A source drifting through the beam of a GMRT dish

Adapted from IOAA 2016 (Bhubaneswar), theory problem 5. We have restated it in our own words and kept the original data. The solution was last checked on 20 October 2026.

The Giant Metrewave Radio Telescope is at latitude 19°06′ N. One of its dishes, 45.0 m across, is held still pointing at a zenith distance of 39°42′ on the northern meridian, so that a point source that crosses the meridian passes along a diameter of its beam. For how long does the source stay within the full width at half maximum of the beam of the dish at 200 MHz? Take the width of the beam to be the resolution of the dish, $1.22\lambda /D$.

AnswerThe source stays within the half-power width of the beam for about 18 minutes.