The radiometer and polarimeters at 80, 35, and 17 GHz for solar observations at Nobeyama

An 80-GHz whole Sun radiometer with a new technique for reducing the atmospheric effect has been completed at Nobeyama. The radiometer has been proved to have the capability of detecting small bursts with the flux density of down to ∼10 sfu (solar flux unit) under usual weather conditions; the detection limit has been lowered by one and a half orders of magnitude compared with that achieved with radiometers of a conventional type. The key point of the new technique is to cancel the quiet Sun component of large flux density (∼ 10⁴ sfu), which deteriorates the detection limit through atmospheric fluctuations, by correlating signals from two small antennas installed on a common equatorial mount with a separation of ∼330 wavelengths. Owing to this new technique, it has been possible even for small flares to determine the microwave spectrum, in association with polarimeters operating at 17 and 35 GHz.