Temperature Measurements On BIMA 6-m Antennas— Part I: Backing Structure

Measurements have been made of the temperature distribution in the antenna backing structures on the 6 m diameter antennas of the BIMA Array at Hat Creek. 32 thermistors were distributed around the backing structure and on one panel and were monitored under varying conditions. Significant gradients were found between the front and rear of the backing structure, across the structure, and radially. These gradients produce distortions of the primary reflector surface that are primarily focusing or pointing errors. The timescales of the temperature changes are short enough to make refocusing and repointing of limited effectiveness. Sunshades and fans together reduce the effects by a factor of 2-4. For the BIMA antennas most of the thermal influences are probably tolerable, but for the mmA the effects may be larger then allowed for by the specifications. Careful design could reduce the problem, but examination of other materials such as CFRP is clearly warranted.