We discuss here the use of far infrared multi-layer low pass mesh filters to thermally reject unwanted heat from theintermediate temperature radiation shields of liquid helium cryostats used to cool FIR and submillimetre detectors.This unwanted thermal background increases both the photon noise of the detector and the helium boil-off from thecryostat. Ideally the detector should be filtered to only those wavelengths that are required with all others beingreflected, not absorbed, by this filter. Conventionally, the solution has been to use a filter based on high frequencyradiation absorption or scattering for this purpose. However, absorption filters are limited in diameter due to warming,with consequent re-emission problems, while scatter filters exhibit a trade-off between low forward scattering and ashallow cut-off edge slope. Here, we present a new form of multilayer low pass mesh filter which is both mechanicallyrugged and maintains its performance for all temperatures between 1 and 300K. These filters function by reflectingall unwanted high frequency power thus reducing the tendency for the filter to heat up. This property combined withthe inherent low emissivity of metal mesh filters reduces the parasitic thermal loading of the cold stage and hencegives enhanced cryogen lifetime (or lower closed cycle cooler operational temperatures).
C. V. Haynes,
"Low pass filters for the far infrared", Proc. SPIE 2104, 18th International Conference on Infrared and Millimeter Waves, 21044U (30 August 1993); doi: 10.1117/12.2298606; https://doi.org/10.1117/12.2298606