Hi Kurt, Thanks for the links to the raw image as well as the calibration images. I downloaded the calibration zip file and then unzipped the contents. The file that was to contain the Darks was empty so I couldn't work with it. The zipping process might have missed the darks. The Flat image looks reasonable and contains some dust shadows as would be expected. The Bias image is very strange as it has a pronounced gradient from top to bottom. Bias images are supposed to be made with zero integration time (which the FITS header says it is) and simply reflect the read noise in the output amplifier as well as a preset pedestal that prevents negative ADU values. Under normal conditions there is a slight gradient from top to bottom as thermal electrons are formed but usually this is very small and is minimized if the temperature is low. In this image the gradient goes from about 800 to over 1600 ADU's. You might want to check with the GRAS people about this image. It isn't very important as for most processing the bias frame isn't used anyway as the bias information would be normally contained in the Dark frame and gets subtracted out when the Dark is subtracted. The only time bias frames are important is when the Darks are scaled for exposure time or temperature difference, which I assume isn't done here. Kurt this is fun. Thank you for supplying the images and letting us work with the images you have paid for. Jerry Foote ScopeCraft, Inc. 4175 E. Red Cliffs Dr. Kanab, UT 84741 435-899-1255 jfoote@scopecraft.com