MIRI Status

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.