wfpc -- Calibration tasks for WF/PC and WFPC2 observations.
The hst_calib.wfpc package contains the "pipeline" calibration code (i.e., the calwfp and calwp2 tasks), as well as routines that are used to produce calibration reference files required by the pipeline, routines to analyze the health and performance of the instruments, and other utility tasks that are specific to WF/PC and WFPC2 data.
The tasks in the wfpc package after STSDAS release 3.1 can deal with extension FITS files as well as GEIS files.
The GEIS format has been the default working data format for WF/PC and WFPC2. Tasks in the wfpc package before STSDAS release 3.1 only operate on this format. Since GEIS is a platform dependent format, it is necessary to convert it, by using the stwfits task, to FITS format for archiving. When users retrieve WFPC2 or WF/PC files from the STScI archive, it is this FITS format (sometimes referred to as "waiver" FITS) they will get. The users then use the task strfits to convert it back to GEIS format.
Because of the limitation of early FITS standard, the "waiver" FITS format for WFPC2 (and WF/PC) is structured in an awkward manner: the four 800x800 images are stacked together as a 3-D image, i.e. an 800x800x4 image, and is stored at the primary header-data unit (HDU). The group parameters of the 4 groups are stored, as a table, in the first extension HDU of this waiver FITS file. Because of this structure, it is difficult for data analysis software to deal with it directly and it is only for the purpose of archiving.
Enhancement of the FITS standard later in the 1990's makes it possible to store WFPC2 data in a FITS format which is both platform independent (therefore suitable for archiving) and easy direct access for analysis software tools. In this structure (sometimes referred to as "extension" FITS), the common header keywords are stored in the primary HDU which has empty data part. Each of the 4 WFPC2 images is stored in each of the 4 extension HDU's. The extension HDU's header holds keywords of detector specific information (such as those in the group parameters of the corresponding GEIS file).
This "extension" FITS format is used for newer HST instruments such as NICMOS, STIS, and ACS. Although for WFPC2 the file extracted from the archive will continue to be waiver FITS, some users may find converting to extension FITS desirable for local storage, data analysis, and data exchange between different platforms. Unfortunately, there is no tool right now to directly convert waiver FITS to extension FITS. There does exist a tool (distributed with pyraf) called readgeis to convert from the GEIS format to extension FITS. As before, strfits/strwfits can be used to convert from/to waiver FITS to/from GEIS format.
Tasks in wfpc package since STSDAS release 3.1 can operate on extension FITS, as well as GEIS files, and therefore provide more motivation for users to use the extension FITS format both for data analysis and storage.
It should be cautioned that for a wfpc task requiring both input and output files, their formats should be consistent, i. e. if the input is extension FITS file(s), then the output file should also be specified as (extension) FITS, not GEIS file.
strfits, stwfits, calwfp, calwp2, readgeis
For additional information about GEIS files, type "help geis".