[PRejto (Peter)]

Hi Greg,

I'd be quite curious about your result of only calibrating with a flat.

I think the pix map ought to be used only after applying a flat + bias (as substitute dark). Obviously you wouldn't use the map before or after using a dark as that would attack the same pix 2X. I would apply it after the flat since the flat will remove crud and light irregularities. My reasoning would be that more correct pixels would be neighboring the removed pix for the interpolation. There are probably better reasons but that's the one that comes into my head.

I've been redoing my darks this morning and I'm noticing something that seems odd. I've done 22 12 min darks. After the first 10 or so the mean value starts progressively dropping, as does the min value (but not the max). I don't see why this should be happening. It's as if the longer the CCD is running the lower the min count is: Started min = 1232, after 10 frames min= 1225, after 20 frames min = 1209, after 27 frames min = 1189. This seems really odd! Any idea?

Thanks,

Peter

EDIT: I wrote to Terry Platt at SX. He said it is "just the reference voltage in the A-D that is drifting slowly as the chip warms up. It doesn't affect the darks – it just shifts the bias point slightly."

Hmmm.I'm not sure why it wouldn't affect the darks. Wouldn't the average total ADU be drifting lower and thus effect the amount that the dark applies in calibration?