The Beam Shape of the NRAO 900-ft Telescope and its Influence on 21-cm Line Measurements

Drift scans through the lunar center were taken in the 21-cm hydrogen line with the NRAO 300-ft tele- scope when the moon was near the Galactic plane. With the HPBW of 10 min of arc the line strength at the center of the moon should have decreased to essentially zero, but instead it decreased to about half its value in the absence of the moon. We attribute this result to the telescope's surface irregularities; the directive gain of the telescope can be approximated as a sum of two Gaussian beams of roughly equal integrated strength, one with the HPBW= 10 min of arc and one with the HPBW being probably several degrees. Twenty-one- centimeter line shapes, areas, and velocities will be seriously influenced by the large contribution from the latter beam only in regions where hydrogen occupies much of its area.