Wavelength Calibration and Resolving Power of the JWST MIRI Medium Resolution Spectrometer

Abstract

Context. The Mid-Infrared Instrument (MIRI) on-board the James Webb Space Telescope will provide imaging, coronagraphy, low-resolution spectroscopy and medium-resolution spectroscopy at unprecedented sensitivity levels in the mid-infrared wavelength range.The Medium-Resolution Spectrometer (MRS) of MIRI is an integral field spectrograph that provides diffraction-limited spectroscopy between 4.9 and 28.3 µm, within a FOV varying from ∼13 to ∼56 arcsec square. The design for MIRI MRS conforms with the goals of the JWST mission to observe high redshift galaxies and to study cosmology as well as observations of galactic objects, stellar and planetary systems. Aims. From ground testing, we calculate the physical parameters essential to general observers and calibrating the wavelength solution and resolving power of the MRS is critical for maximizing the scientific performance of the instrument. Methods. We have used ground-based observations of discrete spectral features in combination with Fabry-Perot etalon spectra to characterize the wavelength solution and spectral resolving power of the MRS. We present the methodology used to derive the MRS spectral characterization, which includes the precise wavelength coverage of each MRS sub-band, computation of the resolving power as a function of wavelength, and measuring slice-dependent spectral distortions. Results. The ground calibration of the MRS shows that it will cover the wavelength ranges from 4.9 to 28.3 µm, divided in 12 overlapping spectral sub-bands. The resolving power, is R 3500 in channel 1, R 3000 in channel 2, R 2500 in channel 3, and R 1500 in channel 4. The MRS spectral resolution optimises the sensitivity for detection of spectral features with a velocity width of ∼ 100 km s −1 which is characteristic of most astronomical phenomena JWST aims to study in the mid-IR. Based on the ground test data, the wavelength calibration accuracy is estimated to be below one tenth of a pixel (0.1 nm at 5 µm and 0.4 at 28 µm), with small systematic shifts due to the target position within a slice for unresolved sources, that have a maximum amplitude of about 0.25 spectral resolution elements. The absolute wavelength calibration is presently uncertain at the level of 0.35 nm at 5 µm and 46 nm at 28 µm, and will be refined using in-flight commissioning observations. Conclusions. Based on ground test data, the MRS complies with the spectral requirements for both the R and wavelength accuracy for which it was designed. We also present the commissioning strategies and targets that will be followed to update the spectral characterisation of the MRS.

Direct emission spectroscopy of exoplanets with the medium resolution imaging spectrometer on board JWST MIRI: I. Molecular mapping and sensitivity to instrumental effects

“…We can conclude that from the tested molecules with the given P-T profile, CH 4 is the most sensitive to photometric errors. Regarding the wavelength calibration error, we find that uncertainty of the current solution (ground solution ∼ 0.1 of resolution element, Labiano et al 2021) suggests that it does not affect the crosscorrelation signal, contributing to maximum a 1-2% reduction. Uncertainty in the absolute wavelength solution again does not affect the result since it would manifest as an error in the radial velocity shift of the planet.…”

“…Observations of point sources are always performed using dithered exposures in order to improve the spatial sampling, as the spatial resolution is not Nyquist sampled for the short wavelength sub-bands (Glauser et al 2010). Following multiple steps of the calibration pipeline (Labiano et al 2016) the data are calibrated (described in detail in Section 2.6) and the detector images are reconstructed into a data cube, where an image is produced for each wavelength bin (Wells et al 2015;Labiano et al 2021). Residual systematic errors in each step of the calibration pipeline pose a significant obstacle to observations of very faint sources, such as exoplanets around a bright star, and careful correction of these residual effects might be critical to the success of the MRS as an exoplanet characterisation instrument.…”