Rederivation of STIS Secondary Echelle Mode Traces

TL;DR

This paper introduces a Gaussian process regression method to redefine echelle traces for the STIS instrument, improving flux throughput and providing updated reference files for multiple modes.

Contribution

A new Gaussian process-based approach for defining echelle traces that accounts for curvature, enhancing calibration accuracy over previous straight-line methods.

Findings

Improved flux throughput by approximately 4%.

Enhanced calibration accuracy near detector edges.

Updated reference files for 9 echelle modes.

Abstract

The STIS echelle gratings can be used with a variety of different central wavelength settings. "Secondary" wavelength settings, designed to cover select absorption or emission lines, have not been calibrated as precisely as their primary mode counterparts. In particular, secondary echelle mode traces (and subsequent extraction regions) have been previously defined using straight line fits to each spectral order. In this work, we define a new general method for defining echelle traces that utilizes Gaussian process regression and accounts for the detailed curvature of each order across the detector. Across a variety of echelle grating and central wavelength settings, we find that this method can improve flux throughput by $\sim4\%$ especially near wavelengths located close to the edge of the detector. We have used this method to provide new traces and update reference files for 9 different echelle modes for both pre- and post-Servicing Mission 4 (SM4; in 2009) observations.