Change tO CCD Imaging Throughput Calibration
A significant change has been made to the adopted flux calibration for STIS/CCD imaging modes. The overall throughput of the STIS/CCD/MIRVIS combination as a function of wavelength has been changed as detailed in the table below. At the same time, the relative throughput of the F28X50LP filter has been increased by about 25% over most of its wavelength range.
Wavelength New/Old Throughput 1500 1.012 2625 1.014 3750 1.016 5008 0.940 6808 0.873 8608 0.833 10408 0.824Note that this new calibration gives our best estimate of the infinite aperture throughput for STIS/CCD imaging modes. Scattered light in the CCD at wavelengths greater than 8000 Angstroms causes aperture corrections to depend very strongly on the spectral energy distribution of the observed target. Sample aperture corrections for 50CCD and F28X50LP observations are given below. A STIS Instrument Science Report giving a full description of the absolute calibration of STIS imaging modes should be available in the near future.
Spectral Aperture radius used for photometry (CCD pixels) Type 5 10 20 5 10 20 50CCD Aperture Corrections (mags) F28X50LP Aperture Corrections (mags) B -0.18 -0.10 -0.06 -0.21 -0.12 -0.08 F -0.20 -0.13 -0.09 -0.24 -0.16 -0.11 M -0.33 -0.24 -0.17 -0.35 -0.25 -0.19New calibration reference files reflecting these changes have been delivered for use in pipeline processing. The only effect of these changes on calibrated STIS files will be different values for the PHOTFLAM keyword. New throughput files have also been delivered for use in SYNPHOT. Users of SYNPHOT should ensure that they obtain the updated files.
Note that these changes have no effect on the absolute flux calibration of STIS spectroscopic modes.