I've been doing BV photometry for a few years now and recently moved on to adding R and I. But I'm finding that with the I filter only, I'm getting many negative pixels in my calibrated images (SBIG ST8XME). What's the best way to handle this, add an offset to my raw science images before applying darks and flats? It would only take about 30-50 ADU.
I think the reason you are getting negative pixels is because you don't have much sky background. Subtracting a dark frame with its noise then gives you positive and negative pixels, with a mean around zero. This is perfectly normal. Do your dark subtraction and flatfielding, and then apply an offset, in that order.
Thanks Arne. In my case I don't think I need to apply any offset since with differential photometry the offset vanishes (var-comp). I'm using Muniwin, and as long as I change the minimum pixel value for bad pixels to include the negative ones (e.g,. min = -50 instead of the default of 0), I get the same photometry compared to if I were to add an offset to the calibrated image and set the lower bad pixel threshold to 0.