URSULA: a twin-beam multiband computer-controlled photometer for astronomical applications

A twin-beam multiband photoelectric photometer has been constructed for ground based astronomical observations. The photometer is integrated to a computer to obtain a high degree of flexibility and efficiency. The structure minimizes the atmospheric transmittance variations. It uses the photon-counting technique to optimize the SNR. Constraints on the precision of astronomical photoelectric photometry are discussed. The philosophy, structure, and possible extensions of the photometer are described. The results of a test on the dynamic behavior of the whole system are also given. This photometer is more flexible, more reliable, and simpler than other similar ones. Four of these machines have been constructed for the Catania, Milan, Naples, and Trieste Observatories.