HSTCAL serves as a stand-alone package written entirely in C for building and running the pipeline calibration tasks for each HST instrument's data; specifically, calacs for ACS, calstis for STIS, and calwf3 for WFC3.
Initially, these tasks were written using C under the IRAF/STSDAS environment, relying on IRAF to perform I/O and other basic interface functions. HSTCAL replaces all the IRAF-based function calls with routines based on CFITSIO, allowing all the pipeline software to be compiled and run without any dependence on IRAF anymore.
HSTCAL vs STSDAS
The calibration software included in HSTCAL replaces the versions previously released under STSDAS with continuing development of the code taking place under HSTCAL, not STSDAS. In addition, all ACS, STIS and WFC3 data retrieved from the HST Archive will have been processed using the HSTCAL version of their calibration software.
The transition of the pipeline code to compile under HSTCAL did not require many changes to the C code in each task itself, with one exception: the computation of the photometry keyword values. Each of the pipeline tasks under STSDAS relied on direct calls to the STSDAS synphot routines to compute the photometry keywords during processing. Those direct calls were replaced by use of a common library to read in the photometry keyword values from a new reference file, IMPHTTAB, containing pre-computed values for the keywords. Each instrument team now generates their own IMPHTTAB reference file based on the latest photometric calibrations and provides those for use in the pipeline by the HSTCAL-based calibration tasks.