An Operational Ruby Maser for Observations at 21 Centimeters with a 60-Foot Radio Telescope

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Abstract
A maser preamplifier designed for radiometry at a wavelength of 21 cm is described. The maser is mounted at the focus of the 60‐ft reflector of the Harvard College Observatory and is being used principally for observations of hydrogen‐line radiation from extra‐galactic nebulae. An automatic gain stabilization system for the maser using a modulated‐noise reference signal is described and an example of an observation made with this stabilization scheme is included. The total input noise temperature of the radiometer is 85°K, without the gain stabilization, and 148°K with the stabilization.