The goals of the Climate Monitoring CubeSat Mission (CM2) are to accelerate climate projection by
obtaining global temperature, tidal and wave measurements with a simple CubeSat-based imaging
spectrograph; and to demonstrate how a high-resolution imaging spectrograph can be deployed on a
CubeSat satellite. In the middle atmosphere (50 - 100 km), beyond the reach of balloons or satellites,
thermal signatures of CO2 radiation and wave activity have been largely missing from climate model
inputs. This paper outlines an instrument to advance the state of the art in atmospheric climate projection
by providing critical global measurements of middle-atmosphere temperatures and waves with a CubeSatscale
imaging spectrograph.
The CM2 will remotely sense middle-atmosphere temperatures and waves at ~90 km by analyzing spectra
of intrinsically bright molecular oxygen emissions at near-infrared wavelengths in the O2 atmospheric
band. The core instrument will be a miniaturized imaging spectrograph based on a monolithic spatial
heterodyne spectrometer (SHS). This spectrograph will have sensitivity and spectral resolution to extract
temperatures with 10° K precision and waves with 4 km scale resolution along a ~200 km cross-track
swath. The SHS is significantly more robust than conventional interferometers, and thus better suited to
space-based observation.
Acquiring high-resolution middle-atmosphere temperature, tidal, and wave data on a daily, global basis will
significantly improve climate models, and will help assess long-term greenhouse gas mitigation policy
impact on upper-atmosphere thermal signatures. The CM2 program will also establish the efficacy of highresolution
CubeSat-based broadband (near-IR to UV) spectroscopy for application to other atmospheric
research missions.
The background and beginning of this project have been described in previous SPIE papers1,2. Since then,
atmospheric modeling work on the project has been completed, and the dual-wavelength monolith has been
fabricated and fully tested3. The full sodium-wavelength SHIELDS unit was assembled and initially tested with a
sodium lamp in 2009, and the project has now turned to that focus in earnest. A full workup with the sodium lamp
has been completed, and the unit has started observing a fluorophore target (UV Green) illuminated with both a
continuous white-light bulb and with sunlight from the day sky. In both cases, a narrow continuum fringe pattern
was observed, but the pattern was too narrow in the spectral direction to extract, as of this writing, the absorption
structure necessary to execute Fraunhofer Line Discrimination (FLD). The problem may be an unnecessarily broad
sodium line filter. Tests with a narrower line filter in the optical path will be completed in July 2010, and
determination of progress and next steps will be made at that time.
Neutral oxygen (O I) is a dominant species between about 250 and 500 km in the thermosphere. A complete thermospheric model requires measurements of the species density ([O]) to incorporate into forward models. One way to measure [O] is to detect Bowen fluorescence at triplet 8446 Å. Bowen fluorescence is generated when thermospheric oxygen absorbs Solar Lyman-β and de-excites through a path eventually leading to 8446 emission.
This emission must be distinguished from the brighter 8446 emission caused by photoelectron (PE) impact, which can be done by measuring the intensity ratio between two branches of the 8446 triplet. An instrument to measure Mid-latitude Bowen fluorescence has been installed at Millstone Hill Observatory. The instrument is a Spatial
Heterodyne Spectrometer (SHS), a novel type of Fourier transform spectrometer. This SHS was first used to observe Na Fraunhofer lines from dayglow at 589 nm, and then observed the 8446 region in an oxygen spectrum tube. Modifications to the system will allow it to observe 8446 emissions from the evening sky.
An imager based upon an etched liquid-crystal Fabry-Perot (LCFP) dispersive element is able to simultaneously sample
four distinct resolution elements in the region 800 nm - 1100 nm, and tune in milliseconds to any one of 30,661 possible
four-color scene images with a spectral resolution of 0.67 nm. Independently tunable quadrants of a single LCFP etalon
are created by etching a transparent conducting layer on the etalon substrate, and one image from each quadrant is
formed on a focal-plane array detector. Designed to weigh less than 20 lbs. in production, the portable, solid-state
camera system is designed to provide fast RGB images of transient spectral phenomena, but many applications are
possible. The fourth image in each four-element image is intended to be a background channel for contrast
enhancement in bright background environments.
The LCFP hyperspectral imager provides high-spectral resolution, allowing detection of short-lifetime atomic spectral
line emissions characteristic of excited or ablating constituents against a bright, broadband, greybody background. High
luminosity via the characteristic Fabry-Perot étendue advantage and an f/0.9 optical system accommodate the tactical
need for a lightweight device with a small footprint. The LCFP dispersive element is tuned with battery-pack power, 0-
10V DC and mA current.
The LCFP hyperspectral technology is easily adapted to Doppler imaging by enhancing the etalon gap and sampling
over a narrower instrument passband. Operation in the MWIR and LWIR is also possible. The camera design creates
multi-spectral images with a small but simultaneously sampled data-cube of narrow bandwidth.
The background and beginning of this project have been described in a previous SPIE paper1. Since then, the Na
monolithic Spatial Heterodyne Spectrometer (SHS) units were constructed, and the housing for the full SHIELDS
unit designed and built. The dual-wavelength SHIELDS was designed, and its construction begun, while the LC
reflectors used in the selection between wavelengths for the dual-wavelength monolith were tested for efficacy in an
instrument-like configuration. Optical modeling and procurement of optical components was completed, making
the Na unit nearly ready for lab tests with a low-pressure sodium source, and then appropriate Na-wavelength
fluorophores. Atmospheric modeling showed the importance of both dealing with the Ring effect -- as it is at least
equal to the fluorescence effect to be measured -- and selecting the best wavelength to observe to mitigate the
effects of vegetative fluorescence and water vapor absorption. The full SHIELDS unit has been assembled and
initially tested in May 2009, and the dual-wavelength monolith will be completed in July 2009.
