Point-by-point design method for mixed-surface-type off-axis reflective imaging systems with spherical, aspheric, and freeform surfaces

In this paper, mixed-surface-type optical system is defined as the optical system comprised of various types of surfaces, including spherical, aspheric, and freeform surfaces. A general point-by-point design method for mixed-surface-type systems is proposed. In detail, methods for spherical system design and optical power assignment are proposed during the point-by-point design process. Additionally, three surface evolutions are proposed: from spherical to aspheric surfaces and freeform surfaces, and from aspheric surfaces to freeform surfaces. A mixed-surface-type off-axis three-mirror system is designed as an example and as a starting point for further optimization. The sphere receives most of the system optical power while the freeform surface has little power, which can reduce system fabrication difficulty.