HDF Superdarks:

The superdark frame is intended to remove spatial structure present in the dark current. The HDF superdark frame is named g141414du.r3h in the HST archive. The HDF delta-superdark frame is named g141351mu.r3h in the HST archive.

The delta-superdark frame is a copy of the superdark frame with hot pixels flagged in the .b3h file and subtracted. Hot pixels were defined as those that were 5-sigma deviant from the mean in 11 dark frames taken at the start of the HDF observations.

For the original superdark, eight sets of 30 dark frames were calibrated using CALWP2.1307. They were combined with the following algorithm to eliminate cosmic rays: The dark calibration image was initially estimated as the median of the 30 input images at each pixel. An iterative technique was then used to improve this estimate. Each iteration proceeded by computing the noise at each pixel using the initial combined image (assuming total readout, a-to-d con- version, etc. noise of 12 electrons). Then masks were generated for each input image with value 1 if the input image was within 4 sigma of the median image, and 0 otherwise. Zeros in each mask were then expanded to a 3 x 3 square pattern, so as to also exclude pixels immediately adjacent to cosmic rays. The masks were then multiplied into their respective images, the resulting images were summed, and finally the result was normalized using the sum of the masks. The few pixels left without data were filled with the median of the 30 input images at those pixels. This procedure was then iterated using the new combined image as the initial estimate with 4 sigma rejection, and then additional iterations were made with 3 sigma and 2 sigma rejection.

30 frame darks were added together in pairs, and then the four resulting files were gcombined with avsigclip with a 4-sigma rejection threshold. The resulting image is normalized to a darktime of 1.0 second, using a darktime value of 1800.00 seconds for each CCD.

Harry Ferguson ferguson@stsci.edu 1/14/96