The Aerospace Corporation’s sensitive Mako thermal infrared imaging spectrometer, which operates between 7.6 and 13.2 microns at a spectral sampling of 44 nm, and flies in a DeHavilland DHC-6 Twin Otter, has undergone significant changes over the past year that have greatly increased its performance. A comprehensive overhaul of its electronics has enabled frame rates up to 3255 Hz and noise reductions bringing it close to background-limited. A replacement diffraction grating whose peak efficiency was tuned to shorter wavelength, coupled with new AR coatings on certain key optics, has improved the performance at the short wavelength end by a factor of 3, resulting in better sensitivity for methane detection, for example. The faster frame rate has expanded the variety of different scan schemes that are possible, including multi-look scans in which even sizeable target areas can be scanned multiple times during a single overpass. Off-nadir scanning to ±56.4° degrees has also been demonstrated, providing an area scan rate of 33 km<sup>2</sup>/minute for a 2-meter ground sampling distance (GSD) at nadir. The sensor achieves a Noise Equivalent Spectral Radiance (NESR) of better than 0.6 microflicks (μf, 10<sup>-6</sup> W/sr/cm<sup>2</sup>/μm) in each of the 128 spectral channels for a typical airborne dataset in which 4 frames are co-added. An additional improvement is the integration of a new commercial 3D stabilization mount which is significantly better at compensating for aircraft motions and thereby maintains scan performance under quite turbulent flying conditions. The new sensor performance and capabilities are illustrated.
The Littrow form of spectrograph has a long and storied history in astronomical spectroscopy since its presentation in 1862 by Otto von Littrow. Light from an input slit traverses the same optical elements in reaching the dispersing element (prism or grating) and returning to a focused, dispersed image at the focal plane. This 1:1 symmetry helps cancel aberrations in the reimaging optics while presenting the dispersing element with the geometry most favorable to dispersion, efficiency and anamorphic scale change. Historically, Littrow spectrographs have not been pushed to high throughputs (fast f/ratios). However in the short- and mid-wave infrared particularly, high index, low dispersion materials like silicon and germanium can be combined effectively into compact, high throughput (<f/2.5), well corrected 1:1 reimaging systems that economize volume and cooling resources and are well-suited for moderately high resolution spectrographic space missions such as atmospheric sounders. We present some high throughput Littrow spectrograph concepts designed for infrared atmospheric sounding missions and incorporating both plane and immersion gratings.
A concurrent engineering approach to the design and analysis of a space-borne Electro-Optical (EO) sensor is presented.
A detailed design of an infrared telescope payload is developed by an interdisciplinary team of mechanical, structural,
thermal, and optical engineers using a Simulation Driven Engineering (SDE) software environment. The telescope
payload design is also integrated with a conceptual level design of the space segment of a mission that incorporates the
payload. The flow of the concurrent design process is described, and design outputs are provided.
A new airborne thermal infrared imaging spectrometer, "Mako", with 128 bands in the thermal infrared covering 7.8 to
13.4 microns, has recently completed its engineering flight trials. Results from these flights, which occurred in
September 2010 and included two science flights, are presented. The new sensor flies in a Twin Otter aircraft and
operates in a whiskbroom mode, giving it the ability to scan to ±40° around nadir. The sensor package is supported on a
commercial 3-axis-stabilized mount which greatly reduces aircraft-induced pointing jitter. The internal optics and focal
plane array are operated near liquid helium temperatures, which in conjunction with a fast f/1.25 spectrometer enables
low noise performance despite the sensor's small (0.55 mrad) pixel size and the high frame rate needed to cover large
whisk angles. Besides the large-area-coverage scan mode (20 km<sup>2</sup> per minute at 2-meter GSD from 12,500 ft. AGL), the
sensor features a scan mirror pitch capability that enables both a high-sensitivity mode (longer integration times using
frame summing, covering a smaller spatial region) and a multiple-look mode (multiple looks at a smaller region in a
single aircraft overpass, for discriminating plume motion, for example).
Achromatic doublet theory is recast for the 1-2.5μm short-wavelength infrared band, suggesting the desirability of
combining barium fluoride with specific high index optical glasses having large differences in primary SWIR dispersion
and small differences in partial SWIR dispersion. Candidate combinations of materials are screened empirically using the
performance of optimized f/3 airspaced achromatic doublets employing barium fluoride as the positive element.
