Lacy G. Cook

Bio: Lacy G. Cook is an academic researcher from Raytheon Space and Airborne Systems. The author has contributed to research in topics: Imaging spectrometer & Spectroradiometer. The author has an hindex of 4, co-authored 6 publications receiving 28 citations.

Imaging Spectrometer Trade Studies: A Detailed Comparison of the Offner-Chrisp and Reflective Triplet Optical Design Forms

Abstract: High quality imaging spectroscopy data is useful for both military and civilian applications. Current state-of-the-art imaging spectrometers typically rely on the Offner-Chrisp (OC) optical form. Making use of a spherically concentric, axially symmetric, and telecentric design, the OC imaging spectrometer provides excellent spectral-spatial uniformity but with many regrets: (1) no real-entrance pupil, (2) relatively slow optical speeds, (3) required convex diffraction grating, (4) narrow field-of-view, and (5) limited scalability. Recently, the Raytheon patented Reflective Triplet (RT) optical design form has produced extremely large format imaging spectrometers of exceptional quality. The RT optical design provides spectral-spatial uniformity comparable to the OC form, but with a number of advantages: (1) extremely large fields-of-view, (2) faster optical speeds, (3) a real-entrance pupil for optimal cold shielding and calibration, (4) use of either a prism or flat diffraction grating operating in collimated space (with an option for both simultaneously in a 2- channel device), and (5) extremely wide spectral range using common reflective optics and multiple focal plane arrays, dispersive elements, and entrance slits. This paper presents a number of detailed designs exemplifying the differences between the OC and RT forms.

Optically fast, wide field-of-view, five-mirror anastigmat (5MA) imagers for remote sensing applications

Abstract: Recent trends in focal plane array (FPA) technology have led naturally to the development of very large format remote sensors that require optically fast, wide field-of-view (FOV) imaging optics. Systems that cover broad spectral ranges, such as multispectral imagers (MSI) and hyperspectral imagers (HSI), require reflective optics to provide aberration and distortion control without the complication of wavelength dependent errors induced by powered refractive elements. These large format systems require even wider fields-of-view than offered by the conventional three-mirror anastigmat (TMA) and four-mirror anastigmat (4MA) designs. Recently, Raytheon has demonstrated in hardware the first-ever aligned and tested five-mirror anastigmat (5MA) imager. The 5MA was designed with an F/3.0 optical speed and a 36 degree cross-scan FOV for use with a large format imaging spectrometer. The 5MA imager has useful features such as: (1) a real entrance pupil to support a full-aperture calibrator or a small scan mirror, (2) an intermediate image for stray light control, and (3) a real exit pupil for optimal cold-shielding in infrared applications. A computer-aided alignment method was used to align the 5MA imager with a final target of balanced wavefront error (WFE) across the full 36 deg FOV. This paper discusses the design and development of the first-ever 5MA imager and some potential air- and space-borne remote sensing applications.

System engineering studies for advanced geosynchronous remote sensors: some initial thoughts on the 4th generation

Abstract: Future operational geosynchronous remote sensors will respond to a broad range of environmental and military/intelligence mission needs. This paper describes initial system engineering design studies for 4th generation operational geosynchronous remote sensors that address notional future mission requirements. Two hyperspectral sensor architectures were considered: an imaging Fourier transform spectrometer and an imaging prism spectrometer. While both imaging FTS and dispersive approaches are viable over a broad trade space, each requires new technology that must be demonstrated low risk by 2017 to enable a mission pathfinder by 2025. To reach this important objective requires that technology risk reduction start now.

