The rings of Uranus

AT least five rings encircle the planet Uranus-as indicated by five brief occultations of the star SAO 158687 that occurred both before and after its occultation by Uranus on 10 March 1977. We observed these events with our three-channel occultation photometer¹, attached to the 91-cm telescope aboard the Kuiper Airborne Observatory (KAO). Both Uranus and the star were contained within a focal plane aperture whose diameter, projected onto the sky, was 46 arc s. A beamsplitter and focal plane television system allowed us to monitor the position of the image in this aperture simultaneously with the photometric measurements. The wavelengths of the three photometric channels used were chosen to yield favourable ratios of starlight to Uranus light² and are given in Table 1. For each channel a cooled photomultiplier (RCA C 31034), connected to a photon-counting system, was used as a detector. The data were recorded as a continuous series of 10-ms integrations of photon counts, and a strip chart recorder displayed the signals from our channels 2 and 3 so that we could monitor the progress of the occultation in real time.