Wide field-of-view lens assemblies covering a full field of view of <100° across the diagonal dimension of the detector have been designed, built, and tested for spaceborne applications for various science objectives like landing operations, docking maneuvers, characterization of unknown regolith and other geological features. The all refractive lenses were designed for both telescopic and microscopic, high-resolution (≥227 l.p./mm), imaging scenarios using a nearly identical bill of materials (BOM). The optical designs have been achromatized and passively athermalized over the visible waveband for optimal performance across typical expected operating temperature range. The opto-mechanical designs have been ruggedized for severe spaceborne environmental conditions. This document demonstrates the design approach, assembly techniques, and test methodology of two different lens configurations using a nearly identical BOM. The first is a wide-angle telescope supporting a 5 Gpixel visible array with a nominal target range from 1m to infinity. The second is a close focus “microscopic” imager, with a nominal target distance of <100mm. A total of four lenses were built and tested; three of which were the telescopic configuration and one microscopic imager. The measured performance of the lens assemblies is near diffraction-limited and meets all flight system level requirements. This design methodology has resulted in significant realized cost reductions and time savings while simultaneously being able to push the envelope with next-generation optical solutions for complex spaceborne applications.
Wide field-of-view (WFOV) lenses covering a full field of view of <120° have been designed and analyzed for spaceborne applications for use in severe environments. The optical designs have been passively athermalized and achromatized over the visible waveband for use during operating conditions and the opto-mechanical designs have been ruggedized for survivability in severe environmental conditions with random vibration levels exceeding 2000 Grms and cryogenic temperature conditions reaching -175°C. To characterize the performance and survivability of the lens assembly for such environments, image quality and random vibration testing was performed on representative optical assemblies, with previous flight experience, before and after exposure to cryogenic liquid nitrogen temperatures. The dynamic test environment consists of low-level random vibration testing in each of the three orthogonal axes (X, Y & Z) and the image quality testing performed includes resolution (MTF), distortion, and effective focal (EFL). Finite element analysis (FEA) was performed to verify survivability to full random vibration spectrums and shock levels; external testing was performed with flight hardware and results were validated. High correlation of characterization measurements before and after cryogenic exposure indicate the optical assemblies' ability to withstand severe temperature environments without any impact to functionality or performance. This document provides an overview of the optical and opto-mechanical lens designs for severe space environments, a comprehensive look into component material characteristics over temperature for various space-grade adhesives used to mechanically retain lens elements, and detailed explanations of the cryogenic test methodology using a custom laboratory test setup to characterize the performance and survivability of lens assemblies and their various components.
Spaceborne optical lens assemblies serve a wide range of applications, including deep-space exploratory missions, earth weather imagery, satellite reconnaissance, and surveillance. These applications demand operation over a wide range of optical performance metrics and environmental conditions. Systems deployed in space cannot afford to use the limited battery or solar power to energize motors required for maintaining focus and diffraction-limited image quality. Therefore, it is vital to develop and characterize passively athermalized solutions for spaceborne optical assemblies that are isolated from direct human contact or have limited access to power. These multi-lens element systems of varying glass materials are susceptible to small optical property changes between glass melt lots and highly dependent on the opto-mechanical assembly method; this creates large sensitivities within the system design that can result in changes in system level performance parameters. Accurate quantification of the optical system performance over the specified operating temperature range prior to flight is critical for mission success. Collins Aerospace, Mission Systems Optronics has developed an environmental test setup and interferometric wavefront measurement approach capable of quantifying the optical performance over temperature of passively athermalized spaceborne optical assemblies. Interferometric wavefront measurements are performed across the operating temperature range; resulting interferograms are analyzed and decomposed in terms of thermal defocus, low-order aberrations, and RMS wavefront error to assess final system compliance. The laboratory prototype system consists of a 633nm common-path interferometer, nitrogen-purged environmental test chamber, and retro-null reflecting pin. We use an f/3 aerospace lens, intended for deep-space application, to demonstrate these measurement techniques.
Adaptive optics systems and their laboratory test environments call for a number of unusual optical components. Examples include lenslet arrays, pyramids, and Kolmogorov phase screens. Because of their specialized application, the availability of these parts is generally limited, with high cost and long lead time, which can also significantly drive optical system design. These concerns can be alleviated by a fast and inexpensive method of optical fabrication. To that end, we are exploring direct-write lithographic techniques to manufacture three different custom elements. We report results from a number of prototype devices including 1, 2, and 3 wave Multiple Order Diffractive (MOD) lenslet arrays with 0.75 mm pitch and phase screens with near Kolmogorov structure functions with a Fried length r0 around 1 mm. We also discuss plans to expand our research to include a diffractive pyramid that is smaller, lighter, and more easily manufactured than glass versions presently used in pyramid wavefront sensors. We describe how these components can be produced within the limited dynamic range of the lithographic process, and with a rapid prototyping and manufacturing cycle. We discuss exploratory manufacturing methods, including replication, and potential observing techniques enabled by the ready availability of custom components.
Standard adaptive optics systems measure the aberrations in the wavefronts of a beacon guide star caused by atmospheric turbulence, which limits the corrected field of view to the isoplanatic patch, the solid angle over which the optical aberration is roughly constant. For imaging systems that require a corrected field of view larger than the isoplanatic angle, a three-dimensional estimate of the aberration is required. We are developing a wide-field imaging Shack-Hartmann wavefront sensor (WFS) that will characterize turbulence over a large field of view tens of times the size of the isoplanatic angle. The technique will find application in horizontal and downward looking remote sensing scenarios where high resolution imaging through extended atmospheric turbulence is required. The laboratory prototype system consists of a scene generator, turbulence simulator, a Shack Hartman WFS arm, and an imaging arm. The system has a high intrinsic Strehl ratio, is telecentric, and diffraction limited. We present preliminary data and analysis from the system.
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