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 0.824
Note 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 -0.19
New 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.
|
