The HST/FOS Wavelength Scale

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 = 4.37Å. 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 GIM errors, is ∼< 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 ends 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.