The Mid-Infrared Instrument for theJames Webb Space Telescope, VI: The Medium Resolution Spectrometer

Abstract

We describe the design and performance of the Medium Resolution Spectrometer (MRS) for the JWST-MIRI instrument. The MRS incorporates four coaxial spectral channels in a compact opto-mechanical layout that generates spectral images over fields of view up to 7.7 x 7.7 arcseconds in extent and at spectral resolving powers ranging from 1,300 to 3,700. Each channel includes an all-reflective integral field unit (IFU): an 'image slicer' that reformats the input field for presentation to a grating spectrometer. Two 1024 x 1024 focal plane arrays record the output spectral images with an instantaneous spectral coverage of approximately one third of the full wavelength range of each channel. The full 5 to 28.5 µm spectrum is then obtained by making three exposures using gratings and pass-band-determining filters that are selected using just two three-position mechanisms. The expected on-orbit optical performance is presented, based on testing of the MIRI Flight Model and including spectral and spatial coverage and resolution. The point spread function of the reconstructed images is shown to be diffraction limited and the optical transmission is shown to be consistent with the design expectations.

The Main Belt Comets and ice in the Solar System

“…The two brightest fluorescence bands of water are the ν 3 band at 2.7 µm and the ν 2 band at 6.3 µm, with fluorescence band g-factors near 3.2 × 10 −4 and 2.4 × 10 −4 s −1 , respectively, when opacity effects are neglected (Crovisier and Encrenaz 1983;Bockelée-Morvan et al 2009;Debout et al 2016). JWST has spectral sensitivity at both bands: the NIRSpec instrument covers 2.7 µm at spectral resolving powers, R = λ/λ, of ∼ 100, ∼ 1000, or ∼ 2700; and MIRI covers 6.3 µm with R ∼ 70 and ∼ 3500 (Kendrew et al 2015;Wells et al 2015).…”

Detecting Proxima b’s Atmosphere with JWST Targeting CO2 at 15 μm Using a High-pass Spectral Filtering Technique

“…The MRS mode of the MIRI (Rieke et al 2015;Wright et al 2015) on JWST utilizes an integral field spectrograph (IFS) that has four image slices producing dispersed images of the sky on two 1024×1024 infrared detector arrays, which provide R= 1300-3600 integral field spectroscopy over a λ=5-28.3 μm wavelength range (Wells et al 2015;Labiano et al 2016). The spectral window is divided into four channels covered by four integral field units: (1) 4.96-7.77 μm, (2) 7.71-11.90 μm, (3) 11.90-18.35 μm, and (4) 18.35-28.30 μm.…”

“…Experiences with data from ISO and Spitzer show that it can be removed down to the noise level (e.g., Lahuis & van Dishoeck 2000). Wells et al (2015) have characterized the fringing of the MIRI detectors in the laboratory and identify three fringe components with scale lengths (in wave number) of 2.8, 0.37, and 10-100 cm −1 , originating from the detector substrate, dichroic, and fringe beating, respectively. The planet CO 2 features also show a regular pattern, but with a characteristic scale length of ∼1.6 cm −1 , which fortunately is significantly different from these fringe components.…”

The MIRI Medium Resolution Spectrometer calibration pipeline

“…The MRS Calibration Pipeline processes LEVEL 2a data (i.e., uncalibrated slopes images) 4 , to produce LEVEL 2b data (calibrated slopes images), and MRS LEVEL 3 data (spectral cubes and 1D spectra) for each observation or association (a set of exposures that are processed together to produce a single LEVEL 3 data product such as mosaics, different visits to the same target, etc.). This process takes place in two sets of steps.…”

“…Thus, to obtain a spectrum for the whole 5 to 28.5 µm range, the observer needs to stitch together 12 sub-band spectra, obtained in three exposures (each exposure with a different GDW position): 1A-2A-3A-4A (exposure 1), 1B-2B-3B-4B (exposure 2), and 1C-2C-3C-4C (exposure 3). Figure 1 illustrates the wavelength coverage of the MRS. A full description of the MRS is given in [4]. …”