REDUCTION OF GROUND SPILLOVER IN THE OWENS VALLEY 5.5-M TELESCOPE

The 5.5-m telescope of the Owens Valley Radio Observatory is used for highly sensitive measurements of the cosmic microwave background radia tion, which are easily contaminated by ground spillover. We describe t he modifications made to the secondary mirror support structure that r educed the total ground pickup of the antenna from 27 to 9 K and decre ased the peak differential ground pickup for our observing technique f rom 4.3 mK to 140 muK. This was achieved by reducing the physical obst ruction, arranging the support leg angles to minimize direct reflectio ns, and installing radiation baffles to control the direction of the r eflection or scatter lobes.