Optical system for astronomical telescope

PURPOSE:To make the F number of an objective lens bright, and to improve curvature of field and coma aberration by satisfying prescribed conditions in a lens system consisting of a front group lens having positive focal length of corrected aberrations, and a rear group lens consisting of positive and negative lenses and having positive focal length. CONSTITUTION:In the lens system constituted of a front group lens consisting of plural lenses, having positive focal length f1, and F number F1 and corrected of aberrations, and a rear group lens disposed with positive and negative lenses by taking an air space, and having positive focal length f2, conditions of the equations (1)-(4) are satisfied. Here, (f) is the focal length of the optical system, and (l) is the distance from the front face of the front group lens to the focus of the front group lens. Thereby, the rear group lens is made usable as a correction lens in an adapter system, the F number of the objective lens is made bright and curvature of field and coma aberrations are improved.