A PROJECT TO IMPROVE THE SENSITIVITY AND THE PHASE STABILITY OF THE λ 8-CM RADIOHELIOGRAPH AT TOYOKAWA

The A 8-cm radioheliograph at Toyokawa has been in operation since 1975 (Ishiguro et al., 1975). Daily solar radio maps are obtained in conjunction with the ). 3-cm radioheliograph (Tanaka et al., 1970) to study the structure of the solar atmosphere between the base of the corona and the chromosphere, coronal holes, S-components emitted from active regions, and radio bursts (Shibasaki et al., 1976, 1978). The antenna system is a T-shaped array of 3-m~ antennas, 32+2 elements on the East-West baseline, 16+1 elements on the North-South baseline, and can be operated as one-dimensional interferometer arrays or two-dimensional T-shaped array. In the present system, ~icrowave signals received by 3-m~ antennas are transmitted to a remote front­ end receiver via a branching network of waveguides (Tanaka et al., 1969). Each transmission line from an antenna is equipped with a rotary phase shifter to sweep a fan beam or a pencil beam. As the mapping of the Sun is performed by scanning, and that it is mechanical, radio maps thus obtained are useful as far as the stationary features in the maps are concerned, but the studies of time-variable phenomena such as radio bursts requiring a sufficient time resolution are considerably limited. On the other hand, the study of low-contrast features such as coronal holes and dark filaments is limited by the dynamic r ange of a radioheliograph. The dynamic range of a radioheliograph is governed mainly by the sensitivity and the sidelobe level. As a scanning system