The Stanford microwave spectroheliograph antenna, a microsteradian pencil beam interferometer

A pencil beam interferometer has been constructed at Stanford, Calif., with multiple beams of 3.1 minutes of arc width to half power (0.8 microsteradian). It is composed of two equatorially-mounted, 16-element, Christiansen arrays of 3-m paraboloids, each 375 feet long (1255 wavelengths at a wavelength of 9.1 cm). The half power beamwidth of the fan beam of a single array is 2.3 minutes of arc. To form the pencil beam, the two arrays are switched together as in a Mills cross. Frequency range is from 2700 to 3350 Mc. Phase adjustment and monitoring are handled by a new technique of modulated, weakly reflecting gas-discharges maintained at the focus of the paraboloids. Television-type scanning yields maps of the sun (spectroheliograms) revealing fine details of the microwave source regions in the chromosphere and corona. All the transient bursts and a large fraction of the steady solar emission at 9.1 cm prove to originate in a small number of highly compact centers, whose brightness temperatures may exceed K. The sensitivity of the instrmnent also allows the thermal emission from the moon ( K) and a number of galactic and extragalactic sources to be studied with high angular resolution. Illumination of the moon by terrestrial radar can be detected. The pencil beam interferometer furnishes the finest beams currently available from pencil beam antennas of any type. Examination of the fundamentals of extracting high resolution details of a source from its radiation field indicates the fitness of pencil beam interferometers, incorporating steerable multielement arrays, for future development to higher resolving power. Adequate technique of phase preservation over wide spacings is available.