MIRI Optics Modules
| The design features an imager and a dual spectrometer. Light enters from the telescope through the periscope to the left. The Primary structure comprises a six strut, isostatic mount interfacing at 3 hardpoints to the 6061 (T6) aluminium alloy optical bench structure |  |
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A model illustrates the compact packaging |
| The instrument uses phase plate coronagraphs. They reject the light of a central source by introducing phase shifts using a quadrant-design plate at the instrument input focal plane. These shifts cause the light from the source to interfere destructively at the detector array. Unlike conventional occulting Lyot coronagraphs, phase plates allow measurements to be obtained very close to the central object. Further from the central object, they provide performance similar to that of a conventional occulting coronagraph. |
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Operation of the spectrometer is kept simple by using a set of dichroics and blocking filters to divide the light into four beams covering different spectral regions. These beams are put into the IFUs.
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and then dispersed by the gratings and reimaged by the camera optics onto the 1024x1024 Si:As IBC focal planes.
The light is divided into four spectral ranges by the dichroics, and two of these ranges are imaged onto each of the two detector arrays. Along the way to the appropriate array, the light is dispersed by a diffraction grating. The gratings are mounted on mechanical turrets with three for each spectral range. A full spectrum is obtained by taking exposures at the three settings of each mechanical turret -- the turrets are ganged together and operated with a single mechanism, and the dichroics allow the same spot on the sky to be distributed to all four spectrometer arms. Thus, only three exposures are required to obtain a complete spectrum. The table below shows the spectral ranges, imaging properties of the IFU, and spectral resolution:
The MIRI mechanisms build on heritage from ISO:
