STIS signal-to-noise capabilities in the ultraviolet

The Space Telescope Imaging Spectrograph (STIS) was designed as a versatile spectrograph for the Hubble Space Telescope (HST) capable of maintaining or exceeding the spectroscopic capabilities of both the Goddard High Resolution Spectrograph (GHRS) and the Faint Object Spectrograph (FOS)aver the broad bandpass extending from the ultraviolet (115 nm) through the visible (1 mu m) STIS achieves performance gains over the aforementioned first-generation HST instruments primarily through the use of large (1024 x 1024) areal detectors in both the ultraviolet and visible regions of the spectrum. Simultaneous spatial and spectral coverage is provided through long-slit or slitless spectroscopy of extended sources. A substantial spectral multiplexing advantage is achieved for ultraviolet echelle spectroscopy. This paper will focus on the key issue of signal-ronoise ratio (S/N) performance with the STIS ultraviolet detectors. Spectra obtained during the first few months of operation illustrate that high S/N spectra can be obtained while exploiting STIS's multiplexing advantage. From analysis of a single spectrum of GD 153, with count statistics of similar to 165, a S/N of similar to 130 is achieved per spectral resolution element in the far-ultraviolet (FUV) for a flat-fielded spectrum. Without flat-fielding, a S/N of similar to 85 is achieved. In the near-ultraviolet (NUV), a single spectrum of GRW +70 degrees 5824, with count statistics of similar to 200, yields a S/N of similar to 150 per spectral resolution element for the flat-fielded spectrum. Without Aat-fielding, a S/N of similar to 100 is achieved. An even higher SM capability is illustrated through the use of the fixed pattern (FP) split slits in the medium-resolution echelle modes. Observations of ED 28 degrees 4211 yield a SM of similar to 250 and similar to 350 per spectral resolution element over an extended spectral region in the FUV and NUV, respectively. For the same spectral region, the S/N without the application of a flat, or the use of specialized iterative solution, yields a S/N of similar to 205 in the FUV and similar to 290 in the NUV. The corresponding S/N from pure count statistics is similar to 285 in the FUV and similar to 380 in the NW. Selective regions of the ED 28 degrees 4211 echelle spectrum with suitable. count statistics yield ever higher S/Ns. These higher S/Ns of similar to 390 in the FUV and similar to 380 in the NUV, are quoted for narrow spectral regions spanning a small areal extent on the detector and may not reflect the SM capability over the full detector. These results verify that STIS is capable of achieving a S/N in excess of 100:1 per spectral resolution element in both the first-order and echelle modes.