ANTENNAS

The main reflector of each VLBA antenna is a 25-m diameter dish which is a shaped figure of revolution with a focal-length-to-diameter ratio of 0.354. A 3.5-m diameter Cassegrain subreflector with a shaped asymmetric figure is used at all frequencies above 1 GHz, while the prime focus is used at lower frequencies. The antenna features a wheel-and-track mount, with an advanced-design reflector support structure. Elevation motion occurs at a rate of 30$^\circ$ per minute between a hardware limit of 2$^\circ$ and a software limit of 90$^\circ$. Azimuth motion has a rate of 90$^\circ$ per minute between limits of $-$90$^\circ$ to 450$^\circ$. Antennas are stowed to avoid operation in high winds, or in case of substantial snow or ice accumulation. In 2007, the St. Croix antenna was taken out of service for three months to repair severe corrosion caused by its location very close to the ocean. See Napier (1995) for further antenna information.