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.
Low resolution spectrograph (LRS)
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.
Medium resolution spectrograph (MRS)
Integral field units (IFUs)
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.
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.
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European Space Agency (ESA), 2000- "JWST (James Webb Space Telescope)," Earth Observation Portal [Updated November 2017]