HI Kurt, Sorry to be such a pain. The Dark image "091206_0141_Darks_300_033.zip" appears to be very similar to the Bias image. Also this Dark image does not have what I would expect to see in a dark image. I wonder if the GRAS people have gotten the calibration images mixed/confused or whatever. As I had mentioned previously the Bias image should have only a very minor if any gradient. The only reason for a gradient in the bias image is if the readout is slow and some thermal electrons are collected. The STL-1100 camera specs from SBIG says that it does take about 26 seconds to download which at 0 C should only generate about 13 electrons per pixel or at a gain of 0.8, 10 ADU's.. The reason I think these images are "troubled" is that the Dark frame has a Mean pixel value of 860 ADU's while the Bias frame has a Mean pixel value of 1157 ADU's. Clearly since the Dark frame signal should include the Bias frame signal it MUST be larger, unless the Bias frame has been already subtracted from the Dark frame. The header of each image does not indicate that any processing has been done on these images although it could have been done and not noted in the header of either. Jerry Foote ScopeCraft, Inc. 4175 E. Red Cliffs Dr. Kanab, UT 84741 435-899-1255 jfoote@scopecraft.com