HDF Superbiases:

The superbias frame is intended to remove structure present in a zero-length exposure. The HDF superbias frame is named g1414255u.r2h in the HST archive.

The superbias was constructed by Silvia Baggett, Krista Rudloff, and Harry Ferguson from 160 individual bias frames.

The raw images were retrieved from the archive in sets of 40 and recalibrated (atod and bias level corrections) using calwp2 version 1307, which removes separate bias levels for the even and odd data columns based on values in columns 9-14 of the extracted engineering datafile (x0h).

The STSDAS wfpc crrej task was used to combine the calibrated frames and remove cosmic rays. The task was run with four iterations and sigmas set to 4,4,3 and 2: pixels are rejected if they are N sigma above or below the initial guess image. The first initial guess image was taken to be the median of the stack; subsequent iterations use the computed average image as the initial guess. In addition, pixels within 1 pixel of a rejected pixel (in a '+' pattern) were discarded if they deviated by more than 2,2,1.5, and 1 sigmas, respectively for each iteration.

The 40-frame superbiases were used in creating superdark images for the same epochs. These superdarks were then normalized to 43.6 sec exposure time and subtracted from the superbiases. The dark-subtracted bias frames were then averaged using the gcombine task with avsigclip and 3-sigma clipping.

Note that this superbias file is different from the standard CDB bias files in that it has most of the dark current subtracted. To use the file correctly, you must update DARKTIME in the header of the frame you wish to calibrate to the value:

    DARKTIME = 60*int((t+16.4)/60) + 43.6.
Calwp2 will then scale the superdark appropriately, and the tiny amount of extra dark current that is in the bias frame will will be taken care of by the superbias.

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

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