Physics
Scientific paper
Jun 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998stis.rept...18b&link_type=abstract
STIS Instrument Science Report 98-18, 99 pages
Physics
Hubble Space Telescope, Hst, Space Telescope Science Institute, Stis, Space Telescope Imaging Spectrograph
Scientific paper
Point source sensitivity curves for the 0.2x0.2 arcsec entrance aperture are derived for the 12 prime echelle medium and high dispersion modes, which collectively span a wavelength range from 1140Å to 3150Å. The sensitivity for each order is determined from observations of spectrophotometric standard stars by fitting a set of spline nodes to the ratio of the observed count rate divided by the standard star flux. Since both the average absolute sensitivity and the shape of the ripple correction are expected to change smoothly with echelle order number, the effects of spectral features are avoided by masking the strongest absorption lines and then fitting smooth functions of order number. In general, the uncertainty in the calibration is determined by the number of observations, by the repeatability among the observations, and by the uncertainty of 3% in the absolute fluxes of the standard stars. A 3% repeatability limit is set by the observed drop in the global count rate between a pair of observations of G191B2B with E140M on consecutive orbits, where the spectra were at the same place on the FUV MAMA within 1 pixel. Since most of the echelle modes have only one observation, the above two 3% uncertainties combine in quadrature to a 4% one sigma expectation for the bulk of the coverage. For the worst case at 1151Å, scatter about the average of +10 to -7% is caused by a low signal level that is near the background. There are greater uncertainties for the particular cases of E140H-1234 and E140H-1598. These calibrations are based on BD+75 325, where a plethora of absorption lines at wavelengths below 1900Å, confuses the analysis and causes errors of 10-20% in the relative flux within one order.
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