Removing the Fringes from Space Telescope Imaging Spectrograph Slitless Spectra11Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5‐26555. Funding of this activity was through the Space Telescope Imaging Spectrograph Guaranteed Time Observations.

Abstract

ABSTRACTUsing what is known about the physical and chemical structure of the CCD detector on the Space Telescope Imaging Spectrograph (STIS) and over 50 calibration images taken with different wavelength mappings onto the detector, we have devised a model function that allows us to predict the fringing of any spectral image taken with the STIS CCD. This function is especially useful for spectra taken without a slit with the G750L grating. The STIS parallel observing program uses this “slitless spectroscopy” mode extensively. The arbitrary mapping of wavelength versus position that results from each source's chance position in the field renders direct calibration of the fringe amplitudes in this mode impossible. However, we find that correcting observed data using our semiempirical fringing model produces a substantial reduction in the fringe amplitudes. Tests using the flux calibration white dwarf standard G191‐B2B show that we can reduce the fringe amplitude in the 9000–10000 Å region from about 20% peak to peak (10% rms) to about 4% peak to peak (2% rms) using the model, while a standard calibration using a “fringe flat” reduces the fringe amplitudes to 3.3% peak to peak (1.7% rms). The same technique is applicable to other astronomical CCDs.