Characterization of Pt/Cr-Ne Hollow-Cathode Lamps for Wavelength Standards in Space Astronomy

The Space Telescope European Co-ordinating Facility (ST-ECF) and National Institute of Standards and Technology (NIST) are collaborating to study hollow cathode calibration lamps as used onboard the Hubble Space Telescope (HST). The project has two main components: First, we have observed the spectra of Pt/Cr-Ne lamps to obtain wavelengths for all emission lines between 115 nm and 320 nm. The observations were made at NIST using the 10.7-m normal-incidence spectrograph and a Fourier transform spectrometer (FTS) optimized for the vacuum ultraviolet (UV). The spectral region corresponds to the Space Telescope Imaging Spectrograph (STIS) Echelle modes. Wavelength calibration of all HST spectrographs has been based on the line list produced by Reader et al. (1990) using a Pt-Ne lamp, despite the fact that STIS and the Faint Object Spectrograph (FOS) use a Pt/Cr-Ne lamp. The addition of Cr is especially significant in the near UV where up to 90% of the observed lines are Cr. However, published Cr wavelengths are not sufficiently accurate for the calibration of STIS and FOS. Our work has established accurate wavelengths for more than 5000 Cr lines. The uncertainty is 2×10 nm for lines in the 110 nm to 180 nm region measured with the grating spectrograph and 10 nm for the 180 nm to 320 nm region observed with the FTS. Second, we are attempting to better understand the performance of hollow cathode lamps and the physical processes involved in their long-term operation. Among the issues we have studied are the dependence of the spectrum on lamp current and cumulative operating time. We have performed accelerated aging tests that simulate operations on STIS using newly made space qualified lamps. Our new Pt/Cr-Ne line list has been successfully applied in the STIS Calibration Enhancement effort; hence these results lead directly to an improvement in the quality of scientific data obtained from existing HST spectrographs. We have also documented the sensitivity of the lamps’ performance to optical alignment in the case of lamps that use a MgF2 lens as window. Our findings constitute important lessons for the design and operation of future UV and optical spectrographs in space. affiliated to the Space Telescope Operations Division of Research and Scientific Support Department of the European Space Agency