Upgrade of a large millimeter-wavelength radio telescope for improved performance at 115 GHz

A 20.1-m-diameter radome enclosed radio telescope, built by Electronic Space Systems Corporation (ESSCO) at the Onsala Space Observatory (OSO) near Gothenberg, Sweden, in 1975, was upgraded by ESSCO in 1992 for improved operation at 115 GHz, increasing the system aperture efficiency from 20% to 40% at this higher frequency. Electrical gain measurements confirm geometric optics predictions of efficiency and improved antenna patterns. The upgrade included replacement of the two inner reflector panel rows with 31-μm panels, stiffening the reflector backstructure based on finite-element structural analysis for a measured rms gravity deformation of 59 μm in the 25° to 70° elevation range, and optical alignment of the reflector surface to 58 μm rms accuracy. This alignment accuracy of approximately 1/345000 of the reflector diameter was accomplished with an electronic angle-measuring theodolite and digital radial strap gauges. Data were downloaded in real time to an on-line portable computer performing surface metrology calculations. Graphical data are presented that compare computer predictions of reflector backstructure gravity deformations to optical measurements, which proved crucial in achieving the stringent alignment accuracies. Measured antenna efficiency data before and after the upgrade are included