This invention relates to an optical system for lantern slide projection, especially for star projectors in planetariums, with the aim of increasing the available light for the projection. The development consists in attaining an increase in brightness and brilliance of the image as well as a reduction in heat generation in the apparatus, through the application of resonator principles.The optical system comprises light sources, condensing lenses which reproduce the lamp filament, and the objective which reproduces the original. A resonator of high transparency which has highly reflective and/or totally reflective boundary surfaces is provided between condensing lens and objective. The boundary surface facing the condensing lens has a light-entrance opening comprising the lamp filament or the image of the lamp filament. The boundary surface lying opposite to that area preferably comprises the original to be projected.The resonator is formed as a glass or hollow body with circular, elliptical or multi-faceted cross-section or as a polyhedron, hollow polyhedron or ball.