Observatory Support
STScI Exposure Time Calculators Oversight

Pysynphot Call Parameters

Observation Mode

The observation mode, or “obsmode” parameter is actually three separate parameters in many pysynphot tasks (instrument, detector and spec_el). Though the values can be specified separately, the ETC typically specifies the entire obsmode in the instrument parameter and leaves the other two blank. Thermback and calcphot both have a single obsmode parameter.

Spectra

CDBS contains a wide variety of input spectrum files. These are used by SYNPHOT and pysynphot for their calculations. Currently pysynphot only supports the functionality needed by the ETC; however, SYNPHOT is still provide a number of functions for generating synthetic spectra and for manipulating spectra. SYNPHOT's capabilities are much more powerful and diverse than is needed for most observers. In general terms, the ETC uses only one continuum and/or up to three emission lines. An input file or synthetic spectra may be red shifted, reddened and/or renormalized.

Kurucz Models

One of the catalogs of spectra described in Appendix B of the SYNPHOT User’s Guide is the Kurucz Model Atmospheres. The ETC maps the spectral types to a set of parameters for a pysynphot “icat” function call. The parameters for the icat call for the Kurucz models are: catalog name (k93models), effective temperature, metallicity and log surface gravity. For example, the expression for a F2V star would be icat(k93models,6890.0, 0.0,4.3).

Table 6 provides with a list of the available Kurucz models and the corresponding stellar parameters for these modes.

Table 6: pysynphot Parameters for Kurucz Model Stars

Bruzual Synthetic Stellar Spectra

These spectra are stored in CDBS and the correct file names can be determined from table B4 in the SYNPHOT User’s Guide.

HST Standard Star Spectra

These spectra are stored in CDBS and listed in the SYNPHOT User’s Guide in Table B3. The tricky part about using these spectra is to identify to identify the best spectra to use. In some cases the spectra is updated with new data or models and these are kept under separate versions. If a new classification is made for the same star, the version number stats with “001”. Each time a particular version is update, this value is incremented by one. The best way to identify which is the most up-to-date and best version of the spectra to use is to look in the CDBS web pages. In here the complete list of the HST calibration spectra can be found under the Trougpht Tables (CALSPEC) link. In this table the recommended spectra, usually the one with the best resolution and broadest wavelength range is listed first, at the left most column.

Non-Stellar Objects

At the time that this document is being written, all of the spectra in this category are stored in the home directory of the ETC itself. Please contact the ETC development team for copies of these files.

Synthetic Spectra

Black body spectra are implemented using the pysynphot function “bb” which takes one parameter, the temperature of the object. For example “bb(5500)”.

Power law spectra are implemented using the pysynphot function “pl” with Jansky units, a reference wavelength of 4000 Angstroms and some user specified exponent. The standard exponent used is –1 which would have an expression of “pl(4000,-1,jy)”.

A flat spectrum is a spectrum with constant energy per either wavelength unit or per frequency unit. Flat spectra are implemented using the pysynphot function “unit” with a value of 1 and either fnu or flam for the form. Note that countrate calculations are done using photons per wavelength unit, as a result, the plots of both forms are not actually flat when used in a count rate calculation.

Emission lines

Up to three emission lines can be superimposed on the input spectrum by the ETC. For a pure emission line spectrum, use the “No continumm” option. Emission lines are specified to the “countrate” task in pysynphot as part of the “spectrum” task parameter.

An example would look like this (Hα and [NII] lines with no continuum):

countrate.spectrum=''(em(6563.0,1.1,3.05E-14,flam)+em(6583.0,0.6,2.85E-15,flam)+em(6548.0,0.6,1.05E-15,flam))''

Sky spectra

Count rates from the sky background are calculated by the same “countrate” pysynphot task used to compute the source counts, the sole difference is in the spectrum expression used in the calculation.

The sky background is derived from two master files that contain the contributions from Earth shine and zodiacal light. The Earth shine contribution is normalized via a multiplicative scale factor, while the zodiacal light contribution is normalized by a surface magnitude specified in Vega magnitudes per square arcsec in the Johnson V band.

Geo-coronal lines are added separately using the “em” function as described in Emission lines, above.

An example would look like this:

countrate.spectrum=''((earthshine.fits*0.5)+rn(spec(Zodi.fits),band(V),22.7,vegamag))''