The general properties of telescopic optical systems consisting of a set of concentric surfaces are discussed. It is shown that if a monocentric system existed for which an incident collimated beam emerged as exactly collimated, then the system would give geometrically perfect imaging of any point in space. It is further shown that a real image of a real object is possible only for monocentric systems containing an odd number of reflecting surfaces. Such perfect collimation is not attainable, but it can be approached, yielding very highly corrected unit-magnification systems. Two such systems, one with a single reflection and one with three, have come into general use for microlithography. Their structure and properties are discussed.