Lunar occultation visibility

Abstract

This paper presents a theoretical and observational study of the visibility of stars during lunar occultations. The theory includes the effects of varying background brightness (from scattered moonlight, night skylight, twilight, daylight, earthshine, and light pollution, as appropriate), telescope parameters (including light-collecting power, magnification, and throughput), and observer quality (based on the Snellen rating, comparative experience, and age). Our model is represented as a set of equations that predict the magnitude of the faintest star that would be visible under a given set of conditions. We report on 1739 visual observations of occultations for comparison with the theoretical model. The theoretical model is well matched by the observations with a typical uncertainty of roughly one-third of a magnitude. Thus, we have confidence that our equations are an accurate model of occultation visibility. We have four recommendations: first, observers should utilize the optimal magnification where the apparent size of the star's disk matches the eye's critical visual angle (this typically is the case for a magnification of roughly 10 power per centimeter of aperture); second, observers should record the magnification used and occultation data bases should include this information; third, occultation predictions should not list the photographic magnitudes of faint stars; and fourth, occultation predictions should routinely present the calculated minimum telescope aperture required to view the occultation as a realistic guide for observers.