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]