The Bi-static Optical Imaging Sensor (BOIS) is a 2-D imaging sensor that operates in the short-wave infra-red (SWIR)
spectral regime over wavelengths from approximately 1.0 to 2.4 microns. The conceptual design of the sensor is based
on integral field spectroscopy techniques. The BOIS sensor utilizes a fiber transition element consisting of multiple
optical fibers to map the 2-D spatial input scene into a 1-D linear array for injection into a hyper-spectral imaging (HSI)
sensor. The HSI spectrometer acquires fast time resolution snapshots (60 Hz) of the entire input target scene in
numerous narrowband spectral channels covering the SWIR spectral band. The BOIS sensor is developed to spatially
observe the fast time-evolving radiative signature of targets over a variety of spectral bands, thus simultaneously
characterizing the overall scene in four dimensions: 2 spatial, wavelength, and time.
We describe the successful design, operation, and testing of a laboratory prototype version of the BOIS sensor as well as
further development of a field version of the sensor. The goal of the laboratory prototype BOIS sensor was to validate
the proof-of-concept ability in the 4-D measurement concept of this unique design. We demonstrate the 2-D spatial
remapping of the input scene (using SWIR laser and blackbody cavity sources) in multiple spectral channels from the
spatial versus spectral pixel output of the HSI snapshot. We also describe algorithms developed in the data processing to
retrieve temperatures of the observation scene from the hyper-spectral measurements.
Kestrel Corporation has previously demonstrated that the Distorted Grating Wavefront Sensor (DGWFS) can successfully reconstruct wavefronts in severely scintillated conditions, and has an ongoing experiment investigating aberrations in the eye using a DGWFS. Existing aberrometers cannot accurately reconstruct wavefronts when large amounts of scattering or scintillation are present and so cannot be used with subjects who have conditions such as cataracts (opacification of the ocular lens). Consequently a large proportion of the population cannot utilize today's diagnostic aberrometers and so do not benefit from otherwise available treatments. As previously reported, a DGWFS has been integrated into an Shack-Hartmann based aberrometer provided by the International Laser Center, Moscow State University, however several issues became apparent regarding data collection from the human eye. Results from laboratory experiments intended to investigate and resolve these data collection issues will be discussed.
The spatial resolution of retinal images is limited by the presence of static and time-varying aberrations present within the eye. An updated High Resolution Adaptive Optics Fundus Imager (HRAOFI) has been built based on the development from the first prototype unit. This entirely new unit was designed and fabricated to increase opto-mechanical integration and ease-of-use through a new user interface. Improved camera systems for the Shack-Hartmann sensor and for the scene image were implemented to enhance the image quality and the frequency of the Adaptive Optics (AO) control loop. An optimized illumination system that uses specific wavelength bands was applied to increase the specificity of the images. Sample images of clinical trials of retinas, taken with and without the system, are shown. Data on the performance of this system will be presented, demonstrating the ability to calculate near diffraction-limited images.
A new significantly redesigned version of clinically applicable adaptive optics multispectral fundus imager is presented. Along with greatly improve adaptive system loop rate, the device performs reliably and is convenient for use in clinical practice. This new imager has allowed us to use new approaches for retina image analysis and obtain original results on the distribution of aberrations in the human eye.
A key element in any adaptive optics system is the deformable mirror used to introduce the conjugate correction. In this paper we will present the results from characterizing a pair of custom 20 element, 38 mm diameter, bimorph deformable mirrors that were specifically designed to provide unusually large stroke to allow correction of significant focus and astigmatism terms in a human fundus adaptive optics imager. Data on the measured correction capability and inherent hysteresis of the mirror shown that the mirrors have 40 μm waves of defocus correction and 20 μm waves of astigmatism correction at 760 nm, with a typical hysteresis at full deflection of 15%. This technology is patented under Patent # 6,331,059 B1.