Future VIIRS enhancements for the integrated polar-orbiting environmental satellite system

Abstract: The Visible/Infrared Imager Radiometer Suite (VIIRS) is the next-generation imaging spectroradiometer for the future operational polar-orbiting environmental satellite system. A successful Flight Unit 1 has been delivered and integrated onto the NPP spacecraft. The flexible VIIRS architecture can be adapted and enhanced to respond to a wide range of requirements and to incorporate new technology as it becomes available. This paper reports on recent design studies to evaluate building a MW-VLWIR dispersive hyperspectral module with active cooling into the existing VIIRS architecture. Performance of a two-grating VIIRS hyperspectral module was studied across a broad trade space defined primarily by spatial sampling, spectral range, spectral sampling interval, along-track field of view and integration time. The hyperspectral module studied here provides contiguous coverage across 3.9 - 15.5 μm with a spectral sampling interval of 10 nm or better, thereby extending VIIRS spectral range to the shortwave side of the 15.5 μm CO 2 band and encompassing the 6.7 μm H 2 O band. Spatial sampling occurs at VIIRS I-band (~0.4 km at nadir) spatial resolution with aggregation to M-band (~0.8 km) and larger pixel sizes to improve sensitivity. Radiometric sensitivity (NEdT) at a spatial resolution of ~4 km is ~0.1 K or better for a 250 K scene across a wavelength range of 4.5 μm to 15.5 μm. The large number of high spectral and spatial resolution FOVs in this instrument improves chances for retrievals of information on the physical state and composition of the atmosphere all the way to the surface in cloudy regions relative to current systems. Spectral aggregation of spatial resolution measurements to MODIS and VIIRS multispectral bands would continue legacy measurements with better sensitivity in nearly all bands. Additional work is needed to optimize spatial sampling, spectral range and spectral sampling approaches for the hyperspectral module and to further refine this powerful imager concept.

Space-based hyperspectral imaging spectroradiometer for coastal studies

Abstract: Resolving the complexity of coastal and estuarine waters requires high spatial resolution, hyperspectral imaging spectroradiometry. Hyperspectral measurements also provide capability for measuring bathymetry and bottom types in optically shallow water and for detailed cross calibration with other instruments in polar and geosynchronous orbit. This paper reports on recent design studies for a hyperspectral Coastal Imager (CI - pronounced "sea") that measures key data products from sun synchronous orbit. These products include water-leaving radiances in the near-ultraviolet, visible and near-infrared for separation of absorbing and scattering coastal water constituents and for calculation of chlorophyll fluorescence. In addition, CI measures spectral radiances in the near-infrared and shortwave infrared for atmospheric corrections while also measuring cloud radiances without saturation to enable more accurate removal of instrument stray light effects. CI provides contiguous spectral coverage from 380 to 2500 nm at 20 m GIFOV at nadir across 5000+ km2 scenes with spectral sampling, radiometric sensitivity and calibration performance needed to meet the demanding requirements of coastal imaging. This paper describes the CI design, including concepts of operation for data collection, calibration (radiometric, spectral and spatial), onboard processing and data transmission to Earth. Performance characteristics for the instrument and all major subsystems including the optics, focal plane assemblies, onboard calibration, onboard processing and thermal subsystem are presented along with performance predictions for instrument sensitivity and calibration. Initial estimates of size, mass, power and data rate are presented.

An Overview of Infrared Remote Sensing of Volcanic Activity

Abstract: Volcanic activity consists of the transfer of heat from the interior of the Earth to the surface. The characteristics of the heat emitted relate directly to the geological processes underway and can be observed from space, using the thermal sensors present on many Earth-orbiting satellites. For over 50 years, scientists have utilised such sensors and are now able to determine the sort of volcanic activity being displayed without hazardous and costly field expeditions. This review will describe the theoretical basis of the discipline and then discuss the sensors available and the history of their use. Challenges and opportunities for future developments are then discussed.

Optical design, laboratory test, and calibration of airborne long wave infrared imaging spectrometer.

