Thermal filtering for large aperture cryogenic detector arrays

Recent advances in the development of large format detector arrays for wide field astronomical applications have required the use of large aperture cryostat windows. An important consequence is the significant increase in thermal loading on the cryogens and in the stray thermal emission reaching the detectors. We have identified that a significant part of this unwanted radiation comes from re-emission of thermal power absorbed by the dielectric substrate of the metal mesh rejection filters. To overcome this we have developed a thermal filter, which preferentially reflects radiation near the 10μm thermal peak. These filters have essentially no absorption, and hence negligible emission, at these wavelengths whilst allowing high transmission of the wanted sub-millimetre bands. We report here on thermal problems with existing metal mesh filter technology and give performance data for the new filters. As a proof of concept we present data for a medium aperture instrument (QUaD1), which utilises a 200mm diameter window, both with and without thermal filters