The `mstools' package contains tasks for working with STIS, NICMOS, ACS, and WFC3 data. Some tasks are "extensions" of existing tasks in the STSDAS system, and support other instruments/file formats as well.
NICMOS, STIS, ACS, and WFC3 data are stored in multi-extension FITS files. Each extension stores a Header-Data-Unit (HDU), which is nothing more than the familiar "image" used by IRAF tasks: a pixel array and its associated header. The first HDU in any of these FITS files is a special one, because it carries a null data array (thus it is made up of a solitary header), and is named Primary Header Unit (PHU).
HDUs can be addressed by their absolute offset in the file. Thus the PHU could be accessed by using the construct "filename.fits", and the 10th HDU in the file by "filename.fits". Alternatively, an HDU can be accessed by the EXTNAME and EXTVER keywords found in their header. For instance, to access the HDU that has EXTNAME="SCI" and EXTVER=1 in its header, use "filename.fits[SCI,1]".
Since STIS, NICMOS, ACS and WFC3 exposures are always comprised of 3 or 5 associated HDUs, these sets of 3 or 5 HDUs are conveniently named IMSETs, and are identified by a common EXTVER number. The EXTNAMEs in each STIS, ACS, and WFC3/UVIS IMSET are SCI, ERR and DQ, and the EXTNAMEs in NICMOS and WFC3/IR IMSETs 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 and a few others. 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 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 these files.