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E. M. Malumuth, R. S. Hill (Raytheon ITSS/NASA GSFC), T. R. Gull, B. E. Woodgate, C. W. Bowers, R. A. Kimble (NASA GSFC), D. Lindler, P. Plait (Advanced Computer Concepts/NASA GSFC), M. Blouke (Scientific Imaging Technologies)
We have developed a model that allows us to defringe slitless 2-dimensional spectra taken with the Space Telescope Imaging Spectrograph (STIS).
Like all thin chip CCDs used in astronomy, the STIS CCD detector acts like an interferometer in the presence of monochromatic light, producing bright and dark fringes where there is constructive and destructive interference. These fringes are especially troublesome for spectra in the near-IR (\geq 7000Å), reaching a peak amplitude of ~20% near 9500Å. Removing fringes from STIS spectra taken with a slit is simply a matter of dividing by an appropriate ``fringe flat''. A suitable flat is obtained by observing a continuum calibration source through the same slit. This however is not possible for slitless spectra, whose wavelength mapping onto the CCD detector varies with position of the object in the field. Beginning with an approximate knowledge of the STIS CCD structure, we have used 50 continuum source spectral flats taken at various central wavelengths to constrain the thickness of the detection layer at each pixel. From this result, we can compute a fringe flat for any astronomical source, no matter how it is positioned in the field. Flats made by this method reduce the fringing amplitude in astronomical spectra by a factor of ~5.
The model fringe flat is a function of the index of refraction of each of four layers in the CCD, the thickness of each layer, and the wavelength of the light hitting each pixel. We show how the CCD structural parameters are incorporated into this ``fitting function'', and describe the procedure used to solve for the free parameters (the thickness of each layer at each pixel). The effectiveness of this technique will be demonstrated using slitless spectra taken as part of the STIS parallel program.
The same technique with a similar fitting function should be applicable to other CCDs used in astronomy.
This work was done with the support of the HST project through the STIS IDT and operations at STScI.