How pysynphot works

The throughput calibration of the HST observatory is represented in a framework consisting of:

  • Component throughput files for every optical component (e.g., mirror, filter, polarizer, disperser, and detector).
  • A configuration table describing the allowed combinations of the components.

In pysynphot, a particular observing mode is specified by a list of keywords, which might be familiar names of filters, detectors, and gratings. The keywords are used to trace the light path through the observatory via the configuration graph file (a.k.a. the TMG file) which helps translating the keyword list into a list of pointers to data files that contain the individual component throughput functions. The grand throughput function of the requested observing mode is then formed by multiplying together the individual component throughput at each wavelength. (See Appendix B: OBSMODE Keywords, Appendix C: TMG, TMC, and TMT Files, and Diaz 2012 for more details on the internal structure and functioning of the configuration graph and component throughput tables.)

To retrieve a particular HST passband, you furnish the passband generator with a couple of keywords, for example, "wfc3,uvis2,f555w". The passband generator uses the keywords to trace a path through the graph file, multiplies together the component passbands it encounters along the way, and returns the passband evaluated on a particular wavelength grid. You can also generate passband in functional form (see Bandpass).

Passbands can then be convolved with spectral data to simulate HST observations of particular targets. Spectra may come from existing files containing lists of fluxes as a function of wavelength, or may be dynamically generated (individually or in combination) as simple blackbody, power-law, or continuum emission spectra of chosen temperatures and slopes (see Source Spectrum and Appendix A: Catalogs and Spectral Atlases).

Most pysynphot data I/O is done via FITS files with binary table extensions, but ASCII tables can also be used. The HST instrument graph (TMG), component lookup (a.k.a. TMC and TMT tables), and component throughput tables are all in FITS format. Input data files, such as passband throughput and spectral data files, may be in either FITS or ASCII format.

The pysynphot database

The pysynphot package is entirely data driven. That is, no information pertaining to the physical description of instruments or their throughput characteristics is contained within the software, but is instead contained within an external “database.” These data must be available in order to run any pysynphot tasks. The data set contains the HST instrument graph, component lookup, and component throughput tables, which are maintained and stored within the HST Calibration Reference Data System (CRDS) at STScI. New versions of these tables are created whenever new or updated calibration information become available for the HST instruments. Users at STScI have automatic access to the pysynphot data set on all science computing clusters. Because the data set is not currently distributed along with this software, off-site users must retrieve and install it separately before they will be able to use pysynphot. This can be done in one of two ways:

  • TAR files: Every time a new pysynphot file is delivered to CRDS, a “snapshot” of the current pysynphot data set is copied into a few TAR files, which can then be retrieved, unpacked, and installed on your system. Instructions on how to do this can be found in Installation and Setup. This method offers the convenience of automatically creating the necessary directory tree for the data.
  • Individual CRDS files: An alternate method is to transfer the individual tables using anonymous FTP (ftp.stsci.edu) to STScI from the directories given also in Installation and Setup.

The best method is perhaps a combination of the two: First-time installers may wish to use the “snapshot” TAR files to initially create and populate the directory structure, and then periodically check the CRDS area at STScI for updates to individual tables.

The instrument graph and component lookup tables are contained in the mtab/ subdirectory and are named *_tmg.fits, *_tmc.fits, and *_tmt.fits. The component throughput tables are logically grouped into subdirectories of comp/ corresponding to each of the HST instruments (acs, cos, fgs, foc, fos, hrs, hsp, nicmos, nonhst, ota, stis, wfc3, wfpc, and wfpc2). Component throughput table names contain a three digit suffix indicating their version number. You can determine which tables are new by comparing either their names or creation dates with the corresponding set of tables installed on your machine.

How Accurate are the Synthetic Photometry Results from pysynphot?

Because the pysynphot package is entirely data driven, the accuracy of its results depends entirely on the accuracy of the bandpass sensitivity curves, and zero points in the pysynphot database. The accuracy of these values is dependent on the instrument and photometric system under consideration.

As a general rule of thumb, synthetic photometry involving photometric systems that have been defined from the ground, or photometry that is given in vegamag, should only be considered accurate to about 5%. The accuracy is a strong function of wavelength and in particular for the available HST Calibration spectra, the accuracy might be about 5% shortward of \(1700 \AA\), where IUE is used, and around 2% over the STIS range. The accuracy is > 5% at the longer IR wavelengths where the dust ring emission dominates (around 2 microns).

Synthetic photometry with the stable HST instrumentation, flying above the atmosphere, when used in HST instrument natural systems, without reference to vegamag, can achieve accuracy much better than 5%; for example, for ACS broad band filters it can be less or about 1% (De Marchi et al. 2004). For more details, see the Data Analysis section in the Data Handbooks for the respective HST instruments.

Can pysynphot be Used for Other Telescopes?

Because the tasks in the pysynphot package are data driven, instrument observing modes can be changed and new instruments added without changing the software. To use pysynphot with non-HST instruments or components you would need to modify (or rebuild) only the instrument graph and component lookup tables. In addition, you also need to set the appropriate telescope collecting area.

For the tables, pysynphot requires:

  • One instrument graph table.
  • One component lookup table.
  • One thermal component lookup table (only needed for thermal background calculations for NICMOS and WFC3)
  • One throughput table for each telescope and instrument component that appears in the graph and component lookup tables.

The names of the instrument graph and component lookup tables to be used by pysynphot are set by setref(). The names of the individual component throughput tables are contained in the component lookup table and are located automatically when needed. See Appendix C: TMG, TMC, and TMT Files for details on the structure of these tables. To build your own instrument graph and component lookup tables, it is perhaps the easiest to either start with a copy of the existing HST tables and modify or add to them, or at least use the HST tables as a model for your own tables.

To make use of your own custom graph and component lookup tables, and telescope area in pysynphot, just change the relevant values in setref().