Lunar occultation measurements of stellar angular diameters

Abstract

Offering an angular resolution which has remained unattained by any other technique for decades, lunar occultations have traditionally been the most productive method for the measurement of stellar angular diameters. Unlike interferometric methods, which are limited in resolution by the size of the aperture or of the baseline between apertures, in a lunar occultation the key to high angular resolution is the phenomenon of diffraction by a straight edge, that occurs at the Moon's limb in a turbulence-free environment. For the reader not familiar with the physics and technical aspects of the lunar occultation (LO) technique, it is sufficient here to show in Fig. 1 some practical examples of occultation lightcurves for sources with different angular diameters. It can be noted that the contrast of the fringes is maximum for a point-like source; it then decreases with the angular diameter, and eventually reaches the regime of a monotonic drop in the signal-as predicted by simple geometrical optics- when the angular extent of the source is large. In practice, the LO method is well suited to measure angular diameters in the range 1 to 50 milliarcseconds (mas). There is no real limitation concerning the wavelength of observation, although at present the near-IR is the region of choice for several different reasons (Richichi 1994).