Parametric geometry analysis for circular-aperture off-axis parabolic mirror segment

Abstract. We investigated the geometrical characteristics of circular-aperture off-axis parabolic (OAP) mirror segments to clarify the meaning of the loosely defined word “center” used in the literature and in documents to describe OAPs. We proposed the elliptical aperture center of an OAP as the definition of the center. The off-axis distance (OAD) is the vertical distance from the reference optical axis to the aperture center. In addition, the OAD can be varied depending on the desired center of a circular aperture to select the part of a parallel beam for focusing. The radius of the circular aperture becomes the minor-axis semidiameter of the elliptical aperture of the OAP. These geometrical parameters were systematically defined, derived, and/or analyzed in the context of optical engineering applications. Based on a set of those fundamental parameters, an intrinsic datum point utilizing the deepest point on the OAP surface was presented. The datum point provides a well-defined reference co-ordinate frame for locating or aligning an OAP within various astronomical telescope designs, instrument manufacturing and assembly processes, and optical system alignment and testing applications.