Optical design of off-axis freeform reflective zoom imaging system with large relative aperture

Off-axis reflective zoom imaging optical system has a wide range of applications in the field of photoelectric detection because of its advantages of chromatic aberration-free, broad-spectrum imaging. The existing off-axis reflective zoom imaging optical system has a fixed pupil diameter, and as the focal length becomes larger, the relative aperture becomes smaller, resulting in a lower signal-to-noise ratio and weaker detection capability. Additionally, aberration correction is vitally important in the off-axis reflective zoom system with large relative aperture. An attempt to improve the performance of an off-axis reflective zoom imaging system with large relative aperture using freeform surface is reported. The F number is 4, and the zoom ratio is 3. The optical design with freeform surfaces shows marked improvements compared with the design with higher order aspheric surfaces.