The concept of a curvature-based wavefront sensor using a distorted grating as the imaging element to capture images of two spatially separated planes onto a single detector has been reported previously. This presentation reports on simulations comparing a Shack-Hartmann (S-H) sensor with a distorted grating wavefront sensor (DGWFS) for a generic adaptive optics (AO) system using a Clear-1 atmospheric model. Using WaveTrainTM simulation software a model of the DGWFS has been developed and integrated into the software. A simulation of a complete AO system including a tip/tilt system, high order correction system, atmospheric model, and a back-propagating laser system has been constructed. The model has then been exercised using various seeing conditions, noise levels, WFS sensitivities, camera systems, and other parameters. A comparison between the performance of the AO system using the S-H sensor and the DGWFS is presented, both in terms of wavefront measurement accuracy, image quality, and as a beam delivery system.
Earlier research reported a comparison of the wavefronts recorded simultaneously by a Shack-Hartmann and a Distorted Grating Wavefront Sensor (DGWFS). In this paper we present the results of a continuation of this earlier work where we have now closed an adaptive optics loop under simulated propagation conditions using the Advanced Concept Laboratory (ACL) at Lincoln Laboratory. For these measurements only one wavefront sensor controlled the deformable mirror at a time. To make direct comparisons between the sensors we took advantage of the ACL's ability to exactly replicate a time varying propagation simulation. Time varying and static comparisons of the two sensors controlling the ACL adaptive system under conditions that ranged from a benign path, D/r0 = 2, to a propagation condition with significant scintillation, D/r0 =9, will be shown using the corrected far field spot as a measure of performance. The paper includes a description of the DGWFS used for these tests and describes the procedure used to align and calibrate the sensor.
Earlier research reported a comparison of the wavefronts recorded simultaneously by a Shack-Hartmann and a Distorted Grating Wavefront Sensor (DGWFS). In this paper we present the results of a continuation of this earlier work where we have now closed an adaptive optics loop under simulated propagation conditions using the Advanced Concept Laboratory (ACL) at Lincoln Laboratory. For these measurements only one wavefront sensor controlled the deformable mirror at a time. To make direct comparisons between the sensors we took advantage of the ACL’s ability to exactly replicate a time varying propagation simulation. Time varying and static comparisons of the two sensors controlling the ACL adaptive system under conditions that ranged from a benign path, D/r0 = 2, to a propagation condition with significant scintillation, D/r0 =9, will be shown using the corrected far field spot as a measure of performance. The paper includes a description of the DGWFS used for these tests and describes the procedure used to align and calibrate the sensor.
A comparison of the wavefronts recorded simultaneously by a Shack-Hartmann and a Distorted Grating Wavefront Sensor (DGWFS) has been completed. The DGWFS is a phase diversity/wavefront curvature type of sensor using a grating to generate the multiple image planes. Data were collected under simulated propagation conditions using the Advanced Concept Laboratory at Lincoln Laboratory. The sensors were arranged such that both recorded a time varying sample of the wavefront at exactly the same time. Dynamic and static comparisons of the two sensors under conditions that ranged from a benign path, D/r0 = 2, to a propagation condition with significant scintillation, D/r0 =50, were completed.
The data show that the two techniques measure static, low amplitude, on the order of a few waves, aberrations with little difference. Under conditions where there are significant aberrations, the wavefronts measured by the two sensors show notable differences with the DGWFS exhibiting a smoothed, low passed, rendition of the wavefront. As the aberrations increase to produce scintillated condition the differences become more pronounced.
Under a NASA Stennis Space Center (SSC) SBIR, technologies required for an imaging spectral radiometer with wavenumber spectral resolution and milliradian spatial resolution that operates over the 8 micrometers to 12 micrometers (LWIR), and 3 micrometers to 5 micrometers (MWIR) bands, for use in a non-intrusive monitoring static rocket firing application are being investigated. The research is based on a spatially modulated Fourier transform spectral imager to take advantage of the inherent benefits in these devices in the MWIR and LWIR. The research verified optical techniques that could be merged with a Sagnac interferometer to create conceptual designs for an LWIR imaging spectrometer that has a 0.4 cm-1 spectral resolution using an available HgCdTe detector. These same techniques produce an MWIR imaging spectrometer with 1.5 cm-1 spectral resolution based on a commercial InSb array. Initial laboratory measurements indicate that the modeled spectral resolution is being met. Applications to environmental measurement applications under standard temperatures can be undertaken by taking advantage of several unique features of the Sagnac interferometer in being able to decouple the limiting aperature from the spectral resolution.
