MIRI spectrometer optical design

MIRI (the Mid InfraRed Instrument) is one of the focal plane instruments of the James Webb Space Telescope. The instrument comprises a camera and a spectrometer module. The instrument plays the following key roles in the JWST science program. Discovery of the "first light". Assembly of galaxies: history of star formation, growth of black holes, production of heavy elements. Formation of stars and planetary systems. Evolution of planetary systems and conditions for life. The MIRI spectrometer covers the spectral range from 5 - 28.3 μm with a spectral resolution better than 2000. The spectral module is an imaging spectrometer with a field of view ≥ 3 arcsec. The spectrometer consists of 4 spectral channels that share 2 detectors of 1024 × 1024 pixels each. In a single measurement one third of the wavelength range of the channels is imaged onto the detectors. By changing gratings the whole spectral range is covered in 3 measurements. In this paper the optical design of the MIRI spectrometer is described