Dark Frames

[mick pinner]

do you have to take a corresponding number of dark frames to light frames,
eg 6x light frames + 6x darks.

[rogerg (Roger)]

No, but it is a good idea to do more than one. I typically average 3 "good" dark frames if doing things manually, or more if it's running automated so I don't have to sit around waiting for them.

By good I mean excluding some that have out of the ordinary hot pixels/cosmic rays? through them. I don't find it such a problwm with the 350D but the SBIG will sometimes pick up abnormalities in some dark frames. I think the difference purely comes down to sensitivity of the camera.

Same for flat and bias frames, I'd usually do 10 of each of those and average them.

Oh, and you can re-use dark frames. I sometimes keep using the same dark frames for the next month or so. But that's only with the SBIG where it's cooled accurately so the conditions are repeatable.

Roger.

[mick pinner]

thank you Roger, can you explain the process of averaging the darks.

[rogerg (Roger)]

It depends where I'm doing it, I'll break my situation down in to SBIG vs 350D.

SBIG: I'm typically using CCDSoft, where it's reduction area allows you to specify against a "Reduction Group":
- Bias frames
- Dark frames
- Flat frames
- Dark frames for flat frames
You basically select the appropriate files for each type and then for each type you choose the combine algorithm (average or median). Then before it does the actual reduction of images it will first combine (stack using average or median as selected) the reduction frames, then use that resulting stacked reduction frame to reduce the actual image.

350D: I do the combining of dark frames manually myself in RegiStar. After converting the .RAW files to .TIF I'll first use RegiStar to combine the dark frames. Then I use photoshop to subtract the combined dark frame from each .TIF image file, I then use RegiStar to register and combine all the reduced .TIF image files, and then again use RegiStar to combine the registered images.

With the 350D it's certinaly a lot easier to just subtract the one from each frame and skip the averaging of the dark frames. I often do this out of lazyness and the fact my DSLR photo's aren't that great anyway, where as the SBIG shots I'm really pushing the limits of what faint objects I'm trying to pick out so want the best possible noise reduction.

I'd like to be able to use CCDSoft for the DSLR stuff too, but for some reason it's never happy with the file format. I get the impression it can't handle large files, relaly not sure. I just get program errors when I try to use TIF or BMP or JPG converted from canon's .RAW.

Roger.