Lunar Flashlight is an innovative NASA CubeSat mission dedicated to mapping water ice in the permanently shadowed regions of the Moon, which may act as cold traps for volatiles. To this end, a multi-band reflectometer will be sent to orbit the Moon. This instrument consists of an optical receiver aligned with four lasers, each of which emits sequentially at a different wavelength in the near-infrared between 1 μm and 2 μm. The receiver measures the laser light reflected from the lunar surface; continuum/absorption band ratios are then analyzed to quantify water ice in the illuminated spot. Here, we present the current state of the optical receiver design. To optimize the optical signal-to-noise ratio, we have designed the receiver so as to maximize the laser signal collected, while minimizing the stray light reaching the detector from solarilluminated areas of the lunar surface outside the field-of-view, taking into account the complex lunar topography. Characterization plans are also discussed. This highly mass- and volume-constrained mission will demonstrate several firsts, including being one of the first CubeSats performing science measurements beyond low Earth orbit.
Stereophotogrammetry typically employs a pair of cameras, or a single moving camera, to acquire pairs of images from different camera positions, in order to create a three dimensional ‘range map’ of the area being observed. Applications of this technique for building three-dimensional shape models include aerial surveying, remote sensing, machine vision, and robotics. Factors that would be expected to affect the quality of the range maps include the projection function (distortion) of the lenses and the contrast (modulation) and signal-to-noise ratio (SNR) of the acquired image pairs. Basic models of the precision with which the range can be measured assume a pinhole-camera model of the geometry, i.e. that the lenses provide perspective projection with zero distortion. Very-wide-angle or ‘fisheye’ lenses, however (for e.g. those used by robotic vehicles) typically exhibit projection functions that differ significantly from this assumption. To predict the stereophotogrammetric range precision for such applications, we extend the model to the case of an equidistant lens projection function suitable for a very-wide-angle lens. To predict the effects of contrast and SNR on range precision, we perform numerical simulations using stereo image pairs acquired by a stereo camera pair on NASA’s Mars rover Curiosity. Contrast is degraded and noise is added to these data in a controlled fashion and the effects on the quality of the resulting range maps are assessed.
The Ultra-Compact Imaging Spectrometer (UCIS) is a miniature telescope and spectrometer system intended for
mapping terrain mineralogy over distances from 1.5 m to infinity with spatial sampling of 1.35 mrad over a 30° field,
and spectral sampling of 10 nm in the 600-2500 nm range. The core of the system has been designed for operation in a
Martian environment, but can also be used in a terrestrial environment when placed inside a vacuum vessel. We report
the laboratory and field calibration data that include spatial and spectral calibration, and demonstrate the use of the
system.
We present the optical design and performance of the Ultra-Compact Imaging Spectrometer (UCIS) currently under
development at Caltech's Jet Propulsion Laboratory. The new instrument demonstrates a low optical bench mass of less
than 0.5 kg and compact size that enables Mars Rover or other in situ planetary applications. UCIS is an F/4, wide field
(30°) design, covering the spectral range 600-2600 nm and is enabled by a simple all aluminum two-mirror telescope and
Offner spectrometer. We discuss here the optical design and alignment method that enables this compact and low mass
imaging spectrometer and demonstrate successful spectrometer alignment with smile and keystone levels at 2-3% of a
pixel width.
KEYWORDS: Spectroscopy, Telescopes, Optical design, Signal to noise ratio, Diffraction, Point spread functions, Sensors, Tolerancing, Space telescopes, Imaging systems
We present the design of a compact, wide-angle pushbroom imaging spectrometer suitable for exploration of solar system bodies from low orbit. The spectrometer is based on a single detector array with a broadband response that covers the range 400 to 3000 nm and provides a spectral sampling of 10 nm. The telescope has a 24-deg field of view with 600 spatially resolved elements (detector pixels). A specially designed convex diffraction grating permits optimization of the signal-to-noise ratio through the entire spectral band. Tolerances and design parameters permit the achievement of high uniformity of response through field and wavelength. The spectrometer performance is evaluated in terms of predicted spectral and spatial response functions and from the point of view of minimizing their variation through field and wavelength. The design serves as an example for illustrating the design principles specific to this type of system.
