Field correctors and focal reducers or extenders

Abstract

The term “field corrector” in telescope systems implies some system placed inside the image plane, but relatively close to it, whose primary function is to correct the field aberrations of the mirror system. If the mirror system does not correct the axial image at the focus in question (e.g. the prime focus of an RC telescope), the field corrector will also be required to achieve this. Normally, field correctors are substantially afocal, i.e. they only have a minor effect on the f/no of the incident beam from the mirror system.