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Session 18 - STIS.
Display session, Wednesday, January 07
Exhibit Hall,

[18.15] Flat Fielding and the Achievable Signal-to-Noise of STIS Ultraviolet Point Source Spectroscopy

M. E. Kaiser (The Johns Hopkins University), D. J. Lindler (Applied Computer Concepts, Inc), R. C. Bohlin, R. A. Shaw (STScI)

The Space Telescope Imaging Spectrograph (STIS) was designed to achieve a signal-to-noise of at least 100:1 per resolution element. In the ultraviolet STIS employs a CsI Multi-Anode Microchannel Array and in the NUV, a CsTe MAMA detector is employed. These detectors are not as ubiquitous as the CCDs employed in the optical region of the spectrum. Consequently the characterization of this detector technology is less thorough. As a result the flat fielding capability of these detectors for flight spectra has been a key issue with the deployment of STIS.

>From analysis of a single spectrum of GD153, with a potential S/N from pure counting statistics of \sim165, a S/N of \sim130 per spectral resolution element is realized over the spectral region from 1280-1458ÅIn the NUV, a single spectrum of GRW+70D5824 with a potential S/N from pure counting statistics of \sim200, yields a S/N of \sim150 per spectral resolution element over the range from 2167 - 2520ÅP>To further test the realizable S/N of a point source spectrum, six spectra of GRW+70D5824 were coadded. This increased the S/N achievable from pure counting statistics to \sim300. Application of the FUV flat field to the combined stellar spectrum yielded a S/N of \sim190 per spectral resolution element. It should be cautioned that the coadded spectra are neither spatially nor spectrally coincident. The slight offset of 4 pixels in the spatial direction will slightly smooth the data, consequently a single point source exposure with sufficient counting statistics may achieve a S/N slightly less than the \sim190 quoted here. For a futher description of the flat field construction and S/N results see M.E. Kaiser et al., 1997 HST Cal. Workshop, S. Casertano et al.,eds.

Using FP split slits to acquire spectra, and the corresponding iterative analysis, has produced even higher S/N spectra (R.A. Shaw et al., these proceedings).

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