Polychromatic RMS geometric image spot sizes appear to increase quadratically with difference in partial SWIR
dispersion between barium fluoride and the complementary glasses. Examples of complex (fast, wide field) systems
demonstrate the utility of the most promising combinations.
We report progress on a high-performance, long-wavelength infrared hyperspectral imaging system for airborne
research. Based on a f/1.25 Dyson spectrometer and 128x128 arsenic doped silicon blocked impurity band array, this
system has significantly higher throughput than previous sensors. An agile pointing/scanning capability permits the
additional signal to be allocated between increased signal-to-noise and broader area coverage, creating new opportunities
to explore LWIR hyperspectral phenomenology.
The Dyson spectrometer form has the potential to deliver good imaging performance, high throughput, and low distortion in a compact configuration suitable for cryogenic infrared applications. The three main requirements for a practical implementation—availability of the required concave diffraction grating, availability of the Dyson lens material, and clearance for slit and focal plane packaging—are now within the state of the art, opening the Dyson form to serious consideration. Several high-performance Dyson designs for the long-wavelength infrared are presented.
A novel thermal-band imager is proposed for space-based Earth science measurement applications such as rock
identification and volcano monitoring. The instrument, MAGI-L (Mineral and Gas Identifier - LEO), would also enable
detection of gases from natural and anthropogenic sources. Its higher spectral resolution, compared to ASTER-type
sensors, will improve discrimination of rock types, greatly expand the gas-detection capability, and result in more
accurate land-surface temperatures. The optical design for MAGI-L will incorporate a novel compact Dyson
spectrometer. Data from SEBASS have been used to examine the trade-offs between spectral resolution, spectral range,
and instrument sensitivity for the proposed sensor.
The Dyson spectrometer form is capable of providing high throughput, excellent image quality, low spatial and spectral
distortions, and high tolerance to fabrication and alignment errors in a compact format with modest demands for weight,
volume, and cooling resources. These characteristics make it attractive for hyperspectral imaging from a space-based
platform. After a brief discussion of history and basic principles, we present two examples of Dyson spectrometers being
developed for airborne applications. We conclude with a concept for an earth science instrument soon to begin
development under the Instrument Incubator Program of NASA's Earth Science Technology Office.
We present a conceptual design for an innovative infrared cross-dispersed spectrograph for the NASA Infrared
Telescope Facility (IRTF) at Mauna Kea. This facility-class instrument will provide a resolving power of up to 80,000 at
1.2-2.5 μm and 67,000 at 3-5 μm with a minimum slit width of 0.25". The instrument employs a silicon immersion
grating in order to reduce the size of the instrument. The design incorporates a 2048×2048 infrared array for the
spectrograph and an infrared slit viewer. The optical design is optimized for the thermal infrared (2.8-5.5 μm).
Aircraft and space-based hyperspectral imaging (HSI) sensors tailored for the reflective or emissive spectral regimes are being designed and developed for a wide variety of military, civil and science applications. Key sensor-level HSI system performance requirements dictate the optical, spectrometer, focal plane and data processing design parameters for a given choice of spectral instrument design and platform altitude. A detailed understanding of the performance/sensor design trade-space that is available facilitates informed decision making and planning. We have developed a spreadsheet-based sensitivity analysis tool for dispersive HSI sensors that enables rapid and meaningful investigation of candidate sensor designs over a wide variety of parametric conditions at a level of detail consistent with the first-order specification of the instrument subsystems. Our approach also facilitates: assessment of a fixed sensor design against varied atmospheric/target phenomenology assumptions, determination of sensor design drivers, and sensor design optimization. Our parametric analysis capability is illustrated by synthesizing a relatively detailed HSI dispersive design based on optical aperture diameter of 70 cm and an orbital altitude of 690 km. These two parameters are borrowed from the IKONOS commercial remote sensing system. As part of this synthesis, sensitivity enhancement by back-scanning is analyzed for the purpose of deriving both the maximum sensor contiguous scan length and the associated precision line-of-sight pitch angle rate control requirements.
