Synphot Data User's Guide
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| Synphot Data User's Guide | |||||
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A.1 HST Calibration Spectra
The directory
crcalcspec$contains the composite stellar spectra that are the fundamental flux standards for HST calibrations. All files are in machine independent binary FITS table format. Information about the pedigree of a given spectrum is in the header of the FITS table file, which can be read with the IRAF hedit task or examined using pyFITS.
In some cases, the spectrum is truncated in the blue (or in the red) at wavelength longer (shorter) than the sensitivity limit of the instrument. As a result, synphot may underestimate the total counts. Users should check that the wavelength range of the spectrum/model they are using is compatible with the wavelength range of the calculation they require.
Table A.1 below summarizes the set of recommended standard star spectra. Columns 2 and 3 give the V and B magnitudes of the stars. More documentation on these stars, e.g. coordinates, finding charts and spectral types can be found in Turnshek et al. (1990), Bohlin, Colina & Finley (1995), and Colina & Bohlin (1997). Column 4 is the computer compatible file name with the plus and minus signs converted to underscores. Thus, the actual CALSPEC file name is the prefix in column 4, plus one of the suffixes in columns 5-8, plus ".fits". For example, a standard that has STIS data is "bd_28d4211_stis_001.fits".
These spectra can be used with synphot tasks by setting either the
spectrumorspfiletask parameters to the name of the desired file, e.g.,crcalspec$alpha_lyr_stis_002.fits. The unit of flux in all files is erg s-1 cm-2 A-1.The pure hydrogen WD model LTE spectra of Bohlin (2000) have been replaced by Hubeny NLTE models calculated with his Tlusty code (Bohlin 2003). These fundamental standards GD71, GD153, G191B2B, and HZ43 are listed by Calibration Data Base System (CDBS) suffix in column 5 of the Table and provide the basis for the secondary flux standards. The observational spectra from columns 5-8 can be compared with the models; and in the case of G191B2B, there is ultraviolet line blanketing at the few percent level. The model calculations extend to 30 microns and cover the long wavelength limits of 2.7 microns for NICMOS, 1.1 microns for STIS and ACS, and 27 microns for JWST. Vega was observed by STIS (Bohlin & Gilliland 2004); and the new composite flux standard alpha_lyr_stis_002.fits consists of IUE data from 1255-1675A, STIS CCD fluxes from 1650-4200A, and a specially tailored Kurucz model longward of 4200A (Kurucz 2003) and from 900-1255A.
Column 6 lists the 20 CDBS suffix names for the second choice standard stars with STIS data from Bohlin, Dickinson, & Calzetti (2001). Tabulated in column 7 are the next best standard star flux distributions, which are composed of FOS spectra in the UV and Oke spectra at the longer wavelengths. Also appearing in column 7 are the three solar analogs that are comprised of FOS observation but do not have "_FOS" in the CALSPEC file name.
The names for the last, but largest, set of standard stars appear in column 8 of the Table. The application of corrections to the original IUE and optical fluxes produces a consistent set of spectrophotometric standards from 1150 to 9200A (Bohlin 1996 and references therein). This set of standards is composed of IUE+Oke data only.
CALSPEC also contains the ultraviolet to near-infrared absolute flux distribution of the Sun (filename: sun_reference_stis_001.fits) to 2.7 microns, which is used to model the IR spectrum of the three solar analog stars. The solar reference spectrum combines absolute flux measurements from satellites and from the ground with a model spectrum for the near-infrared (Colina, Bohlin, & Castelli 1996).
The
SUPPLEMENTAL_CALSPECdirectory contains previous CALSPEC versions and spectrophotometry that may be useful for special purposes.The order of preference for the choice of a standard flux distribution is from left to right in the Table A.1, i.e. from the best in column 5 to the last choice with the lowest quality in column 8.
Table A.2 contains the spectral type of some of the HST calibration sources.
Table A.1: CDBS Flux Standards with Columns in Order of Preference
(1) The unit of flux in all files is erg s-1 cm-2 A-1.
Table A.2: HST Calibration Spectra
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