The primary payload on a small-satellite, the Air Force Research Laboratory's MightySat II.1, is a spatially modulated Fourier Transform Hyperspectral Imager (FTHSI) designed for terrain classification. The heart of this instrument is a solid block Sagnac interferometer with 85cm-1 spectral resolution over the 475nm to 1050nm bands and 30m spatial resolution. Coupled with this hyperspectral imager is a Quad-C40 card, used for on-orbit processing. The satellite was launched on 19 July 2000 into a 575km, 97.8 degree inclination, sun-synchronous orbit. The hyperspectral imager collected its first data set on 1 August 2000, and has been in continuous operation since that time. To the best of our knowledge, the MightySat II.1 sensor is the first true hyperspectral imager to be successfully operated in space. The paper will describe the satellite and instrument, pre-launch calibration results, on-orbit performance, and the calibration process used to characterize the sensor. We will also present data on the projected lifetime of the sensor along with samples of the types of data being collected.
The primary payload on a small-satellite, MightySat II.1, is a spatially modulated Fourier transform hyperspectral imager designed for terrain classification. This imager is the first hyperspectral imager to be successfully operated from space. As part of its year long mission, images have been taken of the Earth's limb and the moon. Analysis of the limb data have shown the presence of large scale structure in the limb while the moon imagery is being used to determine the suitability of using the moon as a vicarious calibration source. The paper briefly describes the satellite and hyperspectral instrument and presents examples of the limb and moon observations and data.
Limited samples of the turbulence structure in the tropopause suggest that conventional models for atmospheric turbulence may not apply through this portion of the atmosphere. This paper discusses the instrumentation requirements, design and calibration of a balloon borne sensor suite designed to accurately measure the distribution and spectral spatial character of the index of refraction fluctuations through the tropopause. The basis for the data system is a 16 bit dynamic range, high data rate sample and hold instrumentation package. Calibration and characterization of the constant current anemometers used in the measurements show them to have a frequency response greater than 170 Hz at the -3 Db point and sufficient resolution to measure a Cn2 of 1 x 10-19 cm-2/3. A novel technique was developed that integrates the over 20 signals into two time correlated telemetry streams. The entire system has been assembled for a flight in the late summer of 2000.
A Fourier Transform hyperspectral imager was integrated onto a standard clinical fundus camera, a Zeiss FF3, for the purposes of spectrally characterizing normal anatomical and pathological features in the human ocular fundus. To develop this instrument an existing FDA approved retinal camera was selected to avoid the difficulties of obtaining new FDA approval. Because of this, several unusual design constraints were imposed on the optical configuration. Techniques to calibrate the sensor and to define where the hyperspectral pushbroom stripe was located on the retina were developed, including the manufacturing of an artificial eye with calibration features suitable for a spectral imager. In this implementation the Fourier transform hyperspectral imager can collect over a hundred 86 cm-1 spectrally resolved bands with 12 micro meter/pixel spatial resolution within the 1050 nm to 450 nm band. This equates to 2 nm to 8 nm spectral resolution depending on the wavelength. For retinal observations the band of interest tends to lie between 475 nm and 790 nm. The instrument has been in use over the last year successfully collecting hyperspectral images of the optic disc, retinal vessels, choroidal vessels, retinal backgrounds, and macula diabetic macular edema, and lesions of age-related macular degeneration.
KEYWORDS: Turbulence, Sensors, Refraction, Atmospheric turbulence, Temperature metrology, Signal processing, Calibration, Correlation function, Data modeling, Information operations
In applications of adaptive optics, understanding the statistical nature of the upper atmospheric turbulence is critical. Today there is a lack of detailed knowledge of upper atmospheric turbulence in the region of the tropopause. Tropopause measurements have been made with simplifications in experimental designs and the potential for experimentally induced artifacts. Recent measurements by various groups indicate the presence of non-Kolmogorov behavior, asymmetric turbulence structure and finite outer scale sizes. Analysis of current balloon borne data collection techniques suggests that signal processing and the balloons own wake may have influenced the sensor. To address some of these concerns, a new measurement platform has been designed that carries 15 constant current probes that are simultaneously sampled at 16 bits at over 1500 samples per second during a controlled descent. All of the signals are then telemetered to the ground without on board processing. The data are then merged with atmospheric data and payload orientation to product key turbulence parameters, including turbulence strength, the inner and outer scales, the temperature structure function, power spectral density of the turbulence, and turbulence isotropic behavior.
