Josephson junction detector for astronomical applications

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Abstract
A wide-band detector for the 1.0 to 1.6 mm atmospheric transmission window has been developed for astronomical observations at the McDonald Observatory 2.7 m optical telescope. For observations of continuum astronomical sources, the relevant figure of merit is the « Noise Equivalent Temperature » or δ Trms. The present δ Trms = 0.25 K for a 1 s observation. The astronomical objects which have been observed so far include the sun, moon, Venus, Jupiter, 3C 273 and the galactic center. With the present detector sensitivity, a 1 mm source of 0.7 flux units would give a signal to noise of one for a 3 h observation at the NRAO 11 m telescope. The detector uses a point contact Josephson junction, with a niobium point against tin. The junction is mounted coaxially in a multimode 5 mm diameter cylindrical cavity which is coupled by a cone to a 5 mm oversize circular wave guide leading up the neck of the dewar. The wide-band mode of operation is used, in which the incident continuum radiation from an astronomical source causes a depression of the junction critical current. The junction is biased with a 20 Ω source resistance, and the change in bias voltage due to the change in the junction critical current is measured. Two modes are used to chop the 1 mm radiation incident on the junction. For bright astronomical sources such as the sun and the moon, a total power chopper at the entrance of the wave-guide alternately opens and closes the radiometer entrance. For faint sources, a differential chopper chops between the source and adjacent sky

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