Recalibration of Pre-SM4 STIS Echelle Throughputs

TL;DR

This paper proposes a simple scaling method to recalibrate the throughput of STIS echelle modes using updated stellar models, improving flux accuracy especially for pre-SM4 data.

Contribution

It introduces a straightforward scaling approach for recalibrating STIS echelle throughputs based on new stellar atmospheric models, applicable to pre-SM4 observations.

Findings

Achieved 0.5-2.4% improvement in throughput accuracy.

Provided updated pre-SM4 throughput calibrations for 8 echelle modes.

Demonstrated the method's effectiveness when calibration data are limited.

Abstract

Recent improvements to stellar atmospheric models have merited updated flux calibration for high priority STIS observing modes. Specifically, in the FUV and NUV, continuum differences of 1-3% are present between the newest models (CALSPECv11) and previous models (CALSPECv04-v07). As a result of these improvements the STIS team has derived updated sensitivity curves and blaze shift coefficients for a variety of echelle modes in order to meet targeted flux accuracies. The first series of echelle sensitivity updates primarily targeted post-Servicing Mission 4 (SM4; in 2009) observations. In this ISR, we investigate instead applying a simple scaling (derived from the ratio of new vs old CALSPEC model continua) to the previously determined throughputs of STIS echelle modes. This alternative approach has a straightforward implementation and provides reasonable accuracy, especially in cases where available calibration data are lacking (e.g., pre-SM4 era). Adopting this scaling approach, we delivered pre-SM4 throughput updates for 8 echelle modes, resulting in typical improvements of 0.5-2.4% across the FUV and NUV.