David W. Warren

IRCS : Infrared Camera and Spectrograph for the Subaru Telescope

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.

Compact prism spectrograph suitable for broadband spectral surveys with array detectors

We describe a novel design for a low-resolution spectrograph that is capable of covering the entire 2.9-13.5 μm region, without scanning, at a resolving power of 20-100. The spectrograph uses two unique curved prisms to disperse radiation onto two 58-element arrays of infrared detectors. It has no moving parts. This spectrograph is an effective replacement for scanning circular variable filter (CVF) spectrometers or for Michelson Interferometers working at low resolution. Because all of the detectors in the spectrograph view an object through the same aperture, time-varying sources introduce no ambiguities into the spectra. The use of BIB detector arrays gives the instrument high sensitivity. We describe a ground-based version of the spectrograph that is currently under construction.

Dyson spectrometers for high-performance infrared applications

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.

Low-resolution array spectrograph for the 2.9- to 13.5-um spectral region

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.

MAKO: a high-performance, airborne imaging spectrometer for the long-wave infrared

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.

Space-based mineral and gas identification using a high-performance thermal infrared imaging spectrometer

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.

Observations of Leonid Meteors Using a Mid-Wave Infrared Imaging Spectrograph

We report broadband 3–5.5 µm detections of two Leonid meteors observed during the 1998 Leonid Multi-Instrument Aircraft Campaign. Each meteor was detected at only one position along their trajectory just prior to the point of maximum light emission. We describe the particular aspects of the Aerospace Corp. Mid-wave Infra-Red Imaging Spectrograph (MIRIS) developed for the observation of short duration transient events that impact its ability to detect Leonid meteors. This instrument had its first deployment during the 1998 Leonid MAC. We infer from our observations that the mid-wave IR light curves of two Leonid meteors differed from the visible light curve. At the points of detection, the infrared emission in the MIRIS passband was 25 ± 4 times that at optical wavelengths for both meteors. In addition, we find an upper limit of 800 K for the solid body temperature of the brighter meteor we observed, at the point in the trajectory where we made our mid-wave IR detection.

MAGI: A New High-Performance Airborne Thermal-Infrared Imaging Spectrometer for Earth Science Applications

A new airborne facility instrument for Earth science applications is introduced. The Mineral and Gas Identifier (MAGI) is a wide-swath (programmable up to ±42° off nadir) moderate spectral resolution thermal-infrared (TIR) imaging spectrometer that spans the 7.1- to 12.7-μm spectral window in 32 uniform and contiguous channels. Its spectral resolution enables improved discrimination of rock and mineral types, greatly expanded gas-detection capability, and generally more accurate land-surface temperature retrievals. The instrument design arose from trade studies between spectral resolution, spectral range, and instrument sensitivity and has now been validated by flight data acquired with the completed sensor. It offers a potential prototype for future space-based TIR instruments, which will require much higher spectral resolution than is currently available in order to address more detailed climate, anthropogenic, and solid Earth science questions.

GMTNIRS: the high resolution near-IR spectrograph for the Giant Magellan Telescope

We present a conceptual design for a powerful, high-resolution near-infrared spectrograph for the Giant Magellan Telescope (GMT). This instrument, the Giant Magellan Telescope Near-Infrared Spectrograph (GMTNIRS), uses silicon immersion gratings as the primary dispersing elements. The design has two modules, one for 1.1-2.4 μm for use under native seeing conditions and one for 3-5.5 μm to be used with adaptive optics. The resulting design is physically compact and allows us to cover entire infrared atmospheric windows in a single exposure with resolving powers up to 100,000.

Mako airborne thermal infrared imaging spectrometer: performance update

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 km2/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, 10-6 W/sr/cm2/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.