nicmos -- Calibration and Analysis of NICMOS Observations.
The nicmos package contains the Routine Science Data Processing (RSDP) calibration codes (calnica and calnicb), routines to generate the calibration reference files and tables required by calnic[ab], routines to generate the parameters needed for observation planning and instrument configuration, and routines to display and analyze NICMOS data. The RSDP pipeline, operated at STScI, provides consistent calibration of NICMOS observations. The purpose of the STSDAS calibration software is first, to provide the calibration parameters used in RSDP, and second, to calibrate NICMOS observations.
NICMOS data are stored in multi-extension FITS files. Each extension stores a Header-Data-Unit (HDU), which is the familiar "image" used by IRAF tasks, composed of a pixel array and its associated header. The first HDU in any NICMOS FITS file is a special one, because it contains only header information with an empty (null) data array, and is called the Primary Header Unit (PHU).
HDUs can be addressed by their absolute offset in the file, with the PHU indexed to unit number 0 (zero). Thus the primary header is accessed by using the construct "filename.fits", and the 9th extension HDU by "filename.fits". Alternatively, HDUs can be accessed by the values of the EXTNAME and EXTVER keywords found in their headers. For instance, to access the HDU that has EXTNAME="SCI" and EXTVER=1 in its header, use "filename.fits[sci,1]".
Since NICMOS exposures are always comprised of 5 associated HDUs, these sets of 5 HDUs are refered to as image sets or IMSETs, and are identified by a common EXTVER number. The EXTNAMEs in each NICMOS IMSET are SCI, ERR, DQ, SAMP and TIME. As an example, the 20th IMSET in a NICMOS MultiAccum file is comprised of 5 HDUs that have the EXTNAMEs above and the common value of 20 in their EXTVER keyword.
Notice that IMSETs are abstract entities, in the sense that there is no way for a generic IRAF task (such as imcopy or imstatistics) to access a given IMSET as a whole. This type of access is performed only by tasks in this package. Notice also that the physical order in which HDUs are stored in a file is not important, since IMSETs are defined by keyword values in the HDU headers, and not by the physical ordering of the HDUs. Thus it is good practice to avoid using the absolute offset naming when describing NICMOS files but rather use the EXTNAME/EXTVER combination.