SHARC II: A Caltech Submillimeter Observatory facility camera with 384 pixels

SHARC II is a background-limited 350 mum and 450 mum facility camera for the Caltech Submillimeter Observatory undergoing commissioning in 2002. The key component of SHARC II is a 12x32 array of doped silicon 'pop-up' bolometers developed at NASA/Goddard. Each I mm x 1 mm. pixel is coated with a 400 Omega/square bismuth film and located lambda/4 above a reflective backshort to achieve >75% absorption efficiency. The pixels cover the focal plane with >90% filling factor. At 350 mum, the SHARC II pixels axe separated by 0.65 lambda/D. In contrast to the silicon bolometers in the predecessor of SHARC II, each doped thermistor occupies nearly the full area of the pixel, which lowers the 1/f knee of the detector noise to <0.03 Hz, under load, at the bath temperature of 0.36 K. The bolometers are AC-biased and read in 'total power' mode to take advantage of the improved stability. Each bolometer is biased through a custom similar to130 MOmega CrSi load resistor at 7 K and read with a commercial JFET at 120 K. The JFETs and load resistors are integrated with the detectors into a single assembly to minimize microphonic noise. Electrical connection across the 0.36 K to 4 K and 4 K to 120 K temperature interfaces is accomplished with lithographed metal wires on dielectric substrates. In the best 25% of winter nights on Mauna Kea, SHARC II is expected to have an NEFD at 350 mum of 1 Jy Hz(-1/2) or better. The new camera should be at least 4 times faster at detecting known point sources and 30 times faster at mapping large areas compared to the prior instrument.