Bolometer array development at the Max-Planck-Institut fuer Radioastronomie

Continuum radiometers based on bolometers have a long tradition at the Max-Planck-Institut fur Radioastronomie (MPIfR) in Bonn, Germany. Arrays of bolometers have been under development since the early 90s. A small 7-element system, operating at 300 mK, saw first light in 1992 at the IRAM 30 m- telescope and has been used successfully by numerous observers at that facility since then. While this array had a conventional 'composite' design, it was obvious that larger arrays, especially for higher frequencies, could take advantage of microfabrication technology. The recent MPIfR bolometer arrays employ a hybrid approach. They combine a single-mode horn array with a planar bolometer array on a single crystal Silicon wafer with Silicon-Nitride membranes. With efficient absorbing structures, the bolometers couple to the single mode of the radiation field collected by the horns, without needing integrating cavities. Readout is provided by NTD-Germanium thermistors that are attached to the absorbers. This paper covers the history of this development, the general aspects of the bolometer arrays, including the coupling to the telescope, and the status of work in progress.