The clear waters of Lake Superior constitute the heart of one of the most significant fresh water ecosystems in the world. Lake Superior is the world's largest lake by surface area (82,100 km2) holding approximately 10% of the earth's freshwater (12,230 km3) that is not locked into glaciers or ice caps. Although Superior is arguably the most significant fresh water ecosystem on earth, questions relating to the lake and its watershed remain unanswered, including the effects of human habitation, exploitation, and economic potential of the region. There is a great diversity of scientific disciplines with a common interest in remote sensing of the Lake Superior ecosystem which have the need for data at all spatial, spectral, and temporal scales-from scales supplied by satellites, ships or aircraft at low spatial, spectral or temporal resolution, to a requirement for synoptic high resolution spatial (approximately 1 meter)/spectral (1 - 10 nm) data. During May and August of 1998, two week-long data collection campaigns were performed using the Kestrel airborne visible hyperspectral imager to acquire hyperspectral data of a broad taxonomy of ecologically significant targets, including forests, urban areas, lakeshore zones and rivers, mining industry tailing basins, and the Lake itself. We will describe the Kestrel airborne hyperspectral sensor, the collection and data reduction methodology, and flight imagery from both campaigns.
Previous papers have described the concept behind the MightySat II.1 program, the satellite's Fourier Transform imaging spectrometer's optical design, the design for the spectral imaging payload, and its initial qualification testing. This paper discusses the on board data processing designed to reduce the amount of downloaded data by an order of magnitude and provide a demonstration of a smart spaceborne spectral imaging sensor. Two custom components, a spectral imager interface 6U VME card that moves data at over 30 MByte/sec, and four TI C-40 processors mounted to a second 6U VME and daughter card, are used to adapt the sensor to the spacecraft and provide the necessary high speed processing. A system architecture that offers both on board real time image processing and high-speed post data collection analysis of the spectral data has been developed. In addition to the on board processing of the raw data into a usable spectral data volume, one feature extraction technique has been incorporated. This algorithm operates on the basic interferometric data. The algorithm is integrated within the data compression process to search for uploadable feature descriptions.
Previous papers have described the concept behind the MightySat II.1 program, the satellite' Fourier Transform imaging spectrometer's optical design, and the design for the hyperspectral imaging payload. Initial qualification testing of the payload has been completed. All component level qualification tests have been finished. The solid block optics, interferometer, camera and telescope where all successfully tested and a payload Critical Deign Review was passed. Early optical testing of the monolithic interferometer has shown that it has the designed spectral resolution of less than 100 cm-1. Bench testing of a custom VME data interface board that operates the sensor in a variety of spatial and spectral resolution modes can transfer data satisfactorily at data rates up to 24.3 Mbytes/sec over a VSB bus to spacecraft solid state memory. Problems in manufacturing the hardened C-40 processors has caused a change to an unhardened version of the C-40 using tantalum foil for protection. This still allows all hyperspectral 'smart' imaging spectrometer demonstrations including a 10:1 data compression technique. The payload is scheduled to be delivered in April 1999 for integration on to the spacecraft bus.
Measurement of atmospheric turbulence progressed though several stages in the last decades but has of recent seen little advance. Uses of lidars, ground based radar and intrusive techniques have all had limitations in their ability to measure the more fundamental atmospheric properties. Be it poor spatial or temporal resolution, difficulty in maintaining the sensor, or the requirement to use a preconceived atmospheric model, all have had shortcomings. Of the several physical atmospheric properties that can be quantified, the inner and outer scale sizes associated with the index of refraction, and hence the other atmospheric properties, are of high interest in the prediction of the performance of various adaptive optical sensors. In this paper, we will discuss a method based on a thin beam optical system to measure the inner and outer scales size that overcomes some of the limitations and assumptions in previous techniques. Based on research originally conducted at the University of Florence, we have extended the theory to optically thin layers that can account for real world design effects. Using this theory the paper will discuss the feasibility of using the technique to measure turbulence scale sizes in the upper atmosphere. Data from laboratory measurements will be shown.
