A radio telescope for the calibration of radio sources at 32 GHz

Knowledge of the flux of celestial sources is important to radio astronomers and telecommunications engineers who wish to accurately calibrate their large aperture antennas. At 32 GHz the flux of celestial objects is given by extrapolations using the spectral indices obtained from measurements at lower frequencies; there have been no direct calibrations. We have therefore developed a program whereby a 5-meter diameter radio telescope at the Owens Valley Radio Observatory (OVRO) operating at 32 GHz will observe a list of radio sources. By accurately calibrating the 5-meter antenna we will be able to have direct measures of celestial radio source fluxes. Accurate calibration of the 5-meter radio telescope must be done in-situ at OVRO; therefore, we have built a radio telescope intended to be a standard gain tool to be used to perform the accurate 5-meter calibration. This telescope uses a highly efficient 1.5-meter offset feed Cassegrainian optics configuration whose aperture efficiency is nearly 80%. The telescope uses a radiometer with a high electron mobility transistor (HEMT) low noise amplifier (LNA), and a pair of feeds to operate in a beam switching mode. When tracking a celestial source, one of the two beams is pointed to the source and the other is pointed to an empty part of the sky. Coherent detection of the difference signal of this rapidly switched signal provides a measure of the signal due to the source. The technical challenge for this standard gain telescope is that it must provide accurate tracking of the celestial source with minimum pointing errors; it must contribute a minimum noise temperature; each beam on the sky has to be nearly equal in performance; and a stable radiometer that can detect signals in millikelvins is required. Such a system is summarized.