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A low-noise X-band radiometer, using a ruby maser preamplifier in radio astronomy measurements, is discussed. The radiometer uses a reflection-cavity maser with voltage-gain bndwidth products as high as 300 Mc at 4.2Â°K. Less than 1 per cent short-term gain instability and dependable performance are obtained. A system noise factor (excluding antenna spillover) of 0.6 db 43Â°K) has been obtained. The system bandwidth is limited to 8 Mc by the intermediate frequency bandwidth, with maser bandwidths of 20-30 Mc available. RMS noise fluctuations of approximately 0.01Â°K with a 12-second integration time and 0.007Â°K with a 42-second integration time are obtained. Gain instability of 0.6 per cent up to 10 minutes and 2 per cent up to 30 minutes has been measured. Electrical and mechanical features as well as measurement and operational techniques are described. Performance data and radio astronomy observations are discussed.