THE BERKELEY-ILLINOIS-MARYLAND-ASSOCIATION MILLIMETER ARRAY

We describe the characteristics of the BIMA millimeter wave array at H at Creek, CA. The array is an aperture synthesis instrument consisting of nine 6 m diameter antennas which may be deployed in three differen t configurations, with spacings ranging from 7 m up to 1.3 km. At an o bserving frequency of 100 GHz these configurations yield maps with ang ular resolutions of 5 '', 2 '', and 0 ''.4, over a 2' field. Larger fi elds may be mapped by using multiple pointings. For all but the oldest telescopes, the surface accuracy is less than or equal to 30 mu m rms , and the aperture efficiency is 77% at 100 GHz. Background emission f rom antenna losses and spillover is very low, about 5 K after subtract ion of the cosmic B nu(2.7 K). Each antenna contains a single dewar wh ich accommodates up to four separate receivers. SIS mixers are cooled to 3.2 K with novel Gifford-McMahon cycle refrigerators. Both the uppe r and lower sidebands bf the first local oscillator are received and s eparated, providing two bands extending from 70-900 MHz on each side o f the first local oscillator. The correlation spectrometer covers a ba ndwidth of up to 800 MHz, and provides up to 2048 channels for each an tenna pair. There are four independently tunable spectral windows (in each sideband), allowing simultaneous observations of several differen t spectral lines. The spectral resolution ranges from 6 kHz to 3 MHz. For a single 8-hr track in one configuration, the sensitivity is appro ximately 1 mJy/beam in the 800 MHz wide continuum. Measurements of atm ospheric phase fluctuations as functions of both time and baseline hav e been made; these indicate that routine imaging at angular resolution s of less than 1 '' at 100 GHz is possible only if self-calibration or some other means of phase correction can be applied. Examples of a fe w recent results are included. We note that 30% of the observing time on the array is granted to visitors.