Synphot Data User's Guide


A.2 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.

There is a table on the CALSPEC web page that summarizes the set of recommended standard star spectra. It includes information about the spectral type, magnitude, color, and CDBS filename for the optimal and secondary choices for standard star model and observed spectra, This table is located at

These spectra can be used with synphot tasks by setting either the spectrum or spfile task 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.

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_CALSPEC directory contains previous CALSPEC versions and spectrophotometry that may be useful for special purposes.

Table A.2 contains the spectral type of some of the HST calibration sources.

Table A.2: HST Calibration Spectra
Star Type
AGK +81 266 sdO
alpha Lyra A0 V
BD +28 4211 Op
BD +33 2642 B2 IV
BD +75 0325 O5p
Feige 34 DO
Feige 110 D0p
G93-48 DA3
G191-B2B DA0
GD50 DA2
GD108 sdB?
GRW +70 5824 DA3
HD 93521 O9 Vp
HZ 2 DA3
HZ 4 DA4
HZ 21 DO2
HZ 44 sdO
LB 227 DA4
LDS 749B DB4
NGC 7293

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