HDF Hot Pixel Removal:

The number of hot pixels is minimized by using a delta-superdark frame in the HDF pipeline; however several hundred remain in each frame after pipeline processing.

Growing pixels are hot pixels that have appeared or disappeared since the deltasuperdark. They are identified by combining a set of images at different dither positions, subtracting a 5x5 pixel median filtered image to remove residual objects, and identifying pixels that deviate from 0 by more than than 5-sigma (as determined by iterstat).

A growing-pixel mask was created for each day using about 48 hours of data roughly centered at UT noon. The masks for each day were logically OR'ed together and OR'ed with the static mask (the list of constant WFPC bad pixels) and the delta-superdark data quality file (the list of pixels that became hot in between the superdark and the delta-superdark).

This procedure still left about a dozen hot pixels per chip. These were identified with a program ``qzap'' similar to the NOAO cosmicrays task, which flagged individual pixels that were more than 6-sigma deviant from their neighbors in a 3x3 box. These pixels were OR'ed into the mask to produce one final mask that flags all pixels that were suspect at any time during the HDF. Altogether roughly 12000 pixels per chip were masked. Because there are 8 or 9 different dither positions per filter, this very conservative approach to masking hot pixels has only a modest effect on the final S/N, reducing it by about 10% in those pixels that were flagged.

Copyright © 1997 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.

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