The DARKDRIFT module is known colloquially as the “jail-bar corrector.” Its purpose is to correct additive offsets between the four read-out channels. When offsets are visible between the read-outs, the effect is a vertical striping or “jail bar” pattern. This pattern may result from drifts over time in the dark-current level that varies from read-out channel to channel. The module is consequently named “DARKDRIFT”.
The darkdrift correction is computed and applied separately for each plane of the input data cube. First, the median is computed for all pixels belonging to each read-out channel, over the entire plane. Second, the average of the four medians (a constant offset) is subtracted from each median to form the correction for each read-out channel. Then, the correction for each read-out channel is subtracted from all pixels belonging to that read-out channel.
The effect is to “even out” the striping caused by the jail bars, while leaving the mean of each image plane unchanged.
In addition to the output FITS data and uncertainty cubes, DARKDRIFT outputs several values for each plane to the darkdrift.tbl file. For the input image the numbers of good and bad pixels, the median and trimmed standard deviation, and the data scale are output for each read-out. The final corrections and uncertainties computed by DARKDRIFT are also tabulated.
The main outputs of the module are the files darkdrift.fits and darkdrift_unc.fits. These files are not available in the Spitzer Heritage Archive, but can be generated by running CUPID.
Figure 5.6 Illustration of darkdrift correction in the top plane of a SL 14 second DCE. The left frame is the input lineariz.fits, while the right frame is the output darkdrift.fits. Note that the prominent jail bars in the input practically disappear in the output. The correction is done on each plane of the DCE separately.