The MPIR 100 mK bolometer array for 2 mm continuum observations

We are developing bolometer arrays for continuum detection in millimeter and submillimeter astronomy [1]. For the 2 mm atmospheric window, where the transmission is comparatively high, a bolometer temperature of about 100 mK is necessary in order to avoid being limited by the system noise. Our new 2 mm array is cooled by a 3He/4He-dilution refrigerator with a base temperature of 30 mK. The substrate for the 19 channel bolometer array consists of a single-crystal silicon wafer with silicon-nitride membranes. Radiation is collected by a single-mode horn array in front of the wafer and coupled into efficient absorbers in the center of the membranes. Resulting temperature changes of the absorbers are measured with NTD-germanium thermistors. In the first stage of the read out electronics, we use JFETs working at 150 K. Cold RF-filters prevent RF interference from entering the bolometer array cavity. The combination of several mesh filters and a short piece of cylindrical waveguide at the end of each horn defines the bandpass for the incoming radiation, which is matched to the 2 mm atmospheric window.