Using progressive design method, we designed and built a wide field of view (FOV) catadioptric lens for LWIR earth sensors in the 14 to 16 μm range. The prototype lens is compatible with 640×480 uncooled FPA and 25 microns pixel pitch. Its full field of view is 170° and F number is 0.86. The f-theta distortion is less than 1%. Besides, the system works well during a temperature range of -40°C~+60°C.
We designed a compact middle-wave infrared (MWIR) lens with a large focal length ratio (about 1.5:1), used in the 3.7 to 4.8 μm range. The lens is consisted of a compact front group and a re-imaging group. Thanks to the compact front group configuration, it is possible to install a filter wheel mechanism in such a tight space. The total track length of the lens is about 50mm, which includes a 2mm thick protective window and a cold shield of 12mm. The full field of view of the lens is about 3.6°, and F number is less than 1.6, the image circle is about 4.6mm in diameter. The design performance of the lens reaches diffraction limitation, and doesn’t change a lot during a temperature range of -40°C~+60°C. This essay proposed a stepwise design method of infrared optical system guided by the qualitative approach. The method fully utilize the powerful global optimization ability, with a little effort to write code snippet in optical design software, frees optical engineer from tedious calculation of the original structure.
The earth sensors on the satellites measure the attitude by observing the discontinuity between earth radiance and cold space background. As IR detector technology advances, the earth sensor has evolved from traditional scanning sensor to static sensors with uncooled infrared FPA. In order to obtain high pointing accuracy of the earth center and avoid the influence of the cloud layer, the optical system should have the following characteristics such as: excellent f-theta linearity, high MTF at corresponding frequency, and high transmittance in the 14 to 16 μm range. In addition, this earth sensor is intended to be used on a LEO satellite, so the optical system must cover a wide FOV larger than 130°, and be as light and compact as possible.
This paper contains the full design process of a wide-angle lens used for LWIR earth sensors from paraxial power allocation calculation to optimization of lens. The lens has a relative large image circle of 13mm, which is compatible with a 640×480 25μm uncooled FPA, and its full FOV is 180°. Its focal length is 4.177 and F number is 0.8. The f-theta distortion is less than 0.25%. By choosing chalcogenide glasses as lens material, the lens has a higher transmittance compare to Germanium lens. Also by analyzing the chromatic aberration of the lens, applying a DOE surface helps to improve the image quality, and pushes the modulated transfer function towards diffraction limitation.