This "dark frame" is a single 10 minute exposure taken with the telescope capped to provide no light entering the image path. The purpose of using dark frames in the calibration process is to reduce the effects of random noise. The Starlight Xpress HX916 CCD camera exhibits very low levels of random noise (because of the nature of its CCD chip and electronics), and so I do not use dark frames in calibration. The difference between a master bias frame and a dark frame are not statistically significant (based on the mathmatics used in image calibration), and using a dark in calibration would actually introduce additional random noise into the final image.