Scattered Light in the STIS Echelle Modes

The STIS echelle spectra obtained during the Early Release Observations (Heap et al. 1997, Walborn et al. 1997, Jenkins et al. 1997) have non-zero residuals in the cores of saturated interstellar lines, indicating the need for a scattered light correction. A rough measure of the magnitude of the needed correction is given in Figures 1 and 2, which show the ratio of the interorder to the in-order flux in different echelle modes in both pre-launch calibration images of a continuum lamp source (Figure 1), and in post-launch images of stellar continuum sources (Figure 2). The interorder and in-order fluxes are computed by averaging the central 200 pixels in the dispersion direction. The amount of scattered light in the interorder region rises toward shorter wavelengths for two reasons: (1) the order separation decreases toward shorter wavelengths, and (2) the amount of echelle scattering is expected to have a inverse dependence on wavelength (e.g., Cardelli, Ebbets & Savage 1990). At the shortest wavelengths the fraction of light scattered into the interorder region can be 10% for the NUV-MAMA and 15% for the FUV-MAMA.