Jul 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004hst..prop10203a&link_type=abstract
HST Proposal ID #10203
Other
Hst Proposal Id #10203 Cool Stars
Scientific paper
Space Telescope Imaging Spectrograph is the most sophisticated space-borne spectrometer ever built, probably the last of its kind for some time to come. A key virtue of STIS is that its medium- and high-resolution echelle modes provide access to broad intervals of the vacuum ultraviolet spectrum in a single shot. Another virtue is the validation of the wavelength scales by periodic observations of an onboard hollow-cathode emission line source. Tying together the different echelle orders by means of the accurately known lamp spectrum enables a wide range of studies that exploit differential comparisons of velocity diagnostics in stellar, interstellar, and even extragalactic spectra. Despite the importance of the wavelength calibrations, however, they are done only infrequently {once a year}. While STIS undoubtedly must be one of the most stable orbiting spectrographs ever designed, possible thermal distortions of the instrument might cause small nonlinear deviations of the wavelength scales and thereby impact the velocity precision. The existing wavecal data sets are separated too far apart in time to isolate short-term thermal fluctuations from long-term secular behavior, and the routine WAVELINEs taken with every grating switch are too underexposed to reveal any differential behavior across the spectrum, aside from a simple zero-point offset. I therefore propose to obtain a series of deep lamp exposures in a few representative modes of the NUV and FUV MAMAs to search for and characterize short-term differential distortions of the echelle formats. This work also will provide an important dataset to test new approaches to derive the basic dispersion relations, such as the effort underway by the Physical Modeling Group at ESA's ST-ECF. The calibration campaign can be scheduled in pure parallel mode with no loss of orbits from the Cycle 13 science program.
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