The graphs show the temperature stages and their stability during the tests. The cryostat (shown in light blue) sets the low temperature and cools the massive structure of the imager (shown in red). The temperature of the detector (dark blue) is controlled by an independent circuit.
Between 2007 and 2010, the cryostat dedicated to testing and approving the on-board imager on the James Webb Space Telescope (JWST), the successor to the Hubble telescope, has been subjected to an number of test campaigns with the aim of commissioning the automatic control systems and testing the imager itself. The Astrophysics Department, which built the Mid-range Infra-red Imager (MIRIM), operating over the mid-infra-red range of 5 to 27 µm, and SACM, who originated the design of the test cryostat, carried out a series of cold tests and low temperature commissioning tests.
This program has been a success for the Saclay built imager, which was qualified at the end of 2009. Despite an excess consumption of helium due to a reduction in the operating temperature since the original specifications were written, the cryostat has successfully maintained the optical systems within the planned ranges of ± 0.01 K and ± 0.1 K on the detector structure. These results have also confirmed the correct operation of the automatic control system which has maintained a constant temperature over several days and faultlessly controlled the cooling and warming sequences for the entire structure, avoiding any contamination of the detector by cryopumped gas residues resulting from uncontrolled degassing.
The photograph shows the operational MIRIM cryostat and the installation of the optical calibration at SAP. The rack on the right contains all the instrumentation and control systems for the cryostat.
