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I. Dashevsky (Space Telescope Science Institute), J. Caldwell (York University)
The Space Telescope Imaging Spectrograph (STIS), installed in February 1997, is currently the only spectrograph in operation on the Hubble Space Telescope. STIS is designed to fulfill numerous scientific objectives using a cesium iodide photocathode Multi-anode Microchannel Array (MAMA) for 115 to 170 nm, a cesium telleride MAMA for 160 to 310 nm, and a Scientific Image Technologies CCD for 170 to 1000 nm, each with a format of 1024 x 1024 pixels. Its capabilities include spatially resolved spectroscopy at low to medium spectral resolution, echelle spectroscopy and solar blind imaging in the ultraviolet, time tagging of photons for high time resolution in the ultraviolet, and direct imaging in the optical.
Data taken with STIS is processed using a program called CALSTIS, which was developed at the Space Telescope Science Institute in Baltimore. However, with so many optical modes there are bound to be a few problems, which are not addressed by the CALSTIS calibration. We devote this presentation to the problem of grating scatter in the primary first order modes.
Light scattered from the grating structure, entrance apertures, and micro roughness of the ruled surface causes a diffused component in the first order modes. The grating scatter is evident as excess photons that contaminate about 30 nm of the shortest wavelengths in the ultraviolet spectrum. Specifically this occurs for data taken with the G230L (NUV-MAMA) grating that covers 157 to 318 nm, and the G230LB (CCD) grating that covers 168 to 306 nm. The scatter is worse for the CCD mode since the NUV-MAMA is less sensitive to red light. We also discuss a simple scatter correction for solar system objects and solar analog stars, as well as correction strategies for general targets, observed with the affected modes.
The support for this research was provided by the Spectrographs Group, Space Telescope Science Institute, operated for NASA by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
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The author(s) of this abstract have provided an email address for comments about the abstract: idash@stsci.edu