In-flight performance and calibration of the Grating Wheel Assembly sensors (NIRSpec/JWST)

TL;DR

This paper evaluates the in-flight performance and calibration of the Grating Wheel Assembly sensors on NIRSpec/JWST, highlighting their evolution from ground testing to space operation for accurate spectral measurements.

Contribution

It presents the calibration process and performance evolution of GWA sensors in space, essential for precise spectral data collection with NIRSpec.

Findings

Sensor calibration improved from ground to space environment

Displacements due to wheel positioning are effectively measured and corrected

Calibration is crucial for accurate spectral extraction and target placement

Abstract

The Near-Infrared Spectrograph (NIRSpec) on board of the James Webb Space Telescope will be the first multi-object spectrograph in space offering ~250,000 configurable micro-shutters, apart from being equipped with an integral field unit and fixed slits. At its heart, the NIRSpec grating wheel assembly is a cryogenic mechanism equipped with six dispersion gratings, a prism, and a mirror. The finite angular positioning repeatability of the wheel causes small but measurable displacements of the light beam on the focal plane, precluding a static solution to predict the light-path. To address that, two magneto-resistive position sensors are used to measure the tip and tilt displacement of the selected GWA element each time the wheel is rotated. The calibration of these sensors is a crucial component of the model-based approach used for NIRSpec for calibration, spectral extraction, and target placement in the micro-shutters. In this paper, we present the results of the evolution of the GWA sensors performance and calibration from ground to space environments.