We report current status of the IR Camera and Spectrograph (IRCS) for the Subaru Telescope. IRCS is a Subaru facility instrument optimized for high-resolution images with adaptive optics (AO) and tip-tilt at 1-5 micrometers . IRCS consists of two parts: one is a cross-dispersed spectrograph providing mid to high spectral resolution, the other is a near-IR camera with two pixel scales, which also serves as an IR slit-viewer for the echelle spectrograph. The camera also has grisms for low to medium resolution spectroscopy. We have just completed the first engineering run about one month before this SPIE conference. It was an initial performance evaluation without AO or tip-tilt to check IRCS and its interface to the telescope. We confirmed the basic imaging and spectroscopic capability we had estimated.
We describe a grism suitable for low-resolution, slitless spectroscopy in the IR region between 3.0 and 5.0 micrometers . The grism is fabricated in silicon using a three-mask, photolithographic process, resulting in an eight-step binary approximation to the normal sawtooth grating profile. Desirable features of this approach include the ability to incorporate aberration correction in the gratin and a gentle ruing relief profile permitting a conformal anti-reflection coating for improved efficiency. To demonstrate the performance of this grism in a practical applications, we have constructed a slitless spectrograph system using an off-the-shelf InSb camera and simple, uncooled, refractive optics. This system is well suited to observing compact, bright, transient phenomena without good a priori knowledge of their positions. We present examples of present instrument performance. An upgrade currently under construction will increase sensitivity by cooling more of the optical path and increasing the aperture of the collecting optics. We plan to use the improved instrument to observe the Leonid Meteor shower in November 1998.
A 1-5 micrometers IR camera and spectrograph (IRCS) is described. The IRCS will be a facility instrument for the 8.2 m Subaru Telescope at Mauna Kea. It consists of two sections, a spectrograph and a camera section. The spectrograph is a cross-dispersed echelle that will provide a resolving power of 20,000 with a slit width of 0.15 arcsec and two-pixel sampling. The camera section serves as a slit viewer and as a camera with two pixel scales, 0.022 arcsec/pixel and 0.060 arcsec/pixel. Grisms providing 400-1400 resolving power will be available. Each section will utilize an ALADDIN II 1024 X 1024 InSb array. The instrument specifications are optimized for 2.2 micrometers using the adaptive optics and the tip-tilt secondary systems of the Subaru Telescope.
We describe the design and performance of an infrared imaging spectrograph that was first used as an airborne sensor in October, 1995. This instrument, called the spatially-enhanced broadband array spectrograph system (SEBASS), is intended to explore the utility of hyperspectral infrared sensors for remotely identifying solids, liquids, gases, and chemical vapors in the 2 to 14 micrometers 'chemical fingerprint' spectral region. The instrument, which is an extension of an existing non-imaging spectrograph uses two spherical-faced prisms to operate simultaneously in the atmospheric transmission windows found between 2.0 and 5.2 micrometers and between 7.8 and 13.4 micrometers (LWIR). ALthough the SEBASS instrument is designed primarily for use from an aircraft platform, it was used in March 1996 for a tower-based collection.
We describe a new form of prism spectrograph system based on aplanatic principles. The basic system is simple, comprising a prism with two spherical refracting surfaces, both operating near their aplanatic conjugates, and a spherical mirror operating near its center of curvature. This form provides a flat, accessible focal surface suitable for use with modern array detectors. Good image quality can be maintained over large wavelength intervals at fast focal ratios, making this form particularly useful for moderate resolution spectrography. Its simplicity, compactness, and tolerance of misalignment make it attractive for space and cryogenic instruments. We present there examples of operating instruments that have been constructed using this new form.
In order to measure the effect of rocket exhaust on stratospheric ozone and aerosol profiles, it is necessary to deploy a space-based mid-UV spectrograph capable of making measurements at high spatial resolution (1 - 2 km) of the intensity and state of polarization of solar light backscattered by the atmosphere. This paper describes the design of an instrument called HIROIG (high resolution ozone imager) which is expected to be deployed in a sun synchronous orbit sometime after 1995. The instrument consists of three identical spectrographs, each one sensitive to light polarized in one direction. Each spectrograph uses a frame-transfer CCD which images the entire 270 - 370 nm spectrum at approximately equals 1 nm spectral resolution. Images re exposed, in the push broom mode, for 140 msec, providing an effective spatial resolution of better than 2 km for typical orbital velocities. The HIROIG field of view is 1000 km cross-track. A ground-based prototype consisting of a single spectrograph has been constructed and the characterization of this instrument is discussed.
