- [*] The HST/FOS wavelength scale
- van der Marel R.P.
- in the `1997 HST Calibration Workshop',
Casertano S, et al., eds.,
Space Telescope Science Institute, Baltimore, p. 443-448, 1997
- © 1997. Space Telescope Science Institute.
All Rights Reserved.
I analyze the accuracy of the FOS pipeline wavelength calibration for
the FOS/RD detector with the G570H grating. I use observations of arc
spectra and external targets that I obtained in the context of studies
of galactic nuclei, complemented with an observation of the planetary
nebula NGC 6833 obtained by the FOS Instrument Science Team. The
combined data were obtained in five visits spread over the period
August 1995 to January 1997. I find that the absolute wavelength
calibration generated by the pipeline for these visits is in error by
0.2-1.2 diodes, in the same direction for all visits. The mean of the
errors is 0.62 diodes, where 1 diode = 4.37 Ang. The error is largest
for the two visits that used the paired apertures. These results
indicate that the grating-wheel non-repeatability is larger than
believed, or that (some of) the dispersion solutions in the pipeline
may need to be modified by a constant offset. There are variations in
the S-distortion between visits at the level of 0.1 diodes. The RMS
shift between arc spectra obtained in the same visit, as a result of
residual GIMP errors, is less than 0.04 diodes. The average
internal/external offset determined from these data is consistent with
the constant value that has been used by the pipeline since 1992. No
dependence is found on aperture or epoch. However, there is an
indication that the internal/external offset varies systematically by
0.15 diodes between the blue and the red side of the grating. If true,
this translates directly into a systematic error in the relative
wavelength scale for all G570H FOS/RD data in the HST Data Archive.
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Last modified December 8, 1998.
Roeland van der Marel,