MIRI Spectroscopic Elements

MIRI offers observing modes for both low-resolution spectroscopy and medium-resolution spectroscopy. Each mode utilizes different spectroscopic elements to produce spectra.  The low-resolution spectrograph (LRS) uses a double prism. The Medium resolution spectrometer (MRS) uses dichroics for sub-band selection and gratings for spectral dispersion. The complex and compact design of the spectrometer allows the selection of three sets of sub-bands via dichroics and gratings mounted on the axes of two wheels.  Image slicers are used to spatially reformat the field of view into a series of slits as necessary for the input to the relevant grating.

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Low-resolution spectrograph (LRS)

Double prism

Main article: MIRI Low-Resolution Spectroscopy
See also: MIRI Optics and Focal Plane

The LRS slit is located at the telescope focal plane (along with the coronagraph masks).  

Figure 1. The mounting bracket for the four coronagraph image-plane masks and LRS slit.

The mounting bracket focal plane module for the 4 coronagraph image-plane masks and LRS slit

Figure Credit: Boccaletti et al. 2015
The LRS shares most of its optics with the imager except the dispersing element and a blocking filter. Its mode (i.e., slit or slitless) is determined solely by the placement location of the target source in the focal plane. Light passing through the focal plane is collimated, forming a pupil image at the filter wheel location that holds a Ge/ZnS double prism (Fischer et al. 2008). 

Expected efficiency for the set of prisms is near 80% from 5 to 10 μm (at cold temperatures) but drops below 25% for wavelengths longer than 12 μm if slit losses are included. To mitigate the effect of the fold-over in the dispersion profile, a filter is mounted on the slit to block light at wavelengths short of 4.5 μm.

Figure 2. Flight model of the LRS double prism assembly

Flight model of the LRS Double Prism Assembly

The flight model of the LRS double prism assembly (DPA), showing the ZnS side. Figure Credit: Fischer et al. 2008


Medium-resolution spectrograph (MRS)

Integral field units (IFUs)

Main article: MIRI Medium Resolution Spectroscopy
See also: JWST Integral Field Spectroscopy

The MIRI MRS has four separate integral field units (IFUs) called channels 1, 2, 3 and 4. Each IFU covers a separate wavelength range between 5 and 28.5 μm. An IFU serves primarily to divide a field of view into multiple slitlets suitable for grating dispersion. Its design has several advantages: (1) easier acquisition of point sources, (2) no slit losses due to vignetting, and (3) allowance for spatial variations as a function of wavelength for which a slit cannot account.

The MIRI IFU design consists of several components, including an entrance pupil, an input fold mirror, an image slicer mirror, a mask carrying exit pupils for the individual sliced images, a mask carrying slitlets for the individual images, and an array of reimaging mirrors behind the slitlets. 

Figure 3. Three dimensional view of the channel 3 IFU

Three dimensional view of the channel 3 IFU

Three-dimensional view of the channel 3 IFU with labels identifying the major components, including an entrance pupil, an input fold mirror, an image slicer mirror, a mask carrying exit pupils for the individual sliced images, a mask carrying slitlets for the individual images, and an array of reimaging mirrors behind the slitlets. Figure Credit: Wells et al. 2015
The IFU itself can be implemented with several different types of technology. The MRS IFU utilizes an image slicer, as illustrated in Figure 4. The slicer has rows that reflect light from different parts of the field of view into different directions. The image slices are then directed through a regular spectrograph slit and diffracted by a grating, thereby resulting in a spectrum for each row. Spatially, the image is sampled in the dispersion direction by the IFU slicing mirrors and in the slice direction by the detector pixels. Spectrally, the width of the slices defines the spectrometer entrance slit and the width of the image (in pixels) of the slice at the detector defines the width of the spectral sample. In the across slice (dispersion) direction, one slice width is matched to the full-width half-maximum (FWHM) of the JWST point spread function (PSF) at the shortest IFU wavelength.

Figure 4. Visual representation of how an IFU image slicer works

Visual representation of how an IFU image slicer works


Figure 5. Channel 1 image slicer hardware

Channel 1 image slicer hardware

The slices are 1 mm wide and 12 mm long. Figure Credit: Wells et al. 2015

Dichroic filter/grating combination wheels

Main article: MIRI Filters and Dispersers

All four MRS channels are observed simultaneously, but each exposure can only cover one-third of the available wavelength range in a single configuration. For complete spectral coverage, three different spectral settings must be observed, termed SHORT (A) 1, MEDIUM (B), and LONG (C). Two dichroic filter/grating wheels have three working positions to move gratings and dichroics simultaneously. In short, light is separated by the dichroics in the Spectrometer Pre-Optics (SPO) and sent to the four IFU channels, where the light is spatially sliced and arranged as a slit. This slit is spectrally dispersed by the gratings and eventually recorded on the detectors.

Figure 6. MRS dichroic and grating wheels

MRS dichroic and grating wheels

Image credit: ESA 2000-


Figure 7. Spectrometer Pre-Optics layout

Figure Credit: Wells et al. 2015

Figure 8. Spectrometer Main Optics (SMO)

Figure Credit: Wells et al. 2015

1  Bold italics font style is used to indicate parameters, parameter values, and/or special requirements that are set in the APT GUI.



References

Boccaletti, A. et al. 2015, PASP, 127, 633 
The Mid-Infrared Instrument for the James Webb Space Telescope, V: Predicted Performance of the MIRI Coronagraphs

Fischer, S. et al. 2008, Proc. of SPIE, 7010, 103
The JWST MIRI double-prism: design and science drivers 

European Space Agency (ESA), 2000- "JWST (James Webb Space Telescope)," Earth Observation Portal [Updated November 2017] 

Wells, M. et al. 2015, PASP, 127, 646
The Mid-Infrared Instrument for the James Webb Space Telescope, VI: The Medium Resolution Spectrometer
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