16442

STIS spectra taken using spatial scanning potentially have a number of potential advantanges. In addition to enabling the collection of more photons before saturating comma scanning an external point source perpindicular to the dispersion direction produces an illumination pattern similar to that produced by an internal lamp illuminating a long comma narrower slit. This may make corrections for IR-fringing and pixel-to-pixel variations more accurate. This proposal will test the repeatability of fluxes determined from STIS spatial scans by taking a series of spectroscopic exposures with the STIS G750L grating of the known exoplanet host 55 Cnc comma trailed in the large 52X2 aperture. Deep contemporaneous lamp flats comma taken through the narrower 52X0.1 aperture comma will be used to remove the fringing at wavelengths beyond about 7000 A. Similar (but more limited) observations obtained in program 15383 yielded rms scatter of order 30 ppm in the total d_commawhite lightd_comma fluxes when d_commade-trendedd_comma to remove systematic instrumental effects. This longer program will include a transit of 55 Cnc e comma in order to gauge how well the transit can be characterized in this observing mode.