PHOTOMETRY WITH A PERIODIC GRID .1. A NEW METHOD TO DERIVE ANGULAR DIAMETERS AND BRIGHTNESS DISTRIBUTION

The interest of observations of point sources with a periodic grid has been recognized for long in astrometry because of the direct relation existing between the position in the sky and the phase of the modulat ed signal. We investigate in this paper a complementary aspect by show ing how observations of an extended stellar or planetary object carrie d out with a periodic grid may also yield valuable information on its size and on the light distribution over its surface. Several categorie s of objects may benefit from this kind of observation depending on th e modulation period, the telescope focal length, and the signal-to-noi se ratio that can be achieved. We discuss the overall principles of th is method and derive an analytical formulation for the 'modulation fun ction' defined in the text. An application based on Hipparcos observat ions of the largest minor planet (1) Ceres is provided as an illustrat ion of the method.