Optical System Design and Distortion Control of Wide Field of View, All-Reflective Imagers

Abstract

Wide field of view (FOV) optical systems allow imaging of large scenes for panoramic photos, surveillance and reconnaissance, or survey missions such as monitoring changes of the earth surface related to global warming. Many current wide field of view systems allow significant optical distortion simplifying the optical design while also allowing compatibility with small, current-technology focal plane arrays. These optical systems are typically refractive, like a fish-eye lens, which has certain downsides such as: limited spectral range, higher absorption, and increased thermal sensitivity. It is also common for wide FOV systems to sacrifice spatial resolution and signal to noise ratio (SNR) in order to maintain a compact system size. The aperture-FOV product, known as throughput, is generally difficult to increase beyond current practice, thus increasing the FOV often comes at the expense of aperture size. This dissertation explores new all-reflective optical designs, system configurations, and applications for wide FOV, high throughput systems with near zero or positive beneficial distortion. These all-reflective, off-axis, unobscured designs utilize freeform mirrors, which allow additional freedom in optical surface form required for precise distortion control over a wide field. Even an optical system with no internal distortion produces distorted images of the earth due to its convex curvatures. The optical system can be designed with positive distortion to counter the negative distortion of the curved-earth to produce images with constant resolution. Four new optical designs are given in this dissertation that are novel because they are all-reflective and have and extreme combination of wide FOV and zero to positive image distortion. These 4 designs have the following f/#, EFL, FOV, and distortion: 1) f/3, 2.6 inch EFL, 120° x 4° FOV, 0% distortion, 2) f/2.5, 2.5 inch EFL, 70° x 4° FOV, +13% distortion, 3) f/2.5, 2.2 inch EFL, 90° x 12° FOV, 0% distortion, and 4) f/2.5, 2.2 inch EFL, 60° x 20° FOV, 0% distortion. These designs are ideal for mid-wave and long-wave infrared systems, not only because they are all-reflective, but they also feature an exit pupil for a cold stop to reduce thermal background signal. Optical design software reports Zernike polynomial aberrations vs field, but the polynomial is contaminated by higher order aberration terms inherent in wide FOV designs. A method is shown to convert Zernike polynomials to monomial aberrations which provide a more direct understanding of the design’s aberration behavior. This process is essential to measure aberration nodes, relating to Nodal Aberration Theory, for wide FOV systems. Distortion in bilaterally symmetric optical systems (off-axis reflective designs) such as smile, keystone, and anamorphism are calculated per surface and their wavefront field maps are analyzed. An in-depth study of all possible 3-mirror design configurations is given highlighting new design possibilities. Scanning optical systems, such as push-broom and whisk-broom satellite sensors are compared using a satellite sensor system model that includes orbital mechanics. It is shown that a wide FOV, small entrance pupil, mechanically simpler push-broom system, can have better performance than current-practice narrow FOV, moderate aperture, whisk-broom sensors. The optical designs in this dissertation have large AΩ product (throughput) requiring large or multiple FPAs. The FPAs needed for these designs are not readily available with today’s technology. These designs also require large freeform mirrors that may be challenging for fabrication and alignment. However it is not the goal of this dissertation to balance optical design with practical or economical FPA, mirror fabrication, and alignment. Instead the goal is to present a series of progressive optical systems that can be made possible with cutting edge FPA and mirror fabrication technology. It is hoped that the optical designs described in this dissertation can be used in a future satellite sensor or other application benefiting from an all-reflective, wide FOV system with zero or positive image distortion.