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Finding that a calibration reference file has changed since your data were calibrated doesn't always mean that you have to recalibrate. The decision depends very much on which calibration image or table has changed, and whether that kind of change to your data is likely to affect your analysis in a significant way. Before deciding to recalibrate, you might want to retrieve the new recommended reference file or table and compare it to the one used to calibrate your data at STScI in order to determine if the differences are important. You can use the table tools in the STSDAS ttools package to manipulate and examine calibration tables; images can be manipulated in the same way as your science data.
The calibration switch keywords reside only in the primary header of NICMOS FITS files: it is critically important to specify extension number zero explicitly when passing file names to tasks like chcalpar or hedit to modify these keywords. For example, to modify calibration keywords in the file n3xe01bhm_raw.fits, be sure to use the complete name n3xe01bhm_raw.fits[0] as input to these tasks. If you do not specify extension zero explicitly, the keywords you modify will end up getting written into the first extension header instead, where calnica will not find them.
An example of using chcalpar on a single NICMOS file is shown in Figure 16.6.
ni> calnica n0g70106t_raw.fits n0g70106t_cal.fitsor:
ni> calnica n0g70106t " "To run calnicb the name of the association table must be given as input:
ni> calnicb assoc_id_asnTo run calnicb on a subset of the *_cal.fits files, it is sufficient to edit the *_asn.fits table and remove the undesired files.
The calibration routines calnica and calnicb will not overwrite an existing output file. If the calibration tasks are run in the directory where the original calibrated files are located, a different output file name must be specified.