Lens Systems for Correcting Coma of Mirrors

Abstract

The only serious optical defect that is outstanding in the usual reflecting telescope system is coma. The various modifications which have been proposed, and in somecases introduced, to correct coma are described and their limitations are pointed out. On the initiative of Dr. G. E. Hale, the writer has investigated the optics of a simple lens system placed near the focus of a parabolic mirror, which will correct the outstanding coma and at the same time will not introduce other aberrations of a serious amount. Employing the third-order equations of general optical theory, a solution has been obtained for a two-piece lens system which eliminates coma and astigmatism but leaves outstanding spherical aberration and distortion. These, however, are not serious in the problems of photometry and astrometry requiring the use of a lens of this type. The third-order solution is found not to be unique, two parameters being undetermined. The double family of lenses is shown in Figure 2. It is not a simple matter to choose the one lens in this double family which will function best under any given set of conditions of focal length and aperture ratio. The difficulty is due to corrective terms introduced by the fifth- and higher-order aberrations. These can oniy be obtained by the ray-tracing methods customary in the computation of complicated lens systems. Corrective lenses have been computed by the writer~ for a number of telescopes, notably the Mount Wil- son 100-inch and 6o-inch, and have proved useful in problems of photometry and astrom~try. V V INTRODUCTIO