UV-VIS-NIR ratiometric reflectance data was obtained for several commonly utilized remote sensing calibration standards used in Fourier Transform Hyperspectral Imaging. We found that single layer reflectance depends on the degree of translucency and hence on the particular choice of background material, from which multiple layer reflectance and extracted absorption and scattering curves logically follow. These data are given as a function of incident wavelength for each calibration standard. Because optical properties are determined by the combination of scattering and absorption, we deconvolved their effects on each material's spectrum.
The Kestrel Corporation visible-near IR band (525 to 1016 nm) airborne Fourier Transform Hyperspectral Imager was modified to include measurement of the polarization characteristics of several ground cover classes. The polarization contrast of typical terrestrial background and target objects was characterized. First, the t statistic was used as an index of class separation to determine whether polarized images were more useful for discriminating several cover classes than unpolarized images. Second, the information present in polarized images which is not present in unpolarized images was identified and described. This was done by regressing polarized and unpolarized images, generating images of predicted values for the polarized images using the regression coefficients, generating images of residuals by subtracting the actual values from the predicted values, and analyzing the statistical separation of cover classes in the residual images. A single polarized image was not more useful for identifying the cover classes than an unpolarized image. A residual image derived from a single polarized image and an unpolarized image provided a mean maximum statistical separation of t equals 18.3 for all cover class combinations. The sum of two orthogonal polarized images provided slightly greater separation, with a mean maximum separation of t equals 23.7.
The MightySat II.1 satellite carries as one of its primary payloads a Fourier transform hyperspectral imager, the first such sensor to be flown in space. Over the last year the sensor has passed its preliminary design and an engineering model of the sensor has been constructed. The model has started to be qualified. To date the sensor has met its weight, volume and power design goals. An unusually high random vibration qualification level has forced the redesign of two mirror mounting techniques. Custom, space qualified, VME electronic camera interface and control cards to handel 20 Mbytes/sec of imagery data has been designed, fabricated, and coupled to a set of four C-40 processors to provide 160 MIPS of onboard processing. Mission operations are now being developed that will demonstrate a 30 m GSD by using the on orbit three axis maneuvering capability of the satellite. The payload is on schedule for a delivery in early 1999 for integration on the bus.
Kestrel Corporation has designed and is now building a dual- band infrared Fourier transform ultraspectral imager for aircraft deployment. Designed for installation in a Cessna 206, this instrument will have a 15 degree FOV, with an IFOV of 1.0 mrad. The target spectral resolution is better than 1.5 cm-1 over 2000 to 3000 cm-1 and 0.4 cm-1 over 850 to 1250 cm(superscript -1$. using 512 spectral channels. The device will use a variety of spectral enhancement techniques to achieve this unprecedented spectral resolution. Computer simulations of the optical systems demonstrates sub-wavenumber resolutions and signal to noise ratios of over 900.
Background clutter and target signatures have traditionally been described by parameters derived form measurements of spatial structure and spectral ratios derived from fixed spectral bandpass images. The advent of hyperspectral imagery requires descriptions of background clutter in a mixed wavelength-spatial or Fourier-transformed (FT) spectral - spatial framework because the data stream may contain simultaneous spatial - spectral, or FT spectral - spatial clutter components. We have developed and tested analytical routines for characterizing the background clutter and target signatures observed by Fourier-transform instruments, without requiring production of a hyperspectral data cube having spectra wavelength and 2D spatial image dimensions. The Kestrel Fourier-transform hyper spectral imager, a Sagnac format interferometer produces a data stream consisting of the Fourier spectra of the background in the in-track focal plane dimension and the spatial information int he cross-track dimension. The temporal data stream thus consists of a time series of frames in FT- spectral vs. spatial dimensions. Spectral wavelength filtering and guard-band subtraction can be accomplished in FT space by binary shift and add algorithms without prior transformation of the data into a hyperspectral data cube. Spatial filtering in the cross track dimension can similarly benefit from efficient binary operations. This paper summarizes some of the target and background clutter characterization algorithms developed and their evaluation against an example atmospheric gas detection scenario.
