Scattered Light in STIS Grating G230LB

TL;DR

This paper characterizes and models the scattered red light in the G230LB grating of the STIS instrument, providing correction formulas to improve ultraviolet spectra accuracy, especially for red objects.

Contribution

It introduces a new empirical model for scattered light correction in G230LB spectra, applicable to both point sources and extended objects, based on calibration observations.

Findings

Scattered light can be modeled as a ramped pedestal proportional to V-band flux.

Correction is unnecessary for stars warmer than G0 spectral type.

Off-slit-center positioning does not significantly affect scattered light properties.

Abstract

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. Recent calibration observations characterize the scattered light using duplicate observations with the MAMA detector and similar grating G230L. The full two-dimensional spectrum contains little helpful information to mitigate the scattered light problem. For one-dimensional, extracted spectra, the scattered light can be approximately modeled as a ramped pedestal 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, correction is superfluous. Off-slit-center positioning appears not to affect the properties of the scattered light. Therefore, we are able to extrapolate correction formulae for extended objects from the point source formulae. Polynomials for flux corrections due to off-center slit positioning in the 0.2 arcsec slit are also tabulated.