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Synphot Call Descriptions

Imaging countrate

The SYNPHOT “countrate” task has three required parameters: spectrum to calculate, magnitude and passband of spectrum, and science instrument. The information for the “magnitude and passband of spectrum” can be (and currently is) specified as part of the “spectrum to calculate” parameter. The other parameter is the “Science instrument” which is one of the three parts of the observation mode or ‘obsmode’.

Spectroscopic countrate

When using the SYNPHOT countrate task and specifying an observation mode that includes a disperser (grating, grism or prism), there is an additional parameter “output”. SYNPHOT will write the countrates, by λ-pixel, to the specified file. Conveniently, countrate still returns to total count rate from the source, which is needed for calculating the global count rates for MAMA detectors.

For extended sources and the sky background, these count rates must then be convolved over the width of the aperture (or the width of the target if smaller). In slitless spectroscopy, we use the conservative simplifying assumption that the background is evenly distributed over the entire detector. The alternative would require knowing the exact placement of the target source within the aperture.

Thermal countrate

The contribution from the thermal background in NICMOS observations is calculated by the “thermback” task in SYNPHOT. Thermback has only one required parameter, the “obsmode”.

An example would look like:

thermback.obsmode = “nicmos,3,f110w”

Effective wavelength

The effective wavelength of an observation is the centroid of the observation in lambda. That is to say, it is the weighted average wavelength of the curve which is the product of the target spectrum and the instrument throughput. The effective wavelength is used in the ETC for any calculation involving a wavelength dependant function such as the fraction of light in a circle of a given radius (for point sources).

The effective wavelength is calculated by the “calcphot” task which has three required parameters: obsmode, spectrum and form. The form is always flam.

An example would look like:

calcphot.obsmode = “acs,hrc,coron,f435w”
calcphot.spectrum = “rn(icat(k93models,3050,0.0,5.0),band(v),20.5,vegamag)”
calcphot.form = “flam”

The output from this example is:

WARNING Spectrum not bounded. Using nearest neighbor: cat
Mode = band(acs,hrc,coron,f435w)
 Pivot Equiv Gaussian
Wavelength FWHM
 4310.291 728.5316 band(acs,hrc,coron,f435w)
WARNING Spectrum not bounded. Using nearest neighbor: cat
Spectrum: rn(icat(k93models,3050,0.0,5.0),band(v),20.5,vegamag)
 VZERO EFFLAM Mode: band(acs,hrc,coron,f435w)
0. 4479.662

Pysynphot access (for COS only)

The cos_etccalc has been provided in STSDAS 3.8 (under PyRAF only) to provide COS ETC users with access to the pysynphot results used by the COS ETC. For convenience, this task is part of the synphot package.

cos_etccalc requires two parameters, the obsmode and the spectrum, and accepts an optional output parameter. It returns all quantities used by the ETC: the pivot wavelength, the effective wavelength, and the total countrate (in counts per second); if a filename is provided in the output parameter, it will write the output spectrum (in counts per second), which is also used by the ETC, to that file. For example:

 
 --> cos_etccalc cos,nuv,g185m,c1986 "rn(bb(15000),band(v),18.6,vegamag)"
 Obsmode: cos,nuv,g185m,c1986
  Spectrum: rn(bb(15000),band(v),18.6,vegamag)
 Pivot Effective Total
 Wavelength Wavelength COUNTRATE (counts/sec)
 1976.985602 1978.638255 4.713177

Important note regarding renormalization in GALEX bandpasses:

The GALEX magnitudes are provided in the AB magnitude system; however, pysynphot presently supports only magnitudes in the Vega system. Therefore, the ETC converts from ABmag to vegamag, using a constant offset correction, when constructing the pysynphot call. Users who wish to renormalize in a GALEX bandpass using the COS_etccalc task must apply the same correction.

The correction values are:

vegamag = ABmag(GALEX_FUV) - 2.128
vegamag = ABmag(GALEX_NUV) - 1.662

Further documentation can be obtained from the help file for the task. (help cos_etccalc)