Three, one-hour darks were obtained, with the LED light source disconnected (images 349912.fits, 349926.fits and 39937.fits, see Data). These were processed by first subtracting the median stacked bias image obtained in Biases, and then reducing each image to a 512x512 pixel image where each pixel is the result of the median of 4x4 pixels from the original image, ignoring any pixel with counts higher than 100 (this process is quite effective in removing cosmic rays from the images). The average of the resulting three, cosmic-ray-free images was obtained, multiplied by the conversion factor of 4.56 electrons/ADU and a d.c. offset added to produce an overscan region with an average of 0 counts. The result is shown in the image below:
A contour plot is provided below, the contours are at 0 (bias region), 40, 80 and 120 electrons per pixel per hour. The corners of the imaging area show the highest counts. The central region is relatively cold. This CCD clearly require careful dark current image subtraction in long exposures.