Steve West, Robert Nagel, David Harvey, A. Brar, B. Phillips, J. Ray, Thomas Trebisky, Richard Cromwell, Neville Woolf, Chris Corbally, R. Boyle, Daniel Blanco, L. Otten
The Vatican Advanced Technology Telescope incorporates a fast (f/1.0) borosilicate honeycomb primary mirror and an f/0.9 secondary in an aplanatic Gregorian optical configuration. We provide a brief technical and performance overview by describing the optical layout, the primary and secondary mirror systems, and the telescope drive and control system. Results from a high resolution wavefront sensor and a current wide-field image taken at the f/9 focus demonstrates the overall fine performance of the telescope.
Kestrel Corporation is designing and building the first Fourier transform hyperspectral imager to be operated from a spacecraft. Performance enhancements offered by the Fourier transform approach have shown it to be one of the more promising spaceborne hyperspectral concepts. Simulations of the payload's performance have indicate that the instrument is capable of separating a wide range of subtle spectral differences. The concept design for the payload has been completed and hardware is in fabrication for an engineering model.
During the past year, Kestrel Corporation has designed and built a low cost Fourier transform visible hyperspectral imager (FTVHSI) for deployment in a light aircraft (Cessna TU-206). The instrument is an imaging spectrometer employing a Sagnac (triangle) interferometer, that operates over a range of 450 - 1050 nm with 256 spectral channels, and a 13 degree FOV with an 0.8 mrad pixel IFOV (450 spatial channels). To aid in the calibration of the instrument, calibration and downwelling signals are recorded with every frame. Installed with the optical instrument are attitude sensors and a scene camera. This auxiliary data allows us to place a hyperspectral slice to within less than 5 m of its true position (using selective availability 'on' and differential GPS). We have performed extensive testing and calibration studies, including data collection conducted synchronously with ground measurements at locations including a White Sands radiometric calibration site. This paper reports some of the calibration studies and their results.
During the past year, Kestrel Corporation has designed and built a low cost Fourier transform hyperspectral imager for deployment in a light aircraft. The instrument is a pushbroom imaging spectrometer employing a Sagnac interferometer. The instrument operates over a range of 350- 1050 nm with 256 spectral channels, and a 13 degree FOV with an 0.8 mrad IFOV. Installed with the optical instrument are attitude sensors, a scene camera, a downwelling sensor and in-flight calibration equipment. This paper will focus on the description of both the optical system and the support equipment used in this revolutionary instrument.
A new hyperspectral imager has recently been developed by Kestrel Corporation for use in light aircraft platforms. The instrument provides 256 spectral channels with 87 cm-1 spectral bandwidth over the 450 nm to 1000 nm portion of the spectrum. Operated as a pushbroom imager, the FTVHSI has been shown to have a IFOV of 0.75 mrad, and a FOV of 0.23 rad. The sensor includes an internal spectral/radiometric calibration source, a self contained spectrally resolved downwelling sensor, and complete line of sight and GPS positioning information. The instrument is now operating from a Cessna TU-206 single engine aircraft.
In this paper we demonstrate the detection limits of the Kestrel Fourier transform hyper-spectral imager (FTHSI) on the MightySat II.I to detect target spectra in a complex natural scene. We simulate the MightySat II.I FTHSI data using a combination of landsat TM based endmember spectra derived from a scene of La Mosquitia, Honduras and library spectra measured in the field at 3 nm spectral resolution. The TM endmember images define the mixing space to produce a simulated hyper-spectral reflectance image. Fractions define how the field measured endmember spectra are mixed in order to produce the simulated hyper-cube. The HIMP model is used to predict the radiance as observed by the FTHSI. Results indicate that this technique allows the detection of tropical camouflage in a natural tropical background when the camouflage is mixed at one tenth of one percent with an accuracy of 95.7 percent. At one percent mixing ratio the detection accuracy rises to 99.7 percent. At five percent and beyond the detection accuracy is one hundred percent. This physically-based non-linear unmixing technique is two orders of magnitude more sensitive than traditional linear unmixing or matched filtering.