Abstract: We discuss and evaluate a long wave infrared imaging spectrometer in terms of its opto-mechanical design and analysis, alignment, testing, and calibration. The instrument is a practical airborne sensor achieving high spectral resolution and sensitive noise equivalent delta temperature. The instrument operates in the 8 to 12.5 μm spectral region with 28.85 nm spectral sampling, 1 mrad instantaneous field of view, and >40° cross track field. The instrument comprises three uniform sub-modules with identical design parameters and performances. The sub-module design is based on a refractive foreoptics feeding an all-reflective spectrometer. The optical form of the spectrometer is a double-pass reflective triplet with a flat grating, which has a fast f/2 and high optical throughput. Cryogenic optics of 100 K is implemented only for the spectrometer. Assembly and thermal deformation and focusing adjustment design are particularly considered for this low temperature. All the mirrors of the spectrometer are opto-mechanical-integrated designed and manufactured by single-point diamond turning technology. We consider the center sub-module as an example, and we present its laboratory testing results and calibration; the results indicate the instrument's potential value in airborne sensing.

Review of the utility of infrared remote sensing for detecting and monitoring volcanic activity with the case study of shortwave infrared data for Lascar Volcano from 2001–2005

Abstract: This chapter provides a historical review of the use of infrared remote sensing for the monitoring of volcanic activity. It (1) examines the theoretical basis for infrared observations of thermally anomalous volcanic features, (2) presents the various sensors that have been used and are currently available and (3) describes the techniques that have been developed to analyse such data. The chapter ends with a case study of data derived from Advanced Spaceborne Thermal Emission and Reflection Radiometer shortwave infrared observations of Lascar Volcano in Chile. This case study aims to highlight the utility of infrared observations in volcano- logical studies, assessing the various techniques that can be used and examining the different factors which influence these analyses. The chapter ends discussing future prospects for volcanic infrared remote sensing.

Large format imaging spectrometers for future hyperspectral Landsat mission

Abstract: This paper describes a design concept for a Landsat-class imaging spectrometer. The challenge is to match the Landsat data parameters, including a 185 Km swath and a 30 meter ground sample distance (GSD) from a 705 Km sun-synchronous orbit with a sensor that has contiguous spectral coverage of the solar reflected spectrum (400 to 2500 nm). The result is a remote sensing satellite that provides global access imaging spectrometer data at moderate spatial resolution. Key design trades exist for the spectrometer, focal plane array, dispersive element, and calibrator. Recent developments in large format imaging spectrometers at Raytheon are presented in support of a monolithic spectrometer approach. Features of the design include (1) high signal-to-noise ratio, (2) well-corrected spectral fidelity across a 6,000 pixel push-broom field-of-view, (3) straightforward calibration of the data to units of absolute spectral radiance, and (4) real-time simulation of Thematic Mapper bands, vegetation indices, and water vapor maps for direct continuous downlink.

Analysis method of the Offner hyperspectral imaging spectrometer based on vector aberration theory.

Abstract: Compact hyperspectral imaging spectrometers with a wide field of view (FoV) have significant application value. However, the aberration field of such imaging spectrometers is sensitive, and varies for different wavelengths when reducing the spectrometer volume. It is difficult to explain the variation in the aberration field using traditional aberration theory. In this study, we extend the vector aberration theory (VAT) to the Offner imaging spectrometer. We deduce the expression of the aberration field decenter vector of the Offner spectrometer based on the real ray-trace method. Furthermore, we derive the expression of the third-order vector aberration of the system. Subsequently, we explain some common phenomena in the Offner imaging spectrometer. This new analysis method can provide useful guidance for designing a compact wide FoV Offner imaging spectrometer. With this new insight, we designed a compact wide FoV Offner imaging spectrometer with a freeform tertiary mirror. Compared to conventional spectrometers with the same specifications, the total length of the spectrometer decreased by 37%, and the volume by 75%. After the tolerance analysis, the freeform optics satisfied the existing machining technology. The analysis method presented in this paper furthers the designer’s understanding of the aberration field of the Offner imaging spectrometer. The method is significant in the design of a compact wide FoV Offner imaging spectrometer with freeform optics.