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

Abstract

The NIRSpec instrument for the James Webb Space Telescope (JWST) is a highly versatile near-infrared spectrograph that can be operated in various observing modes, slit apertures, and spectral resolutions. Obtaining dedicated calibration data for all possible combinations of aperture and disperser is an intractable task. We have therefore developed a procedure to derive a highly realistic model of the instrument's optical geometry across the entire field of view, using calibration data acquired through only a subset of NIRSpec apertures, which nevertheless allows the light paths within the spectrograph to be accurately computed for all apertures and all observing modes. This parametric instrument model thus provides the basis for the extraction of wavelengthcalibrated spectra from any NIRSpec exposure, regardless of observing mode or aperture used. Optimizing the NIRSpec instrument model and deriving its final wavelength and astrometric calibration was one of the most crucial elements of the NIRSpec commissioning phase. Here, we describe the process of re-fitting the NIRSpec instrument model with in-orbit commissioning data, and present its final performance in terms of wavelength accuracy and astrometric calibration.

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

“…For that, we make use of the NIRSpec spectrograph model derived during Commissioning. 9 The model has been extensively described in the literature, 15,16 and, in summary, has two major components: (1) the parameterization of the coordinate transforms between the main optical planes (IFU-FORE, IFU-POST, collimator, and camera); and (2) the geometrical description of their elements (MSA, IFU slicer, GWA, FPA). In particular, the GWA elements are described by their orientational positioning in the wheel, and according to type, with the gratings defined by the individual groove densities and front surface tilt angles, the prism by the front surface tilt, internal prism angle, and the Seidel model based prescription for its refractive index, and the mirror treated as a simple reflective surface.…”

“…The residuals are consistently below 1/10th of a pixel, confirming the excellent performance of the instrument model both for imaging and spectral modes. 9…”

“…The necessary geometrical transforms for this later computation are defined by the NIRSpec instrument model. 9 The position of the grating wheel at the moment the target acquisition image is acquired defines the position of the reflected beam on the focal plane and therefore it must be known to a great accuracy in this computation. NIRSpec's stringent operation mode requires the angular position of the GWA mechanism to be known to a higher accuracy, i.e., approximately 5 and 10 times better than the typical reproducibility for spectral calibration and target acquisition, respectively.…”

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

“…We then obtained images of a dense star field for which highly accurate measurements of the stellar positions exist. These observations of the 'astrometric reference field' allowed us to precisely derive the magnification and distortions of the NIRSpec optical train, which is captured in the parametric instrument model described in [9]. The resulting coordinate transformation between detector, MSA, and sky are necessary to be able to position science targets in the various NIRSpec apertures.…”

“…Since the microshutters are only about 200 milli-arcsec wide, the stars need to be placed with an accuracy of 20 milli-arcsec or better. This was a major hurdle on the path to commissioning the NIRSpec instrument, and required precise tuning of the parametric instrument model [9] to derive the coordinate transformations between the detector, MSA, and sky planes.…”