Corrections for Scattered Light in STIS Grating G230LB

The G230LB grating used with STIS’s CCD detector scatters red light. In red objects, the scattered light mingles with the ultraviolet signal, causing spurious short-wavelength flux and weakening absorption features. Here, we report on calibration observations that characterize the scattered light using duplicate observations with the MAMA detector and similar grating G230L. For one-dimensional, extracted spectra, the scattered light can be approximately modeled as a linear ramp whose amplitude is proportional to the object’s V-band flux. We present formulae for scattered light corrections. For stars warmer than G0 spectral type, no corrections are necessary. Off-center positioning appears not to affect the properties of the scattered light. Therefore, we are able to provide scattered light correction formulae for extended objects as well as point sources. Polynomials for flux corrections due to off-center slit positioning in the 0.2-arcsecond slit are also tabulated.