Using a new microsat called MightySat II as a platform, Kestrel Corporation is designing and building the first Fourier transform hyperspectral imager (FTHSI) to be operated from a spacecraft. This payload will also be the first to fly on the Phillips Laboratory MightySat II spacecraft series, a new, innovative approach, to affordable space testing of high risk, high payoff technologies. Performance enhancements offered by the Fourier transform approach have shown it to be one of the more promising spaceborne hyperspectral concepts. Simulations of the payload's performance have shown that the instrument is capable of separating a wide range of subtle spectral differences. Variations in the return from the Georges Bank and shoals are discernible and various types of coastal grasses (sea oats and spartina) can be isolated against a sand background.
Hyperspectral sensing systems are being developed for applications spanning astronomy, space object identification, remote sensing, and surveillance. We've developed an interactive, spreadsheet-based computer model which can be used to predict the performance for a type of hyperspectral sensor referred to as spatially modulated imaging Fourier transform spectrometers. The Hyperspectral Imager Model Program (HIMP) includes parameters which allow the specification of numerous target, atmospheric, instrumental, geometrical, and detector characteristics, as well as a variety of graphical outputs. HIMP may easily be modified or altered for a wide range of applications and scenarios.
Kestrel Corporation and the Florida Institute of Technology have designed, and are now manufacturing, a Fourier transform visible hyperspectral imager system for use in a single engine light aircraft. The system is composed of a Sagnac-based interferometer optical subsystem, a data management system, and an aircraft attitude and current position sybsystem. The system is designed to have better than 5 nm spectral resolution at 450 nm, operates over the 440 nm to 1150 nm spectral band and has a 2D spatial resolution of 0.8 mrad. An internal calibration source is recorded with every frame of data to retain radiometric accuracy. The entire system fits into a Cessna 206 and uses a conventional downward looking view port located in the baggage compartment. During operation, data are collected at a rate of 15 Mbytes per second and stored direct to a disk array. Data storage has been sized to accommodate 56 minutes of observations. Designed for environmental mapping, this Fourier transform imager has uses in emergency response and military operations.
High spectral resolution observations of several common camouflages have been made as part of a utility study for a Fourier transform visible hyperspectral imager (FTVHSI). Two types of data were collected. First were non-imaging spectral reflectance measurements made with a spectral resolution better than 0.3 nm over the 350 nm to 1150 nm band. Second were hyperspectrally resolved two dimensional hypercubes of the samples using a FTVHSI. These second data have a spectral resolution of 270 cm-1 over a band of 370 nm to 1030 nm and a spatial resolution of about 2 cm. The data were taken against representative foliage backgrounds that ranged from grass, to tropical forest vegetation, to an arid mesa. The data show both macro and micro spectral differences between the camouflage and the backgrounds that are apparent in the hyperspectral renditions but missing in broad band observations.
Over the last several years the practical implementation of adaptive optics to compensate for the atmospheric distortions in large telescopes has become a reality. Of the elements that must be considered in the design of an adaptive optics system, the expected atmospheric turbulence is one of the most important. The usual method for estimating these criteria is to use standard atmospheric models or site specific adaptations of these models. An implicit assumption in these models is that the atmosphere can be treated as an isotropic mass and that the index of refraction variations follow Kolmogorov theory. An analysis of two of these features, smoothed data vice a set of individual turbulence profiles and the influence of a partially non-Kolmogorov atmosphere, was performed for a large adaptive optics system. The results show that performance expectations can vary significantly. Smoothed data tends to over estimate atmospheric effects up to 50%. Non-Kolmogorov effects are less significant introducing differences on the order of 10% for zenith observations. The conclusion is that the designer must pay careful attention to the atmospheric model and the method in which it is employed. The use of multiple phase screens created directly from sonde data are recommended.
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