STIS signal-to-noise capabilities in the ultraviolet

Mary Elizabeth Kaiser, Ralph C. Bohlin, Don J. Lindler, Ronald L. Gilliland, Vic S. Argabright, Randy A. Kimble

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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) over the broad bandpass extending from the ultraviolet (115 μm) through the visible (1 μm). STIS achieves performance gains over the aforementioned first-generation HST instruments primarily through the use of large (1024 × 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-to-noise 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 ∼165, a S/N of ∼130 is achieved per spectral resolution element in the far-ultraviolet (FUV) for a flat-fielded spectrum. Without flat-fielding, a S/N of ∼85 is achieved. In the near-ultraviolet (NUV), a single spectrum of GRW +70°5824, with count statistics of ∼200, yields a S/N of ∼150 per spectral resolution element for the flat-fielded spectrum. Without flat-fielding, a S/N of ∼100 is achieved. An even higher S/N capability is illustrated through the use of the fixed pattern (FP) split slits in the medium-resolution echelle modes. Observations of BD 28°4211 yield a S/N of ∼250 and ∼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 ∼205 in the FUV and ∼290 in the NUV. The corresponding S/N from pure count statistics is ∼285 in the FUV and ∼380 in the NUV. Selective regions of the BD 28°4211 echelle spectrum with suitable count statistics yield ever higher S/Ns. These higher S/Ns of ∼390 in the FUV and ∼380 in the NUV, are quoted for narrow spectral regions spanning a small areal extent on the detector and may not reflect the S/N 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.

Original languageEnglish (US)
Pages (from-to)978-990
Number of pages13
JournalPublications of the Astronomical Society of the Pacific
Issue number750
StatePublished - Aug 1998

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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