Cemented doublet field compressor/corrector: a new optical component of general utility

Abstract

Field correcting lenses have been used to increase the fields of view of large telescopes since at least Ross's work in 1935. In recent years, due to the advent of electronic image sensors, there has been movement toward combining field correction with focal length reduction in so-called field compressor/corrector or telecompressor lenses. Such lenses are now commonly employed in both professional and amateur astronomy. This paper demonstrates that field compressor/corrector lenses can be of great utility in a wider context -- the design of optical instruments. In the finite conjugate and relay applications prevalent in optical instrument design, standard achromats are often employed because there is currently no better alternative short of custom lenses. I show that there exists a particularly attractive family of cemented doublet compressor/correctors that can be combined with standard achromats to greatly improve the imaging performance obtainable in these applications. I propose that these field compressing/correcting doublets should be made available from stock as standard optical components. I introduce a novel viewpoint for analyzing the imaging capability of a lens that makes it simple to visualize the performance obtainable with a lens when it is employed over a wide range of fields and focal ratios. Using the new viewpoint, I demonstrate the performance improvement obtainable when employing the new compressor/correctors. I also show that the new lenses are flexible, that is, that one may obtain their advantages while employing them with a range of achromats or over a range of focal lengths, and that the alignment tolerances between the achromat and the field compressor/corrector are modest.