Modeling a space borne imaging system is key in predicting mission utility and exploring the sensor design trade space. It is important to capture critical real world phenomena in the modeling as accurately as possible to optimize design parameters. As a step toward optimal design of spectral imaging systems, this work presents simulation techniques that were used to model a panchromatic imaging system and predict well-known image quality metrics for a range of values of a key optical design variable - the effective focal length (EFL).We designed a at desert scene that included 7% and 15% reflectance panels and generated simulated images for a range of EFLs. The panels were used to calculate a sensor signal-to-noise ratio (SNR). The simulation incorporated a summer atmosphere with a collection time and geometry set to produce zenith solar and nadir collection angles. Platform motion and height with a given integration time for known detector parameters were also incorporated to produce images with changing EFL. A point spread function (PSF) of a typical optical system was incorporated that was scaled according to the EFL for a constant aperture diameter in order to capture the optical resolution changes. The PSF along with smear from the platform motion and integration time introduced realistic image blur to enable the relative edge response (RER) for the system to be estimated from the simulated images. The simulation used the Digital Imaging and Remote Sensing Image Generation (DIRSIG) model that incorporates ray-tracing techniques and physics based radiation propagation modules. The SNR, RER, and GSD calculated from the synthetic images were used to estimate the National Imagery Interpretability Rating Scale (NIIRS) rating for a range of EFLs. An optimum EFL was found by this process which included the compensating effects of resolution, sampling, and noise as the EFL changed. This technique is currently being expanded to assess trades for sensor design in order to optimize optical payload designs for multispectral and hyperspectral imaging systems.
Protective windows and domes on air vehicles such as aircraft and missiles must be efficient aerodynamically, and also they must be acceptable from an optical standpoint. Flat windows have essentially no effect to the optical performance, however they are extremely blunt and not efficient aerodynamically. Concentric domes are reasonably efficient aerodynamically, and if the imaging sensor optics gimbals around the center of curvature of the concentric dome, then the optical aberrations can be easily and effectively corrected for all fields of regard away from the nose. For high speed applications such as with missiles, concentric domes have been a standard for many years. Unfortunately, for extremely high speed applications where aerodynamically induced drag is a problem, concentric domes are simply not adequate, and more aerodynamically efficient shapes such as tangent ogives, must be used. For many applications using today's state of the art IR focal plane arrays the optical performance must be close to diffraction limited. While correction of the residual optical aberrations of a concentric dome is quite trivial, the highly aspheric shape of a tangent ogive introduces significant asymmetrical aberrations which change dramatically with field of regard. In this paper we discuss some recent developments using binary optics for correcting these optical aberrations. With the approaches outlined herein, the heretofore impossible task of imaging through a tangent ogive pointed dome is now shown to be possible.
A Scophony Infrared Scene Projector (IRSP) is being developed for use in evaluating thermal-imaging guidance systems. The Scophony IRSP is configured to be a very high frame rate laser-scanned projection system incorporating Scophony modulation. Scophony modulation offers distinct advantages over conventional flying-spot scanning, for example, longer pixel dwell times and multiple pixel projection. The Scophony IRSP serves as the image projection system in a 'hardware in the loop' therminal-phase guidance simulation. It is capable of projecting multiband, target engagement scenarios with high fidelity using Aura's proprietary software/electronic control system. The Scophony IRSP utilizes acoustooptical (AO) devices to produce the required imagery at four separate wavelengths simultaneously. The four separate scenes are then combined and projected into the imaging guidance system.
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