Abstract.We describe an aperture synthesis radio telescope optimized for studies of the Galactic interstellar medium (ISM), providing the ability to image extended structures with high angular resolution over wide fields. The telescope produces images of atomic hydrogen emission 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 and 3.4 at the respective frequencies. A full synthesis can achieve a continuum 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 δ K per channel, for total spectrometer bandwidth B MHz and declination δ. The tuning range of the telescope permits studies of Galactic and nearby extragalactic objects. The array uses 9 m antennas, which provide very wide fields 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 incorporated 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 correlator systems in detail, together with calibration and observing strategies that make this instrument an efficient survey machine.

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A 408‐MHz earth rotation synthesis telescope is described, which has been constructed as a second frequency channel on an existing 1420‐MHz telescope. Both frequencies are received simultaneously. The angular resolution at 408 MHz is 3.5 arc minutes, and the field of view is 7°. Sensitivity is 3 mJy/beam area (1 Jy = 10−26W m−2 Hz−1). A dual‐frequency, prime focus feed receives both hands of circular polarization at each frequency. The signal transmission system is designed so that the signal phase from its 408‐MHz input to its 30‐MHz output is entirely unaffected by changes in the length of the single coaxial cable which joins each antenna to the central processor. The functions of phase rotation, signal delay equalization, and correlation are performed in a digital signal processor, using microcomputer software rather than hardware. The quadrature output is also produced by computation in real time, using a band‐limited form of the Hilbert transform. Some observational results are presented.