A pick-off mirror in front of the JWST OTE (Optical Telescope Element) focal plane directs the MIRI field of view toward the imager MIRIM. A small fold mirror adjacent to the imager light path picks off the small (8 x 8 arcsec) field of view of the spectrometer. A second fold in the spectrometer optical path is used to select either light from the telescope or from the MIRI calibration system.
The imager module MIRIM has a combined field of view for the imager and coronagraph / low resolution spectrometer modes. The coronagraph masks are placed at a fixed location on one edge of the imager field. The light is collimated and, at the pupil image formed by the collimator, a filter wheel holds the imaging filters, Lyot masks and filters for the coronagraph, and a pair of prisms for the low resolution 5-10 µm spectroscopy. A camera then images the field of spectrum onto the single 1k x 1k detector.
The medium resolution spectrometer module is split into a short wavelength (5 to 12 µm) and a long wavelength spectrometer (12 to 28 µm) each covering 2 of 4 wavelength channels split by dichroics. Each channel consists of an Integral Field Unit whose output is collimated and then dispersed by a dedicated first-order diffraction grating. The spectra from pairs of the IFUs are then combined by two cameras onto 1k x 1k detectors. The spectrometer optics fall naturally into 2 subsystems—a Spectrometer Pre-Optics which consists of the system of dichroics and image slicers with associated fold and re-imaging mirrors, and a Spectrometer Main-Optics consisting of the gratings and camera systems for the 2 spectrometers.