The MOONS cryostat has arrived in Edinburgh – and it’s big. This is a key milestone for the project and allows much of the major assembly to really get under way.
MOONS will be unique in providing wavelength coverage out to 1.8 microns, with a high multiplex. But this comes at a cost: to work at these wavelengths the entire spectrograph must be cooled to 130 K, while the multiplexing leads to large optical beams. Both of these factors result in this huge cooled vacuum vessel. We believe that MOONS will have the largest cryostat yet used in any astronomical instrument – although we are happy to be corrected on this, so please let us know!
The MOONS vessel in numbers:
- Internal Volume: 16m3
- Dimensions: 4m x 2.6m x 2.4m
- Mass: 3100kg
- Operating pressure: <1×10-6 mBar resulting in 400 tonnes of vacuum loading
Cooling this beast
We will use liquid nitrogen to precool the system and then closed-cycle coolers to maintain the bench temperature and to provide the lower temperatures required for the infra-red detectors (40K). Cooling a cryostat of this size is challenging: we will need around 4 tonnes of liquid nitrogen per cool down! We require new infrastructure at the ATC to store this much coolant and so progress is also well underway for installing a new nitrogen Dewar outside the main lab.
The optical bench, which sits vertically inside the centre of the instrument has already had much of the cooling pipework and heaters installed. This will now be mounted inside the radiation shield, which will in turn be placed inside the MLI blanket, which finally all goes inside the vessel. We are aiming for the first cool down of the system in the next few months and then we can begin installing all the optics.
The vessel has been made for MOONS by AS-Scientific. It was a challenging design with two large open-sided doors required to give easy access to both sides of the optical bench.