Design of an oblique camera based on a field-dependent parameter

In this paper, a new parameter, field focal length (FFL), is introduced to characterize the optical properties for each field of an optical system. A freeform three-mirror off-axis reflective system, working as an oblique camera, is found to have various FFLs across the field angle of 30°. With the help of FFL, the ground resolution (GR) of this camera is analyzed under two working modes. One of the two working modes has more uniform GR when the field with larger FFL value aims at the object far away from the camera. By using FFL to control the GR, an oblique camera system is designed that has a constant GR within 99.41% over the field of view.