Introduction

Abstract

Some years ago I was asked to design a two-mirror optical system with a variable magnification, which would focus radiation from an extended object onto a detector. The problem was almost trivial; so much so that it appeared to be solvable by analytic means as well as by the usual numerical techniques. I made the attempt. Many months later — indeed, long after the original purpose of the design had been forgotten — and after an extensive excursion into the never-never land of cubic polynomials. I had a solution. Not a solution to the original problem, I hasten to add, but nevertheless a solution.