James Webb Telescope moves further towards completion

The imaging system of the James Webb Telescope, the successor to Hubble, has completed vital cryogenic testing at the Science and Technology Facilities Council’s (STFC’s) Rutherford Appleton Laboratory (RAL) in Oxfordshire.

Essentially acting as Webb’s eye, the Mid-Infrared Instrument (MIRI), contains a camera, medium-resolution integral field spectrograph and coronagraphs and it covers the wavelength range of 5 to 28 microns. 

It will be placed much further from the Earth than Hubble — around 1.5 million km away at the Larange point, where the combined gravitational pull of the earth and sun will keep it in a solar orbit. 

Crucially though, it will be too far away for emergency maintenance missions, as was required with Hubble.

‘All of the testing and verification programmes have to be so rigorous because there’s no chance of servicing and fixing anything if it were to go wrong,’ said Paul Eccleston, project lead of the Assembly, Integration and Test (AIT) arm of MIRI.

STFC is leading the European team that is developing MIRI in a partnership with NASA’s Jet Propulsion Laboratory (JPL). The recent testing is the culmination of more than 2,000 individual tests by the respective partner institutions on the different components.

‘This is the first time it’s been tested as a system to check cross-compatibility issues, making sure that all the interfaces between the modules are correct and that they all work together,’ Eccleston said.

One of the main challenges for the testing team was the requirement for all the instrumentation to be cooled to around 7k to facilitate the relatively long wavelengths MIRI picks up.

‘We’re looking for the heat given off by cold bodies, so if the instrument itself were any warmer than 15k the detector would be blinded by the self-emission from the rest of the instrument — what we’re looking for is that infrared glow given off by the stars in the distant universe.’

As well as cooling the instruments themselves, the team has to cool the testing chamber down to 40k to simulate the space environment that the telescope will be placed in. This means getting a room 3.5m by 5.5m down to a pressure of 7 millibars (around 10 billion times lower than atmospheric pressure), which takes around 10 days. The room itself resembles something like a small planetarium, with simulated stars of various compositions, to mimic the different features Webb will hope to pick up.

Eccleston explained that the testing chamber essentially serves two main purposes.

‘There’s the verification stage, which is proving that everything works and meets requirements. But we also need to understand exactly what the data that’s going to come out of it when it flies really means and that’s what the calibration activities are aimed at.

‘All of our ground-support equipment is tied back to standards and known quantities to give us a reference point so that, when we fly and the data comes back, we can know exactly what it is we’re looking at.’

The MIRI team is now preparing the instrument for delivery to NASA’s Goddard Space Flight Center, where it will be integrated with the other instruments, the telescope and eventually the spacecraft.