The Stanford five-element radio telescope

The Stanford radio telescope array is a fast image-forming interferometer using earth rotation synthesis to produce a brightness map of the continuum radiation from a portion of the sky using data obtained in one 10-h observation. The array consists of five 18.3-m diam paraboloid antennas mounted on an east-west line in such a way that pairs of antennas span all spacings from 1 to 9 times the unit spacing of 22.9 m. At the operating wavelength of 2.8 cm (10 690 MHz) the half-peak beamwidth is 18".8 and the grating-lobe spacing is 4\´.2, both in the east-west direction. The antennas, which were constructed at Stanford, are equatorially mounted and have a range of motion in declination from the south horizon to the north pole and in hour angle from horizon to horizon or ±5 h, whichever is less. Both hour angle and declination motions are controlled by chain-drive mechanisms. The electronic receiving system includes tnnnel-diode preamplifiers followed by mixers and an IF system with a passband from 10 to 70 MHz; both upper and lower sidebands are accepted. The local oscillators at the antennas are phase locked to a reference signal which is distributed from a centrally located oscillator. A system for monitoring variations in the electrical lengths of the reference-signal cables is incorporated using modulated reflectors at the five antennas. The IF signals from the antenna pass through 9-b variable delay lines and the signals from each of the ten possible antenna pairs are then fed into ten analog multipliers. An on-line computer samples the output waveform of each multiplier five times per second and digitally filters the data to estimate the complex correlation of the signals averaged over any desired time interval. The computer also sets the delays, monitors various equipment, and controls the data recording and operator displays. In addition, it can be used off-line to perform the Fourier transformation or similar processing required to derive a map of a radio source. The sensitivity with the tunnel diode preamplifiers gives a signal-to-noise ratio of 5 to 1 for a point source of flux density 3 × 10^-28W ċm^-2Hz^-1; this assumes a system noise temperature of 1000 K, an a- ntenna aperture efficiency of 30 percent, and an observing time of 10 h. An increase in sensitivity by a factor of 10 will be obtained by the use of uncooled degenerate parametric amplifiers.