4 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 limits of 2$^\circ$ to 90$^\circ$. Azimuth motion has a rate of 90$^\circ$ per minute within a range of $-$90$^\circ$ to 450$^\circ$. Antennas are stowed to avoid damage in high winds, or in case of substantial snow or ice accumulation. See Napier (1995) for further antenna information.