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.The CCC (Fig. 3) acts as a front cover for MIRI. It is open when the observatory is launched and remains open for about one week to allow air to escape from the instrument. Then, as the instrument cools, the CCC is closed to keep contaminants from entering the instrument and freezing onto the optical surfaces. When Webb is sufficiently cold, the CCC will be open for normal operations. However, during portions of coronagraphic target acquisition, it will be closed to prevent the bright target star from saturating the detector as it is placed in the center of a coronagraph. The CCC has completed all environmental and lifetime qualifications.

The focal-plane system consists of the focal-plane modules, which contain the detectors and the focal-plane electronics. MIRI has three 1024 x 1024, arsenic-doped silicon (Si:As) detectors, which differ from the detectors in all the other Webb instruments, due to the longer-wavelength operating range. One detector is used for the imager, the coronagraphs, and the low-resolution spectrometer. The remaining two detectors are used for the medium-resolution spectrometer. Two different anti-reflection coatings are used to optimize the detector performance for short and long wavelengths. The flight, flight-spare, and engineering pathfinder detectors are shown in Figure 4. The detector performance is excellent, with demonstrated read noise of 14 electrons and dark current of approximately 0.1–0.2 electrons per second. The focal plane electronics are undergoing final design modifications as a result of thermal-vacuum testing. The modifications will ensure reliable communication of data to the rest of the Webb system. When modifications are complete, the focal-plane system will be delivered to the EC for integration into the instrument at RAL.

Because MIRI operates at longer wavelengths than the other instruments on the observatory, it must operate at a colder temperature. A cryocooler will maintain the detectors at 6.7 K and the MIRI instrument at below 10 K. The cooler has very complex interfaces across the whole of the Webb observatory. For that reason, it is being developed separately, and built up in stages that are integrated with the observatory construction. The MIRI optical system and complete MIRI cooler system will be tested together only at a late stage.

Detailed preparations are underway for the cryogenic performance test of the flight model. This test is expected to begin in early 2011 and will last for 2–3 months. Upon successful completion of this test campaign, MIRI will be delivered to Goddard Space Flight Center for integration into the integrated science instrument module.