Status of the STIS Auto-wavecal Exposures

TL;DR

This paper evaluates the current performance of STIS auto-wavecal exposures, analyzing lamp fading effects and suggesting increased exposure times at short wavelengths to maintain calibration accuracy.

Contribution

It provides an assessment of the STIS wavecal calibration process and recommends adjustments to exposure times to counteract lamp fading effects.

Findings

Wavecal spectra generally yield reasonable SHIFTA values.

Lamp fading affects short-wavelength calibration accuracy.

Increasing exposure times at short wavelengths can improve calibration reliability.

Abstract

We discuss the behavior of the default ``wavecal'' spectra obtained together with most STIS spectroscopic exposures, which are needed for proper wavelength calibration of the science data. Because the Pt/Cr-Ne lamps used for the wavecals have been fading (especially at the shortest wavelengths), some changes in the default lamp and/or exposure time have been implemented in recent years to maintain accurate calibrations. To assess whether additional changes might be appropriate, we examine the distribution of the SHIFTA1, SHIFTA2 values derived from the wavecals (the x and y offsets of the spectral image on the detector), we re-visit the wavelength-dependent fading of the lamps, and we perform simulations to estimate the exposure times that would be needed to obtain accurate SHIFTA values. While the current wavecals do appear to yield reasonable SHIFTA, increases in the default exposure times for some of the shortest-wavelength settings would help to ensure reliable wavelength zero points as the lamps continue to fade.