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MIRI keeps cool under low pressure
by Staff Writers
Paris (ESA) Feb 11, 2014

The JWST MIRI instrument will be cooled by a unique cryocooler system (shown here), which provides cooling remotely: the cold head is close to the MIRI detectors which are located approximately 20 metres from the cryocooler compressor and electronics. The cryocooler is developed by Northrop Grumman Space Technology (NGST) under contract to JPL. Image courtesy Northrop Grumman.

The Mid-Infrared Instrument (MIRI) on JWST has performed beautifully during its first cryo-vacuum test campaign carried out at NASA's Goddard Space Flight Center towards the end of last year. An examination of data recorded during those tests confirms that the instrument is in good health and performing well.

In August last year preparations began for the first ISIM (Integrated Science Instrument Module) test campaign to be performed in a vacuum and at cryogenic temperatures. Fitted with two instruments - MIRI and FGS/NIRISS (the Fine Guidance Sensor / Near-InfraRed Imager and Slitless Spectrograph) - ISIM was installed in the Space Environment Simulator (SES), a large cryogenic test chamber at Goddard.

The doors to the chamber were closed on 29 August and after about 5 days the vacuum (approximately 10-6 mbar) was achieved, ISIM and the instruments were checked to be functioning and in good shape, and the cool-down then began.

Nearly twenty days later, the ISIM reached a temperature of -233 C (40.15 K) - the same temperature it will experience in space. However, this temperature is not low enough for the advanced detectors on MIRI. To ensure that the detectors are not blinded by temperature-driven parasitic light and currents, astronomers require the instrument to be cooled down to around -266 C.

This is barely seven degrees above absolute zero - the latter being the lowest temperature possible, as defined by the laws of physics. To achieve this level of cooling, a dedicated cooling system, called the MIRI cryocooler is being developed by NASA JPL.

During the almost three-week cool down of MIRI, team members on-site at Goddard, across Europe and elsewhere in the US monitored the health of the instrument, 24 hours a day, seven days a week.

Once the Optical Telescope Simulator (OSIM), ISIM, MIRI and FGS/NIRISS all reached their operating temperatures, the scientists and engineers resumed their tests, constantly monitoring the state of the instruments, as well as conducting their experiments and analysing the data.

Every indication showed that MIRI performed well, however, not even scientific experiments deep in the Goddard cryogenic test chamber are immune from external perturbations and on 1 October all testing was put on hold as the American government shut down.

Once the furlough ended, testing started again on 17 October. To reduce the impact of the unscheduled 'break', the sequence of tests was revised and heavily streamlined. The most critical activities were given the highest priorities and tested accordingly.

Despite this interruption, and due to the great efforts from all of the teams, the tests at operating temperature were successfully completed on 29 October and all of the primary goals of the first ISIM test campaign were reached. Warm-up then started and was finished in the week of 11 November, after which this first ISIM cryo-vacuum test campaign was officially declared complete.

This was the first time MIRI could be tested at its operating temperature since the instrument-level testing at RAL in Europe in 2011. The data gathered by the MIRI team have shown that the instrument performed beautifully and is in good health.


Related Links
Integrated Science Instrument Module on JWST
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