A primary goal of the astrobiology program is the search for fossil records. The astrobiology exploration strategy
calls for the location and return of samples indicative of environments conducive to life, and that best capture and
preserve biomarkers. Successfully returning samples from environments conducive to life requires two primary
capabilities: (1) in situ mapping of the mineralogy in order to determine whether the desired minerals are present;
and (2) nondestructive screening of samples for additional in-situ testing and/or selection for return to laboratories
for more in-depth examination. Two of the most powerful identification techniques are micro-imaging and
visible/infrared spectroscopy. The design and test results are presented from a compact rugged instrument that
combines micro-imaging and spectroscopic capability to provide in-situ analysis, mapping, and sample screening
capabilities.
Accurate reflectance spectra should be a measure of reflectance as a function of wavelength only. Other compact
multispectral microimagers use separate LEDs (light-emitting diodes) for each wavelength and therefore vary the
angles of illumination when changing wavelengths. When observing a specularly-reflecting sample, this produces
grossly inaccurate spectra due to the variation in the angle of illumination. An advanced design and test results are
presented for a multispectral microimager which demonstrates two key advances relative to previous LED-based
microimagers: (i) acquisition of actual reflectance spectra in which the flux is a function of wavelength only, rather
than a function of both wavelength and illumination geometry; and (ii) increase in the number of spectral bands to
eight bands covering a spectral range of 468 to 975 nm.
The continuing development of new and fundamentally different classes of imaging spectrometers has increased the complexity of the field of imaging spectrometry. The rapid pace at which new terminology is introduced to describe the new types of imaging spectrometers sometimes leads to confusion, particularly in discussions of the relative merits of the different types. In some cases, multiple different terms are commonly used to describe the same fundamental approach, and it is not always clear when these terms are synonymous. Other terminology in common use is overly broad. When a single term may encompass instruments that operate in fundamentally different ways, important distinctions may be obscured. In the interest of clarifying the terminology used in imaging spectrometry, we present a comprehensive system for classification of imaging spectrometers based on two fundamental properties: the method by which they scan the object spatially, and the method by which they obtain spectral information.
The University of Florida is developing a mid-infrared camera for the 10.4-meter Gran Telescopio CANARIAS. CanariCam has four science modes and two engineering modes, which use the same 320 x 240-pixel, arsenic-doped silicon, blocked-impurity-band detector from Raytheon. Each mode can be remotely selected quickly during an observing sequence. The pixel scale is 0.08 arcsec, resulting in Nyquist sampling of the diffraction-limited point-spread-function at 8 μm, the shortest wavelength for which CanariCam is optimized. The total available field of view for imaging is 26 arcsec x 19 arcsec. The primary science mode will be diffraction-limited imaging using one of several available spectral filters in the 10 μm (8-14 μm) and 20 μm (16-25 μm) atmospheric windows. Any one of four plane gratings can be inserted for low and moderate-resolution (R = 100 - 1300) slit spectroscopy in the 10 and 20-μm regions. Insertion of appropriate field and pupil stops converts the camera into a coronagraph, while insertion of an internal rotating half-wave plate, a field mask, and a Wollaston prism converts the camera into a dual-beam polarimeter.
Although the throughput and multiplex advantages of Fourier transform spectrometry were established in the early 1950's (by Jacquinot and Fellgett , respectively) confusion and debate arise when these advantages are cited in reference to imaging spectrometry. In non-imaging spectrometry the terms throughput and spectral bandwidth clearly refer to the throughput of the entire field-of-view (FOV), and the spectral bandwidth of the entire FOV, but in imaging spectrometry these terms may refer to either the entire FOV or to a single element in the FOV. The continued development of new and fundamentally different types of imaging spectrometers also adds to the complexity of predictions of signal and comparisons of signal collection abilities. Imaging spectrometers used for remote sensing may be divided into classes according to how they relate the object space coordinates of cross-track position, along-track position, and wavelength (or wavenumber) to the image space coordinates of column number, row number, and exposure number for the detector array. This transformation must be taken into account when predicting the signal or comparing the signal collection abilities of different classes of imaging spectrometer. The invariance of radiance in an imaging system allows the calculation of signal to be performed at any space in the system, from the object space to the final image space. Our calculations of signal - performed at several different spaces in several different classes of imaging spectrometer - show an interesting result: regardless of the plane in which the calculation is performed, interferometric (Fourier transform) spectrometers have a dramatic advantage in signal, but the term in the signal equation from which the advantage results depends upon the space in which the calculation is performed. In image space, the advantage results from the spectral term in the signal equation, suggesting that this could be referred to as the multiplex (Fellgett) advantage. In an intermediate image plane the advantage results from a difference in a spatial term, while for the exit pupil plane it results from the angular term, both of which suggest the throughput (Jacquinot) advantage. When the calculation is performed in object coordinates the advantage results from differences in the temporal term.
