The Astrolib Synthetic Photometry simulator (pysynphot), and its astropy version Synphot, simulates photometric data and spectra as observed by HST as well as constructs complicated composite spectra from various grids of model atmosphere spectra, parameterized spectrum models, and atlases of stellar spectrophotometry.

These packages use a set of two basic types of FITS files to perform all their calculations. These are:

  • Component throughput tables (extension *_syn.fits): each such table gives the transmission curve for an individual element or "component" in the optical path of HST+Science-Instrument. For the most up-to-date files in use, please check CRDS.
  • Thermal emissivity files  (extension *th.fits): each such table lists the thermal emission curves for the infrared detectors in WFC3 and NICMOS. For the most up-to-date files in use, please check CRDS.

These files are selected and combined according to the information contained in three master tables:

  • TMG Table (extension *_tmg.fits): is the Master observatory configuration or "master graph" table. It provides with the relationship of the components to one another as encountered by a light ray tracing each of the many distinct paths from the HST Primary Mirror to the different science instruments detectors. For the most up-to-date TMG file in use, please check CRDS.
  • TMC Table (extension *_tmc.fits): is the Master component lookup table. It contains the full directory path and name of each component throughput table (*_syn.fits) used. For the most up-to-date TMC file in use, please check CRDS.
  • TMT Table (extension *_tmt.fits): is the Master thermal component lookup table. It contains the full directory path and name of each component's thermal emissivity file (*_th.fits) that should be used by all the infrared detectors. Only WFC3 and NICMOS have thermal emissivity files. For the most up-to-date TMT file in use, please check CRDS.

Pysynphot and Synphot software use these files to estimate the total throughput of any given observing mode or configuration. All these data, or updates, can be found in the MAST Archive. Note that the Exposure Time Calculators use a dedicated installation of these files, which is updated on the ETC release schedule and may not be the same as the files listed in CRDS.

Pysynphot and Synphot expect all files to be in a specific directory structure. All the throughput and thermal emissivity files are separated by instrument.  These have names according to the optical component they represent. The TMC, TMG, and TMT  tables are kept in a directory, called "mtab." They are uniquely named using crds.uniqname, which generates rootnames using the schema on the FILES AND TABLE STRUCTURES page.

If you are using Synphot or Pysynphot you might want to obtain the latest version of the data it uses in its calculations. Since the set of pysynphot data files is rather large, they are distributed in separate gzipped tar files (listed below) and can be obtained from Note that there can be issues with certain browsers (e.g., Safari). Alternatively, you can do the following:

          ftp to
          username is anonymous, password is your email address
          cd to pub/hst/pysynphot and download the desired file(s).

Please Contact the HST Help Desk with any Questions