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Space Telescope Imaging Spectrograph
Updates to STIS Spectroscopic Throughputs

On 2005-Apr-04, a number of new photometric throughput (pht) and time-dependent sensitivity (tds) reference files were delivered that provide significant improvements to the absolute flux calibration for 1D extractions of STIS low-dispersion, PRISM, and echelle modes.

Throughputs for G140L and G230L have increased by about 2% at most wavelengths (thereby giving extracted fluxes that are lower by about 2%), with somewhat larger changes at the shortest wavelengths of each grating and in the vicinity of the Lyman-alpha line.

Throughputs for CCD gratings have not changed with this release. Once corrections for time-dependent-sensitivity (TDS) changes and charge transfer inefficiencies (CTI) are made, the absolute flux accuracy for all STIS low-dispersion observations done using the 52X2 aperture should have relative accuracies of 3% or better and an absolute accuracy of better than 5% (see discussion in Bohlin et al. 2000, AJ 120,437).

For E140H and E140M the new sensitivities are typically 3 to 20% higher, with substantially larger changes at the very shortest wavelengths near 1140 Angstroms. E230H and E230M sensitivities are changed by less than 10% at most wavelengths. TDS corrections are now included for all echelle observations, and for echelle observations using primary wavelength settings and the 0.2X0.2 aperture, typical flux accuracy should be ~5%, although isolated individual observations may occasionally be sufficiently miscentered in the 0.2X0.2 aperture to have a more noticeable flux loss.

The new NUV MAMA PRISM throughput curves differ from the previous ones by up to 15%, and extracted x1d PRISM fluxes now also include TDS corrections. The absolute accuracy of these observations should now be about 5%. A new and more accurate photometric correction table (pct file) which corrects the flux for the vertical extent of the chosen extraction region has also been delivered. An error in the previous prism pct file, which could cause errors in 1D-extracted fluxes of up to 25% for slitless PRISM data, has been corrected.

Note that there are a number of issues for the calibration of extracted fluxes for point source spectra that have yet to be fully addressed.

We hope to address most of the above issues in the next few months and deliver a final flux calibration for 1D spectral extractions by the end of the year.