi guys,

I have a two part question with darks frames for dslr's. I noticed far less noise with my latest image that happened to use 48 dark frames, compared with previous images that use 12-24 dark frames. I was wondering what people generally use for dslrs? I know there may not be a magic number as it will depend on your setup, just after your experiences.

I'm building up quite a collection of darks frames, as I take them at every imaging session. So my second question is can I reuse them, assuming I can match the sensor temp they were taken at with my light frames? This way I could spend more time during the night acquiring light frames.

Thanks,
Aaron

P.S. I use a Canon 1D mk3, which is pretty old now and can be noisy compared to todays models. My scope is an 8" Meade lx90. I'm trying to find the limits of my equipment and resist heading down the ccd path...for now ;)

[lazjen]

The more dark frames you have, the better, but you get diminishing returns after sometime.

You can re-use your dark frames - I've been building a library as well. I usually use dark frames that are within a few degrees of the light frame. You may need to re-evaluate them in the future as the camera sensor changes, but I suspect they should last quite some time.

If I do flats, I generally just do the dark flats at the same time, since it's fairly quick to do.

[aarong]

Thanks Chris. Good to know. Sounds like building a dark frame library is a good job for a cloudy night.

[nebulosity.]

I use around 30 dark frames on average, Building a dark library is a good idea.

Cheers
Jo

[rcheshire]

You can reuse them, if you build libraries by temperature. There is a limit to the temperature range that an averaged dark frame is effective without producing too many artifacts.

Fresh darks are best and libraries are OK, particularly if you dither your light frames while imaging. A dithered stack will hide most of the temperature mismatch artifacts, among several other benefits - Backyard EOS.

Because you are averaging the noise by producing a master/super-dark - more frames is better. One image processing program suggest 10 as a minimum, but experience shows us that more is better. Taking longer darks than lights for the purpose of dark scaling (for which you must subtract the bias) is advanced stuff, if you need it.

There is a school of thought that says in-camera reduction is more effective per frame and just as effective despite half the exposure time - expect a big discussion/firestorm over that one.

If you have the time, segregate your darks by temperature. I don't think it's agood idea to mix them.

Hope this helps.

[traveller]

Just what I do, not sure if it's right or not.
I use darks and lights on a 1:1 ratio and try to match the exposure times.
Cheers,
Bo

[Marios]

That is correct in the case of the libary you would catergorize these Darks by temp and exposure time. 0.30 1, 2, 5, 10 minutes and so on.

[aarong]

Thanks all for the great info!

Cheers,
Aaron