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Greg
I have been reading this with interest - flats were driving me mad a few months back. I have them more under control now (I think).
I use another of Peter's lightboxes. To avoid shutter artifact, I asked Peter to include a potentiometer to step down the intensity. I have also used a t-shirt between scope and box.
My routine is as follows:
1. Take a flat using box - I aim for 20k ADU except for Blue filter which needs 25k. I get the exposure right using the Sky Flats plug in for Maxim.
Be careful with filter wheels - sometimes Maxim uses the same filter even though I have told it to move to a different filter. That throws a big spanner into the works.
2. Manually take another 9 or so flats with the same exposure time. The plug in does shift the exposure time slightly between flats so I do it manually to keep it fixed for dark subtraction.
3. Repeat for each filter using the same binning as I am imaging.
4. Take darks with same exposure length.
5. Take a few bias frames.
6. Calibrate flats with darks and bias.
7. Then take long darks - or use a library dark. I had problems relying on the scaling function in Maxim. For some reason, the scaling works for light frames but introduces noise when applied to flats.
I do all this at the end of an imaging run or begining if I am early setting up.
The other lesson I have learned is that flats don't help much if you have a low signal to noise ratio in your light frames. I was stacking 3 x 5 minutes light frames of galaxies and wondering why the flat cured the dust bunnies but didn't quite fix the apparent uneven exposure and in fact made it worse by highlighting the edges at the expense of the centre - especially in colour. The real issue was there wasn't enough signal to play with.
Hope that helps
Pete
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