Telescope Objective without Spherical Aberration for Large Apertures, Consisting of Four Crown Glass Lenses

Abstract

A telescope objective of 10-cm focal length may be produced with practically zero spherical aberration for all wave-lengths of the visible spectrum and for apertures up to f:2 by combining four thin lenses of crown glass (n = 1.523) of the same converging power. The lens surface curvatures approximate those of minimum aberration lenses but are in integral half diopters as follows: 1st lens, 2.5 and 0 diop.; 2nd, 4 and −1.5 diop.; 3rd, 6 and −3.5 diop.; and 4th, 7.5 and −5.0 diop. Detailed results are given for the linear spherical aberration Δf/h12, where h1 is the radius of the zone where the rays enter the first lens, as a function of h12, as a function of index of refraction n, and as a function of the spacing of the fourth lens from the third. This spacing permits final adjustment to secure the least possible aberration. Minimum aberration lenses are shown to be more effective in securing zero aberration than aplanatic lenses suggested by Dunoyer.