Static Fourier transform spectrometers have the ability to combine the principle advantages of the two traditional techniques used for imaging spectrometry: the throughput advantage offered by Fourier transform spectrometers, and the advantage of no moving parts offered by dispersive spectrometers. The imaging versions of these spectrometers obtain both spectral information, and spatial information in one dimension, in a single exposure. The second spatial dimension may be obtained by sweeping a narrow field mask across the object while acquiring successive exposures. When employed as a pushbroom sensor from an aircraft or spacecraft, no moving parts are required, since the platform itself provides this motion. But the use of this narrow field mask to obtain the second spatial dimension prevents the throughput advantage from being realized. We present a technique that allows the use of a field stop that is wide in the along-track direction, while preserving the spatial resolution, and thus enables such an instrument to actually exploit the throughput advantage when used as a pushbroom sensor. The basis of this advance is a deconvolution technique we have developed to recover the spatial resolution in data acquired with a field stop that is wide in the along-track direction. The effectiveness is demonstrated by application of this deconvolution technique to simulated data.
Using a recent theory of scintillation, including the development of a probability density function (PDF) for the irradiance, a theoretical analysis is carried out on the system performance of a coherent array receiver in terms of the scintillation index, the implied carrier-to-noise (CNR), and signal fade statistics. We also describe a field experiment in which measured data from a wave reflected from a rough (Lambertian) target located 1000 m from the transceiver was used to calculate the scintillation index, CNR, and probability of fade. The results of this study support the theoretical models.
There is an upper bound on the effective size of receiving aperture for both the improvement in carrier-to-noise ratio and the reduction in signal fading of a conventional coherent detection system due to the atmospheric turbulence-induced finite transverse coherence length of the received laser field for laser radar and communications. But a coherent array detection system, which uses multiple independent apertures/receivers whose IF's are electro-optically co-phased in real time and then added, can overcome the effective aperture limitation and mitigate the signal fading. This paper presents experimental comparison of the performance of an eight-aperture coherent array detection system and the conventional coherent detection system. The field tests were conducted over two-km range (round-trip). The carrier-to-noise ratio and the fade statistics of the IF signals were investigated for both systems. The results show a gain factor of up to six on the mean carrier-to-noise ratio of the IF signal from the eight-aperture coherent array detection system compared to the conventional coherent detection system even though the two systems collected the same power of laser signal. Also shown is a factor of 100,000 reduction in probability of fade of the IF signal.
In this paper, a theoretical comparison is made of the mean carrier-to-noise ratio (CNR) for a coherent (heterodyne detection) equal gain (EG) optical array receiver system with that predicted by a conventional single-aperture monolithic coherent detector system. Our analysis shows that the mean CNR for an EG array receiver system improves significantly over that of a single aperture system. Experimental data taken from a recent outdoor experiment over a range up to 1 km between target and transceiver are also presented and compared with the theory for a particular eight-element EG system developed at the University of Central Florida. Optical signals received by the EG receiver array are launched into eight single mode optical fibers. Phase compensation between the individual receivers is accomplished by wrapping the fiber around PZT cylinders that are controlled by phase compensating electronics.
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.
The detection and processing of laser communication signals are drastically affected by the fading induced onto these signals by atmospheric turbulence. One method of reducing this fading is to use an array of detectors in which each of the detector outputs are added together coherently. This requires measuring the phase difference between each of the receivers and co-phasing each of the detector outputs. This paper presents experimental verification at the Innovative Science and Technology Experimentation Facility over an outdoor range of a 1.06 micron eight element coherent receiver used to mitigate the effects of fading. The system is composed of a 60 mw Nd:Yag laser used as the transmitter and a 27 MHz AO modulator used to frequency shift the transmitted beam. The receiver is composed of eight 1 cm lenses launching the eight received optical signals into eight signal mode optical fibers. Phase compensation between each of the eight receivers is accomplished using single mode fibers wrapped around PZT cylinders that are controlled by phase compensating electronics. The carrier-to-noise (CNR) ratio was measured on a single channel and was then compared with the CNR obtained from the coherent sum of the eight channels. The improvement of the CNR for the coherent sum as compared to a single channel was then compared against theoretical predictions.
