Background and Scattered-light Subtraction in the High-resolution Echelle Modes of the Space Telescope Imaging Spectrograph1
نویسندگان
چکیده
We present a simple, e†ective approach for estimating the on-order backgrounds of spectra taken with the highest resolution modes of the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. Our scheme for determining the on-order background spectrum for STIS E140H and E230H observations uses moderate-order polynomial Ðts to the interorder scattered light visible in the two-dimensional STIS MAMA images. We present a suite of high-resolution STIS spectra to demonstrate that our background-subtraction routine produces the correct overall zero point as judged by the small residual Ñux levels in the centers of strongly saturated interstellar absorption lines. Although there are multiple sources of background light in STIS echelle mode data, this simple approach works very well for wavelengths longward of Lya At shorter wavelengths, the smaller order separation (j Z 1215 A ). and generally lower signal-to-noise ratios of the data can reduce the e†ectiveness of our background estimation procedure. Slight artifacts in the background-subtracted spectrum can be seen in some cases, particularly at wavelengths of Most of these are caused by echelle scattering of strong spectral [1300 A . features into the interorder light. We discuss the limitations of high-resolution STIS data in light of the uncertainties associated with our background-subtraction procedure. We compare our backgroundsubtracted STIS spectra with the Goddard High Resolution Spectrograph (GHRS) Ech-A observations of the DA white dwarf G191-B2B and the GHRS Ðrst-order G160M observations of the early-type star HD 218915. We Ðnd no signiÐcant di†erences between the GHRS data and the STIS data reduced with our method in either case.
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