darks, flats and bias frames

[dmahon]

I haven't even got my scope/mount/camera yet, but have been reading, lots. Including Making Every Photon Count (which is excellent so far). I'm reading about darks, flats, bias frames, etc at the minute.

I understand why you would want to take these away. I also understand why you need to take bias from darks/flats and darks from flats - otherwise you would take things away from the light image more than once. However, if, like many people seem to do, you don't use flats, could you just take dark frames alone, ignoring bias frames - as the bias will be included in the dark? In effect you will have (dark+bias) frames, and you are going to remove the (dark+bias) from the light frame all in one go. Is that right?

[themos]

Yes.

But bias frames are useful if you want to "scale" your darks. For instance, if you shoot 2 min lights and 3 min lights, you don't have to shoot 2 min and 3 min darks. You can shoot just 3 min darks and "scale" these back to 2 mins. But you need a bias frame to do that.

[Ben Ritchie]

The other use for bias is in some CCDs with very low thermal noise, where you may simply subtract bias rather than darks.

While it's not perfect, in many situations it's convenient and 'good enough' as it avoids the need to spend time taking darks. The ICX285 chip in assorted Artemis, Atik and Starlight-Xpress cameras is a good example.

[Trull]

There's a worthwhile discussion of this in the Nebulosity manual - freely downloadable Nebulosity