We describe a slitless spectrograph designed for use in the IR region between 2.6 and 5.2 micrometers . The dispersing element is a grism fabricated in silicon by binary-optical techniques. This approach permits the incorporation of aberration correction into the grating element. When combined with simple, all-silicon field and camera optics, the grism forms zero-order images and 3.5 mm- long dispersed spectra on a 256 by 256 element array of 38 micrometers InSb detectors. Interchangeable field lenses provide for 8:1, 6:1, and 4:1 reduction to a final focal ratio of f/2.5.
A preliminary design for an imaging spectrograph that simultaneously covers the 2.15- to 5.2-μm spectral region with a resolution of 0.01 to 0.03 μm is described. The instrument is an extension to two dimensions of an existing infrared spectrograph that uses one-dimensional arrays of infrared detectors. In the two-dimensional version, light entering the spectrograph through a slit is dispersed onto a 256- x 256-pixel InSb array by a novel spherical-faced prism and mirror combination. The use of a prism rather than a grating disperser allows more than a one-octave spectral interval to be covered with no moving parts. In addition, the prism optical efficiency remains high over the entire band covered. The simplicity and ruggedness of the design make it ideal for a space-borne instrument.
This paper describes a preliminary design for an imaging spectrograph that simultaneously covers the 2.15-5.2-micron spectral region with a resolution of 0.01 to 0.03 micron. Light entering the spectrograph through a slit is dispersed onto a 256 x 256 pixel InSb array by a novel spherical-faced prism/conic mirror combination. The use of a prism rather than a grating disperser allows more than a one-octave spectral interval to be covered with no moving parts. In addition, the prism optical efficiency remains high over the entire band covered. The simplicity and ruggedness of the design make it ideal for a space-borne instrument.
Parametric studies of the imaging properties of sensor systems using high-resolution charge-coupled device (CCD) arrays require that the spatial frequency content of the images presented to the CCD be well characterized. We describe a laboratory system based on an all-reflective 1:1 optical relay which provides diffraction-limited images over a useful field up to 100 sq mm area, at any wavelength, at speeds of up to f/5. The design form employed, an Offner relay, has a number of inherent advantages. Additional features have been incorporated for the convenience of experimenters. While presently static, the system can be adapted to simulate scanned sensors. The imaging capability of this relay was verified using a digital camera system built around a large-area (1320 by 1035 pixel), high-resolution (6.8 sq micron pixel size and pitch), Kodak 'megapixel' CCD array.
the prime focus of the Wyoming Infrared Observatory (WIRO) 2.3-m telescope. The detector is a 64 x 64 element HgCdTe array. A microprocessor-based control board residing on the dewar clocks the CCD multiplexer, controls the double-correlated sampling, and digitizes the detector signal. All voltage levels and clocking sequences can be adjusted by software in real time. The data acquisition computer communicates with the control board over a modified RS-232 link at an adjustable rate (usually 50 kilobaud). This allows virtually any computer to be used for data acquisition with a minimum of difficulty. The optics are optimized for the study of extended sources of low surface brightness, with maximum optical throughput. The f/2 primary is followed by a liquid-nitrogen-cooled Wynne corrector and two cold-filter wheels with a capacity of 12 individual filters and a 90-degree CVF segment. The positions of the lens, the instrument, and the filter wheels are adjusted by stepper motors. The plate scale is 2.06 arcseconds per pixel.
This paper describes the optical system and the electronics of a newly developed low-resolution IR spectrograph, designed for ground-based and airborne observations. The spectrograph covers the entire 2.9- to 13.5-micron spectral region simultaneously, without scanning, at a nominal resolving power of 50 and a minimum resolving power of 20. The new spectrograph equals in spectral coverage to circular variable filter spectrometers that contain three filter segments. Because all of the detectors view the source through the same aperture, telescope tracking errors do not result in spectral ambiguities such as those that can arise in scanning spectrometers.