MIRI Status

Scott Friedman, friedman@stsci.edu, & Gillian Wright, gsw@roe.ac.uk

The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope is highly versatile, offering imaging through nine filters, four separate coronagraphs, low-resolution slit spectroscopy (LRS), and medium-resolution, integral-field-unit spectroscopy (MRS). Covering 5–28.3 microns, MIRI complements the other instruments on Webb, all of which operate at wavelengths shorter than about 5 microns.

Assembly and testing of MIRI is proceeding along several fronts. Through a collaboration among the ten countries of the nationally funded MIRI European Consortium (EC) and JPL, various pieces of the optics, mechanisms, structure, and detectors are being prepared and tested as the instrument gradually takes shape. The final instrument assembly, followed by a comprehensive thermal-vacuum performance test in a flight-representative environment, will take place at the Rutherford Appleton Laboratory (RAL) in England this year.

MIRI has a modular design and the subassemblies have endured a series of environmental tests, verifying performance at a range of temperatures, as well as acoustic, vibration, and lifetime tests, as appropriate. Each MIRI subassembly is then subject to a formal qualification review, and the majority of these reviews have already been held. Pre-ship reviews, held prior to delivery for integration at RAL, will verify that each subsystem meets its requirements, at least to the extent that it can be tested before assembly into the full instrument.

The mechanisms in an instrument are always of great interest because they must work in order to achieve full science performance. MIRI has four mechanisms: an imager filter-wheel assembly (FWA), two dichroic-grating-wheel assembles (DGAs), and a contamination control cover (CCC).

The FWA (Fig. 1) holds all imager filters, four narrow-band, coronagraphic filters and their associated Lyot stops, the double prism that disperses the light for the LRS, a neutral density filter, and an opaque blank. The FWA will be the most heavily used mechanism in the instrument.

The two DGAs hold dichroic beamsplitters and the twelve gratings used in the MRS. Obtaining complete spectral coverage over the full bandpass of the medium-resolution spectrometer requires three separate rotations of the DGAs. These mechanisms have recently been fully assembled and integrated into the MRS pre-optics assembly (Fig. 2). The FWA and the DGAs have completed flight-acceptance environmental qualification, and lifetime tests of these mechanisms are proceeding in parallel with flight instrument assembly.