Design of lens-hood in the space fisheye optical system

Abstract

Due to the extra wide field of view, fisheye optical systems are appropriately applied in space camera for scouting large-scale objects with near-distance. At the same time, because of the violent sunlight linger within the field of view more than other optical system and more stray light occur during the period, to design proper lens-hood can effectively reduce the sunshine time. Another distinct characteristic of fisheye optical system is the first protrude lens, which is contrived with negative focus to trace the ray with angle about even above 90 degree of incidence. Consequently, the first lens is in danger of damaging by scratching when operating the camera during the ground experiments without lens-hood. Whereas on account of the huge distortion which is the third mainly characteristic of fisheye optical system, to design appropriate lens-hood is a tough work comparing with other low-distortion optical system, especially for those whose half diagonal field is more than 90°. In this paper，an research carried out on the design lens-hood for fisheye is proposed. In the way of reverse ray-tracing, the location on the first lens and point-vector for each incident ray can be accurately calculated. Thus the incident ray intersecting the first lens corresponds to the boundary of the image sensor form the effective object space. According to the figure of the lens and the incident rays, the lens-hood can be confirmed.In the proposed method, a space fisheye lens is presented as a typical lens, whose horizontal field and vertical field are 134°, diagonal field is up to 192°, respectively. The results of design for the lens-hood show that the lingering time of sunshine is shorten because of obstructing some redundant sunlight, and the first outstanding lens are protected in the most degree. 1．INTRODUCTIONTo keep the large-scale objects under surveillance within near-distance, such as the solar battery boarding, the cameras with fisheye optical system are widely applied on spacecraft due to the extremely wide field of view. The first lens of the fisheye optical system bulges outward, which is called fisheye. Thus the fisheye optical system is often at the hazard of being scratched by careless operation during the ground experiments. Meanwhile, the sunshine on the fisheye lens lasts more time and produces more stray light when the spacecraft circles the sun, which affects the images and shorten the camera's life time. Accordingly to design the lens-hood in the fisheye optical system for space camera is a necessary step.Yet it is unsuitable to use the traditional conic lens-hood when the half diagonal field is more than 90°. Usually the petaloid lens-hoods are designed. As a result of the natural huge distortion, to contrive the lens-hoods closest to the object space for fisheye optical system is quite more difficult than for the low-distortion optical system.