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An earth-rotational synthesis radio telescope is now operating at a wavelength of 21 cm at Fleurs, near Sydney, Australia. The telescope produces a map of a 1.5 square-degree region of sky in an observing time of 12 to 16 h. In the first stage the resolution is 40" and the sensitivity (5 times rms noise fluctuations) is about 70 × 10-29. A full range of scanning angles is available for mapping sources between declinations 80°S and 30°S, but mapping of a lower quality is possible at lower declinations. The telescope consists of two independent arrays used consecutively. One is in an east-west line and the other is in a north-south line. Each consists of two 13.8-m-diam paraboloids and thirty-two 5.8-m paraboloids arranged as a compound grating interferometer. The paraboloids track a source for 8 h and the phases of signals from each paraboloid are rotated continuously to compensate for the movement of the source across the sky. 64 cosine and 64 sine Fourier components of the brightness distribution over the source are extracted simultaneously and each is recorded every minute. After an observation of several hours, a digital computer makes the necessary transformations and produces a map.