The Middle Way of Cooling Instruments
“Closed cycle” coolers are now available to cool instruments, and they are simpler systems with less maintenance needs. No daily liquid cryogen fills are required, and there is basically no worry about ice plugs in your system leading to a destructive failure. All of this makes cooling the instrument much easier.
We use “Cryotiger” coolers at Anderson Mesa that work off of the Joule – Thomson effect. This may sound intimidating, but it’s very similar to how your fridge at home works:
→ Compress a gas, which causes the gas to get hot
→ Pass the compressed gas through radiator to get rid of that heat
→ Transport the ambient temperature compressed gas to where you want stuff to get colder ( At home, it’s your freezer, at the Mesa it’s inside an instrument [NASA42 at the 42”] )
→ Once at the desired cold location, let the gas expand quickly and it will get colder as it expands.
The only difference for our astronomical uses is that the working gas being compressed/expanded is more exotic than the gas in your fridge’s compressor, which is how it gets NASA42’s detector down to about -100 C (-148 F).
But, these coolers can be temperamental. The compressors are large, and get very hot, and can’t be moved around with the telescope and must stay in a fixed position. 50-foot-long lines carry the cooling gasses between the compressor and the instrument, and contaminants can sneak into those lines through the various seals and fittings in the lines leading to their eventual replacement. The gas being compressed is usually flammable, and is a mixture of propane and other similar gasses to be able to reach ultra-cold temperatures, so significant handling care is still required.