Time-domain multiplexed SQUID readout of a bolometer camera for APEX

We describe the development and fabrication of the sub-millimeter APEX (Atacama pathfinder experiment) bolometer camera (SABOCA) and its installation at the APEX radio telescope in Chile. SABOCA has 37 bolometers for a wavelength of about 0.35 mm. The bolometers with superconducting transition-edge sensors are read out by superconducting quantum interference devices (SQUIDs). Time-domain multiplexing was used to compress 40 readout signals into four output channels, which are further amplified by superconducting quantum interference filters (SQIF). The multiplexer readout circuit is described in detail. One spare SQUID and two spare bolometers have been used as reference to evaluate contributions to the bolometer noise figure. SABOCA's electrical noise equivalent power is about 7 × 10 − 16 W Hz − 1/2 , whereas the optical noise equivalent flux density measured at APEX is better than 0.35 Jy Hz − 1/2 (1 Jy = 10 − 26 W m − 2 Hz − 1 ). The measured bolometer noise exceeds the theoretical estimation by factor of two, possibly caused by excess noise in the transition-edge sensor. Nevertheless the achieved instrumental sensitivity meets the specification given by the APEX board. SABOCA was commissioned as the facility instrument in the spring of 2009.