Pre-Flashing WFC3/IR Time-Series, Spatial Scan Observations

TL;DR

Pre-flashing WFC3/IR detectors reduces systematic effects in time-series observations by rapidly reaching equilibrium, but offers limited advantages over existing modeling techniques and involves additional operational complexity.

Contribution

This study demonstrates that pre-flashing significantly reduces systematics in WFC3/IR spatial scan data, but concludes it is not necessary given current modeling capabilities.

Findings

Pre-flashing reduces systematic amplitude by a factor of ~7.

Pre-flashing achieves marginal improvement in white light curve precision.

Pre-flashing does not warrant future use due to operational complexity.

Abstract

Spatial scan observations using WFC3's IR channel exhibit time-dependent systematics (in the form of a ramp or hook) that have been attributed to the effects of persistence. The amplitude of these systematics is often two orders of magnitude larger than the signal sizes of interest and, therefore, must be carefully modelled and removed. The goal of this calibration program (CAL-15400) is to mitigate these systematics by continuously illuminating the detector while repeatedly reading it out during Earth occultation (termed pre-flashing). Compared to standard observations, we are able to reduce the amplitude of the systematic effect by a factor of $\sim$7 (from 1.30% to -0.19%), thus confirming our hypothesis that the detector more quickly reaches an equilibrium state when subjected to higher flux levels. Compared to the latest modeling techniques (Zhou et al., 2017), we achieve a marginal improvement in the white light curve precision ($\Delta$rms = -8{\pm}9 ppm); therefore, pre-flashing is an equally effective means to mitigate WFC3's instrument systematics. We conclude that pre-flashing does not warrant future consideration due to the increase in the number of channel select mechanism (CSM) motions, effort required to implement, and equivalent ability to model instrument systematics with current techniques.