In-flight performance and calibration of the grating wheel assembly sensors (NIRSpec/JWST)

Abstract

The Near-Infrared Spectrograph (NIRSpec) on board of the James Webb Space Telescope will be the first multiobject 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.

Astrometric and wavelength calibration of the NIRSpec instrument during commissioning using a model-based approach

“…Before changing to the next grating, the GWA was 'spun around' once, and a REF lamp exposure was taken at each position. These extra observations were used for the grating wheel sensor calibration (see [5] for a full list of data).…”

“…the position and rotation of each quadrant) since they could have moved during launch. After computing the grating wheel sensor calibration for the mirror (see Section 4 and [5]), the imaging model was complete.…”

“…A detailed description of the grating wheel sensor calibration is provided in [5]. Here, we only provide a short summary.…”

“…The procedure of fitting this part of the model is described in section 3. The second part of the model is the grating wheel sensor calibration described in [5] and summarized in section 4. The remaining FORE optical transforms are fit in the astrometric calibration step by registering the positions of stars on the detector with an astrometric catalogue as described in 5.…”

In-orbit commissioning of the near-infrared spectrograph on the James Webb Space Telescope

“…This is important, because it allowed a quick and accurate calibration of the position sensors on the GWA. As described in more detail in [10], this is an essential pre-requisite for the precision of the NIRSpec parametric instrument model that informs the coordinate transformations between sky, MSA, and FPA used during target acquisition, as well as for the extraction and calibration of NIRSpec spectra.…”