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I'm not sure closed-cycle coolers can get as cold as evaporation of liquid helium? Herschel was cooled to ~1.7 K [0] while MIRI will only be cooled to 7K [1]. Also, with regard to reliability, evaporation of liquid helium is a pretty foolproof design, while closed-cycle systems can potentially break and end a mission early.

[0] https://en.wikipedia.org/wiki/Herschel_Space_Observatory#Ins...

[1] http://jwst.nasa.gov/miri.html

1 comments

Are the individual instruments cooled to that temperature though? Because if they are not, then the difference doesn't matter.

Had a look at the SPIRE manual [0][1] and it seems that Herschel does include an Helium cooler to cool SPIRE and PACS to 300mK.

On the other hand a closed-cycle system can also extend the mission, since you won't run out of cooling at a fixed point in time.

[0] http://herschel.esac.esa.int/Docs/SPIRE/html/spire_handbook.... [1] http://herschel.esac.esa.int/Docs/Herschel/html/ch02.html