The Ellipsoidal Mirror Used As An Objective Lens

Problems associated with scientific uses of high-speed photography are many. This paper deals with the problems encountered in using long focal length relatively fast objective lenses at short conjugates (less than 20:1) and gives one solution. The problems arise when small subjects must be photographed from many feet away. A long focal length lens (500 MM or longer) is required in order to achieve enough magnification to resolve fine detail (thousandths of an inch) and a reasonably fast speed (f/5.6 or faster) is required when a non-light emitting subject is photographed at very short exposure time (millisecond or microsecond range). All lens aberrations are corrected for infinity unless otherwise stated by the manufacturer. At conjugates of approximately 12:1 aberrations start to degrade the image noticeably when fast relative apertures are used. An ellipsoidal mirror corrected for the conjugates and f number needed, is one solution. Subjects covered are: determining when an ellipsoid is required, how it should be specified, how it can be tested, and design and performance of a completed system.