hst_calib: HST Science Instrument calibration packages.

• synphot: Tasks for synthetic photometry and modelling instrument response.

Introduction

The `st4gem.hst_calib' package contains the tasks that are used to calibrate data from each of the HST Science Instruments (SIs), and to model the combined response of the various detectors and optical elements. It also contains tasks to derive the instrument calibrations and construct the calibration reference files. The tasks are divided into nine packages: seven that are specific to a particular instrument, and two that contain more general HST calibration tools. A quick summary is given in Table 1 below; a more detailed summary can be found in the following sections.

            Table 1.  HST Calibration Packages
+-----------------------------------------------------------+
| Package | Description                                     |
+-----------------------------------------------------------+
| synphot | General tools for modelling instrument response |
+-----------------------------------------------------------+

USING GEIS FILES

Perhaps the most daunting aspect of HST data reduction and analysis for novice users of ST4GEM is the complexity of the data structures. GEIS format (also called ST format or "group" format) data is described in some detail in the "ST4GEM Users Guide" and in the on-line help file (type "help geis"). The most important point is that GEIS files can contain more than one image in a single datafile; each image is stored in a separate "group" within the file. As well, the parameters (or descriptors) that are common for all groups in the image are stored in FITS-style keywords in the ASCII header, but parameters that are group-specific are stored within the binary image data. While this subtlety is transparent to most ST4GEM tasks, it is an important point to remember.

It is also important to realize how GEIS files are used for each instrument. FOC data files have only one group per file, and WFPC and WFPC-2 files usually have four groups---one per CCD detector. The spectrographs (FOS and HRS), on the other hand, store each read of the diode array in a separate group. Thus, one file could contain hundreds of groups, and the sequence could define a spectral time series, a single spectrum as the counts accumulate, or an acquisition image, depending upon the instrument observing mode. Table 2 summarizes the way that groups are used for each HST instrument.

            Table 2.  HST Data Structure Summary
+--------------------------------------------------------------+
| Instrument | Group Use                                       |
+--------------------------------------------------------------+
|   FOC      | One group per file                              |
|   FOS      | One group per scan, possibly hundreds per file  |
|   HRS      | One group per scan, possibly hundreds per file  |
|   WFPC     | One group per CCD detector, up to four per file |
|   WFPC-2   | One group per CCD detector, up to four per file |
+--------------------------------------------------------------+

If you use non-ST4GEM tasks in the IRAF environment for your analysis, be aware that operations must be performed explicitly on each group in the data file; the default is usually to operate on only the first group of multi-group files. On the other hand, most ST4GEM tasks either perform the specified operation on all groups, or make some explicit provision (usually in the task parameters) to define the group(s) on which they operate. Novice users may find the tasks in the `toolbox' packages especially useful, and may profit from practicing some basic operations on test HST data, examples of which can be found in the directory "scidata$".

GENERAL CALIBRATION UTILITIES

There are two packages with tasks that are useful for two or more HST science instruments, and both are implicitly loaded when `hst_calib' is loaded. Placing these tasks in a separate package merely serves (in this case) to emphasize their utility for more than one instrument, and to make the instrument package menus manageable.

The `ctools' package contains several general, calibration-related tools, such as a generic calibration parameter editor, a vacuum-to-air wavelength correction task, and a task for computing encircled energy in a PSF, etc. See the `ctools' package help for details.

The `synphot' package provides a number of very powerful utilities for computing the throughput of all optical pathways in the OTA, COSTAR, and the instruments themselves (including all the gratings and filters). It can also model the response of each instrument to a variety of sources, including standard stars, black-body and power-law spectra, and user-defined spectra. Since the instrument sensitivities are part of the `synphot' database, it is particularly useful for deriving count-rates and exposure times for observing proposals. Further details can be found in the package help file, and in the "Synphot Users Guide" (available from the ST4GEM Group, send e-mail to: hotseat@stsci.edu).

Most of the general plotting and analysis tools that users need to display, analyze and interpret their HST data are available within other packages, such as the `graphics', `analysis', and `toolbox', packages.

INSTRUMENT-SPECIFIC PACKAGES

Tasks that are usually useful for only one SI can be found in a package named after that instrument. It is within these packages that the Routine Science Data Processing (RSDP) calibration code (i.e., the "calXXX" or "calibration pipeline" tasks) can be found. Other tasks in the instrument-specific packages enable users to, e.g., examine FOS or HRS acquisition images, perform post-pipeline data reduction (e.g., for spectropolarimetry), or derive accurate coordinates for stars on WFPC images.

Tasks that are used to derive the instrument calibrations, and to produce the calibration reference files for the Calibration Data Base System (CDBS), are usually found in sub-packages within the instrument packges. Generally, these tasks are of little interest to any except the instrument teams, as they are used to analyze the health and quantum efficiency of the instruments, perform trend analysis, and re-format data for CDBS. However, these tasks are distributed off-site for users who wish to create their own reference files (e.g., WFPC flat fields), or who wish to know more about how the calibrations are derived. See the package help for each instrument, the HST instrument handbooks, and the "ST4GEM Calibration Guide" for further details.

See also

acs, cos, ctools, foc, fos, hrs, nicmos, stis, synphot, wfc3, wfpc

Type "help geis" for more information about GEIS format files.