While waiting for the next clear, moonless night I have been experimenting with the Auto-Grab function in CCDOps with my STXL16200 and I have noticed something that I wanted to get some input on. Compare the two images below. Both are 20 second "light" frames that were taken with the cover on the camera (Sorry - No M42). The Test5, Clear003 image was taken without any dark frame subtraction, and the Test6, Clear003 image had an auto dark frame subtraction. The cooler was running at -30 Deg C for both images. In my mind, the Test6 image with the dark frame subtracted is much grainier than the Test5 image. I would have expected just the opposite to be the case. Is this increase in graininess due to the fact that I am subtracting a dark frame from a dark image, or can I expect to see a similar increase in the graininess with real images when I do an auto dark subtraction ? I don't normally do auto darks because I process my images in Maxim DL and I will do an auto-calibration of the raw images with a stack of dark frames that I take separately.