We describe an aperture synthesis radio tele- scope optimized for studies of the Galactic interstellar medium (ISM), providing the ability to image extended structures with high angular resolution over wide elds. The telescope produces images of atomic hydrogen emis- sion using the 21-cm H i spectral line, and, simultaneously, continuum emission in two bands centred at 1420 MHz and 408 MHz, including linearly polarized emission at 1420 MHz, with synthesized beams of 1 0 and 3:4 0 at the respective frequencies. A full synthesis can achieve a con- tinuum sensitivity (rms) of 0.28 mJy/beam at 1420 MHz and 3.8 mJy/beam at 408 MHz, and the 256-channel H i spectrometer has an rms sensitivity of 3.5B 0:5 sin K per channel, for total spectrometer bandwidth B MHz and declination . The tuning range of the telescope permits studies of Galactic and nearby extragalactic objects. The array uses 9 m antennas, which provide very wide elds of view of 3.1 and 9.6 (at the 10% level), at the two frequencies, and also allow data to be gathered on short baselines, yielding extremely good sensitivity to extended structure. Single-antenna data are also routinely incorpo- rated into images to ensure complete coverage of emission on all angular scales down to the resolution limit. In this paper we describe the telescope and its receiver and corre- lator systems in detail, together with calibration and ob- serving strategies that make this instrument an ecient survey machine.