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.
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.
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.
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.
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.
Spatially modulated Fourier transform spectrometers (FTS) have a throughput advantage over dispersive spectrometers, since an FTS does not require a slit in order to achieve spectral resolution. The traditional implementation of FTSs employs a scanning Michelson interferometer, but since this interferometer is temporally modulated, it is difficult or impossible to use with a target whose spatial and/or spectral signature is changing rapidly. The less common spatially modulated approach to FTS allows all spectral channels to be acquired simultaneously, but cylindrical optics are required to create an imaging version of this type of spectrometer [an imaging fourier transform spectrometer (IFTS)]. This combination of cylindrical and spherical optics, used to achieve both spectral and spatial resolution, increases the difficulty of understanding and controlling the aberrations. An understanding of the effects of these aberrations is essential to developing a spatially modulated IFTS with good spectral and spatial resolution. A spatially modulated IFTS based on a Sagnac interferometer is described, and the effects of aberrations on the spectral resolution, spatial resolution, and modulation are discussed.
The Florida Institue of Technology and the Phillips Laboratory have developed several advanced visible (0.4-0.8 micrometers ) imaging fourier transform spectrometer (IFTS) brassboards, which simultaneously acquire one spatial and one spectral dimension of the hyperspectral image cube. The initial versions of these instruments may be scanned across a scene or configured with a scan mirror to pick up the second spatial dimension of the image cube. The current visible hyperspectral imagers possess a combination of features, including (1) low to moderate spectral resolution for hundreds/thousands of spectral channels, (2) robust design, with no moving parts, (3) detector limited free spectral range, (4) detector-limited spatial and spectral resolution, and (5) field widened operation. The utility of this type of instrument reaches its logical conclusion however, with an instrument designed to acquire all three dimensions of the hyperspectral image cube (both spatial and one spectral) simultaneously. In this paper we present the (1) detailed optical system designs for the brassboard instruments, (2) the current data acquisition system, (3) data reduction and analysis techniques unique to hyperspectral sensor systems which operate with photometric accuracy, and (4) several methods to modify the basic instrument design to allow simultaneous acquistion of the entire hyperspectral image cube. The hyperspectral sensor systems which are being developed and whose utility is being pioneered by Florida Tech and the Phillips Laboratory are applicable to numerous DoD and civil applications including (1) space object identification, (2) radiometrically correct satellite image and spectral signature database observations, (3) simultaneous spactial/spectral observations of booster plumes for strategic and surrogate tactical missile signature identification, and (4) spatial/spectral visible and IR astronomical observations with photometric accuracy.
In this paper we present (1) the optical system design and operational overview, (2) laboratory evaluation spectra, and (3) a sample of the first observational data taken with HYSAT. The hyperspectral sensor systems which are being developed and whose utility is being pioneered by the Phillips Laboratory are applicable to several important SOI (space object identification), military, and civil applications including (1) spectral signature simulations, satellite model validation, and satellite database observations and (3) simultaneous spatial/spectral observations of booster plumes for strategic and surrogate tactical missile signature identification. The sensor system is also applicable to a wide range of other applications, including astronomy, camouflage discrimination, smoke chemical analysis, environmental/agricultural resource sensing, terrain analysis, and ground surveillance. Only SOI applications will be discussed here.
Optical interference filters were required for the Wind Imaging Interferometer (WINDII), a Canadian instrument to fly on NASA's Upper Atmosphere Research Satellite (UARS). The WINDII instrument is a Michelson interferometer which measures wind speeds and temperatures in the upper atmosphere by analyzing airglow emissions. The original perception of the optical filters as a straightforward, low-risk element of the WINDII system, and the evolution of this view into a realization of the challenge and complexity of the filter requirements, are presented. The challenges discussed include the tight manufacturing tolerances required to achieve a wide field of view simultaneously with a narrow passband, and the problem of drifting of the filter passband over time. Changes in the scientific approach in order to relax the tolerances, tilting of the filters relative to the instruments's optic axis, and manufacturing changes in order to minimize the passband drift, are described. Other innovations in the filter design are also discussed, including a filter which incorporates a correction for chromatic aberration, and a 'split' filter which has two separate passbands.
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