Monitoring of the wavelength calibration lamps for the Hubble Space Telescope

The Space Telescope Imaging Spectrograph (STIS) and the Cosmic Origins Spectrograph (COS) are the two optical-UV spectrographs on board the Hubble Space Telescope. To determine the wavelength scale for individual science observations, internal arc lamp spectra accompany most observations of external targets. Here we present a detailed analysis of the changes in the COS and STIS internal lamp fluxes and spectra over time, and also compare our results to pre-launch ground testing, and to laboratory accelerated aging testing of similar lamps. Most of the analysis presented here focuses on the behaviour of the lamps in the far-UV (FUV). We find that the STIS LINE lamp has faded by a factor of ~15 in the very short FUV wavelengths (1150-1200A) over the 13-year period on which STIS was in space, a much steeper fading than predicted from accelerated aging tests in the laboratory. We also find that all STIS lamps have faded during the period in which the spectrograph was not operational (2004-2009) thus pointing to on-orbit conditions as an additional and important cause of lamp fading. We report that the COS P1 lamp output appears to decline with usage with a similar slope as the LINE and HITM1 lamps on STIS. Finally, we recommend switching from the LINE to the HITM2 lamp for a more efficient wavelength calibration of the STIS settings covering the very short FUV wavelengths.