Commissioning Report


Some results may change as codes (be it this package, ASTROLIB, or their dependencies) change. It is recommended that the commissioning tests be re-run from time to time and this report updated accordingly, as needed.

This report covers the commissioning tests of stsynphot against ASTROLIB PYSYNPHOT. Given that ASTROLIB PYSYNPHOT was tested against IRAF SYNPHOT in TSR 2009-01 (Laidler 2009), we can infer that any agreement to ASTROLIB PYSYNPHOT is also an agreement with IRAF SYNPHOT.

As stsynphot is not a straight port from ASTROLIB nor IRAF, but rather a new implementation designed to use astropy.modeling, a 100% agreement between the difference software is impossible. However, an extensive commissioning process was performed to verify that the results produced by stsynphot are either as good as, or better than, the results obtained using ASTROLIB PYSYNPHOT for the same calculations.

This runs all the commissioning tests in addition to regular unit tests:

pytest --pyargs stsynphot docs --remote-data --slow --html=/path/to/report.html

Since this involves detail comparison with ASTROLIB PYSYNPHOT using various spectra and observing modes, it is important that the tests have access to the ASTROLIB software and CRDS Database. Here are the commissioning test results (might take time to load as it contains results for over 10,000 tests) for the following software versions:

  • stsynphot 0.1b2

  • synphot 0.1b2


  • Astropy 1.3

  • Scipy 0.17.1

  • Numpy 1.11.1

  • Python 3.5.2

  • pytest (version not recorded)

  • pytest-html (version not recorded)

To ensure repeatability, the graph and component tables are fixed at a specific version, as follows. Since we are only testing for software agreement, the fact that these tables might be outdated is irrelevant:

  • 07r1502mm_tmg.fits

  • 07r1502nm_tmc.fits

  • tae17277m_tmt.fits

Tests were only written for HST instruments that are currently active, i.e, ACS (HRC, SBC, and WFC), STIS (CCD, FUV-MAMA, and NUV-MAMA), and WFC3 (IR, UVIS-1, and UVIS-2). The specific observing modes tested were ported directly from ASTROLIB’s commissioning tests against IRAF. Reddening laws that are no longer supported (e.g., gal1) were replaced with something else that were similar (e.g., gal3); This should not affect the integrity of the tests as both software under test use the same reddening law anyway.

For each observing mode, the wavelength set and flux/throughput in internal units (Angstrom and PHOTLAM) were compared for all bandpass, source spectrum, and observation involved. Exceptions are made for known differences, for instance, when ASTROLIB uses an arbitrary default waveset while Astropy does not (those comparisons are skipped). For an observation, its count rate and effective wavelength were also compared. When applicable, bandpass thermal source and background count rate were compared as well.

For stysnphot 0.1b2, a total of 10,026 tests were run and only 160 (1.6%) failures found. All the failures are understood and explained below:

  • Astropy models computed very small flux values, while ASTROLIB set those to zeroes. This is especially true for blackbody source, for which ASTROLIB has some IRAF-like (32-bit) “rounding to zero” behaviors.

  • Both Astropy and ASTROLIB models computed very small flux values (anywhere from 1.8E-26 to 2E-311 PHOTLAM) but they are not within 1% agreement. This is not surprising given that 1% of such small values is too small to be realistic for comparison.

  • Expected source spectrum waveset differences between Astropy and ASTROLIB for blackbody and flat source spectra. In stynphot, the former model includes more “tail” region, while the latter does not use arbitrary default waveset anymore.

  • Thermal source calculations in stsynphot has a different (and arguably better) sampling, resulting in flux values that do not agree with ASTROLIB within 1%. Despite this, thermal background results still agree.

In conclusion, these commissioning tests have shown that synphot and stsynphot are robust enough to replace ASTROLIB PYSYNPHOT and IRAF SYNPHOT for most cases.

Future Work

Here are some possible follow-up action items for this commissioning test report:

  1. Instrument teams, who wish to utilize stsynphot for their Exposure Time Calculators, should independently run comparison tests for the observing modes and the source spectra that are relevant to them. Using those results and the information provided here, they should decide for themselves whether this software is good enough for their needs.

  2. The known failures reported here may be revisited so that they could be quantified and understood further. More understanding could result in code changes that would lead to even better agreement.