Study On Large Aperture Space Optical Imaging System With Ring-Sector Pupil

To improve the capability of space optical remote sensing systems, an optical imaging system with higher resolution is required. One method is to enlarge the aperture of optical system, which is limited by optical glass material available, weight, cost and delivery capacity. Thus it is necessary to explore new principle, method and technique to improve the resolution without increasing the aperture. In this work, a ring-sector encoded aperture-based primary mirror is investigated. The result is helpful for breaking a new path for large aperture optical systems. And it is beneficial to advance the development of spaceborne optical remote sensors.The proposed ring-sector pupil encoding technique is to sparsely encode the primary aperture of large optics. Since it forms an intermediate blurring optical image on detector, the final sharp image is necessarily recovered with digital decode procedure. This technique combines optical design and digital image processing together so that substantial mass reduction of its large aperture primary and high imaging performance equivalent to its corresponding full-aperture optics can be concurrently implemented, but also its optical transfer function is low-sensitive to large defocus range. And consequently the ring-sector pupil structure has excellent prospect for practical applications.At first, this paper overviews in detail the state of art of research home and abroad about high resolution and large aperture optical remote sensors. It is point out that vigorous development of high resolution remote sensing satellite and exploration of new both theory and technology is urgent in our country. The imaging procedure and the point spread function (SPF) and modulation transfer function (MTF) of encoded pupil optics are respectively analyzed and derived from Fraunhofer diffraction theory. Through analysis of the structural types of aperture encoder and its associated MTF distribution feature, design philosophy about large aperture sparse pupil, i.e. ring-sector pupil structure, is put forward.Then, through discussion of the geometrical characteristics and structural style of ring-sector pupils and analysis of their pupil function and corresponding SPF and MTF, why our proposed ring-sector pupil has the same resolution power as its corresponding full pupil is explained clearly. The affluence of its primary structural parameters, i.e. width-diameter ratio and sub-aperture angle, on MTF is investigated. Its low-sensitivity to defocus aberration is analyzed and shown. From the viewpoint of practical application, its manufacturability is analyzed and it is shown, through finite element analysis, that the ring-sector pupil structure has a favourable and marked effect on light weight.Further, according to the application features of high-resolution large-aperture optics, an on-axis three-mirror telescopic objective with a bias field of view and a catadioptric objective with all spherical surfaces are optimally designed, and ring-sector pupil is applied to these designed lenses. Their optical imaging and digital image recovering are simulated for on-axis scene under different defocus. From the good recovered images, it is shown that the suggested ring-sector pupil structures allow large aperture optics low-sensitive to defocus and, to some extend, to widen its field of view.Finally, an experimental ring-sector encoded pupil system is manufactured. Imaging results for star points and resolution test plate with ring-sector pupil of different filling factor verify that the imaging resolution power from the combination of ring-sector pupil optical imaging and digital image restoration can be compared with that of its corresponding full-aperture, and that it may have obvious advantage of potential applications.