Improved Destriping and Background Removal

Sometimes, a very bright extended source can ``fool'' DESTCRDD or BACKCRDD into thinking that the background level in a given detector data stream is higher than it actually is. If this artificially high background is subtracted from the data, the resulting images can display lower background levels on the in-scan sides of the source than on the cross-scan sides. The applications DESTCRDD and BACKCRDD contain filtering algorithms which can be tuned to reduce these sorts of effects, but if necessary, the ``Quality'' system used by IRAS90 can also be used. This requires the user to identify ``source regions'' which are to be excluded from the estimation of the destriping constants and background levels. There are several ways in which this can be done; for instance the KAPPA applications ZAPLIN can be used to manually identify and remove source regions from an initial map:

• Follow the procedure described in section to produce a first estimate of the map.
• Use KAPPA:ZAPLIN (with parameter ZAPTYPE set to ``bad'') to create a copy of the image in which the area containing the offending source has been replaced with ``bad'' values.
• Use SIMCRDD to generate simulated CRDD files using the output from KAPPA:ZAPLIN as the trial sky input, and the original CRDD files to define the position and PSF for each sample. The simulated samples corresponding to the source area will hold ``bad'' values.
• Use SETQUAL to assign a quality name (let's assume you use the quality name ``SOURCE'') to the samples in the original CRDD files which fall in the source region. To do this the input NDFs should be the original CRDD files, and the mask NDFs should be the ones generated by SIMCRDD. The SETQUAL function should be ``HS''. This ensures that the quality ``SOURCE'' is held by all the samples which are bad in the CRDD files generated by SIMCRDD (i.e. all samples falling within the source region).
• Run DESTCRDD on the original input CRDD files, specifying ``.NOT.SOURCE'' for parameter QEXP. This causes all the samples with quality ``SOURCE'' to be excluded from the estimation of the detector backgrounds. Note, it does not mean that these samples will be excluded from the output CRDD files!
• Run BACKCRDD on the output CRDD files from DESTCRDD, specifying ``.NOT.SOURCE'' for parameter QEXP. This causes all the samples with quality ``SOURCE'' to be excluded from the estimation of the CRDD file backgrounds.
• Run MAPCRDD again on the output CRDD files from BACKCRDD.

The ICL procedure ``MASTER'' described in appendix contains an alternative approach to improved destriping in which the source regions are identified automatically. In addition, MASTER uses the extra trick of running the data through DESTCRDD and BACKCRDD twice.