This is raw HIRES FITS image file of a dark (shutter closed) exposure of usually 0 or 1 second duration. This image is used to derive a column-to-column bias pattern for the HIRES CCD.
The 1998 HIRES CCD has an offset (above the baseline) of about 8 DN for a 1x2 binned image (see plot below). This offset tends to vary systematically with column number (but not row number). The pattern is derived by taking the median value for each column in a short dark image.
This is an optional user input. The makee package contains archive bias patterns for the 1x2, 1x1, and 2x2 binning configurations. These will be used if a short dark is not provided by the user. Since this pattern does not change significantly with time (year to year), using these default patterns should not adversely affect your result.
Here is a plot of the column-to-column bias pattern for 1x2 (col,row) binning from April 1998: bias1d_1x2.ps . Also see: bias1d_1x1.ps . and bias1d_2x2.ps .