The background and beginning of this project have been described in a previous SPIE paper1. Since then, the Na
monolithic Spatial Heterodyne Spectrometer (SHS) units were constructed, and the housing for the full SHIELDS
unit designed and built. The dual-wavelength SHIELDS was designed, and its construction begun, while the LC
reflectors used in the selection between wavelengths for the dual-wavelength monolith were tested for efficacy in an
instrument-like configuration. Optical modeling and procurement of optical components was completed, making
the Na unit nearly ready for lab tests with a low-pressure sodium source, and then appropriate Na-wavelength
fluorophores. Atmospheric modeling showed the importance of both dealing with the Ring effect -- as it is at least
equal to the fluorescence effect to be measured -- and selecting the best wavelength to observe to mitigate the
effects of vegetative fluorescence and water vapor absorption. The full SHIELDS unit will be assembled and tested
in March 2009, and the dual-wavelength monolith completed in May.
Neutral oxygen (O I) is a dominant species between about 250 and 500 km in the thermosphere. A complete
thermospheric model requires measurements of the species density ([O]) to incorporate into forward models. One
way to measure [O] is to detect Bowen fluorescence at triplet 844.6 nm. Bowen fluorescence is generated when
thermospheric oxygen absorbs Solar Lyman-β and de-excites through a path eventually leading to 844.6 nm
emission. This emission must be distinguished from the brighter 844.6 nm emission caused by photoelectron (PE)
impact, which can be done by measuring the intensity ratio between two branches of the 844.6 triplet. An
instrument to measure Mid-latitude Bowen fluorescence has been installed at Millstone Hill Observatory. The
instrument is a Spatial Heterodyne Spectrometer (SHS), a novel type of Fourier transform spectrometer. This SHS
was first used to observe Na Fraunhofer lines from dayglow at 589 nm, and is to be turned to the 844.6 region.
With simple readjustments, it can also be reset for other visible and near IR wavelengths.
Fraunhofer Line Discrimination (FLD) is a passive optical spectroscopy technique with potential for battlefield remote sensing of aerosol targets, as well as other military and academic applications. The Spatial Heterodyne Interferometer for Emergent Line Discrimination Spectroscopy (SHIELDS) will provide real-time remote sensing using FLD. The unit will be contained in a man-portable box to provide heads-up detection of dangerous chemicals in target clouds. The spectrometer employed will be the monolithic Spatial Heterodyne Spectrometer (SHS). One SHIELDS unit will feature a monolithic SHS to look at the 589-nm Solar Fraunhofer doublet. A second monolith will be built, using novel designs, to look at several different Fraunhofer lines of interest, all in the visible (H-b, Mg, H-a). The finished monoliths will be tested on laboratory targets, and the final complete SHIELDS unit will be
further tested in the field.
We propose a novel switchable circular-to-point converter (SCPC) device based on the holographic polymer dispersed liquid crystal (HPDLC) technology. An SCPC device converts the Fabry-Perot ring pattern into one point or point array, while an external electrical field on the SCPC will deactivate the conversion. By designing an Indium Tin Oxide (ITO) ring-pixel pattern on the SCPC that match the Fabry-Perot circular interference pattern, we are free to select different single rings of Fabry-Perot ring pattern and convert it into different points that is easy to be detected or collected. Stacking different single SCPC elements will give us a random Optical Switch with application in Lidar detection and optical telecommunication.
Scientific Solutions Inc. (SSI) has developed a tunable liquid crystal Fabry-Perot (LCFP) etalon system comprised of a resolving and a suppression etalon in tandem. The 30-micron resonant cavity spacing of the resolving etalon provides for high spectral resolution while the system maintains the significantly broader free spectral range of the 6-micron gap suppression etalon across the tunable region. An applied electric field alters the ordinary refractive index of nematic liquid crystal cells within each etalon cavity, thereby
altering the resonant properties of the etalons, allowing for system tunability over several orders of interference. This system acts as a tunable optical filter with an operating range from 700nm to 1100nm.
Testing of the LCFP etalon system with both a high resolution Czerny-Turner monochrometer and a stabilized ND:Yag laser demonstrate a FWHM of 0.67nm to 1.03nm. System transmission reaching 70% of polarized light is achieved with tunability over one free spectral range in approximately 30 milliseconds. The free spectral range of the tandem etalon system ranges from 27nm-36nm over the operating range, and
allows for 40 randomly selectable spectral channels per free spectral range. This system is designed for use in spectral imaging systems, initially for the semiconductor industry, but is equally applicable to the earth remote sensing community.
This paper discusses the scientific motivation for, design of and buildup of the Spatial heterodyne Spectrometer (SHS) for a sounding rocket mission to study the Cygnus Loop, a prototypical middle-ages supernova remnant. The results of that mission are also presented. The goal of the flight was to obtain a radial velocity-resolved spectrum of the C IV (lambda) 1550 emission line from the Cygnus Loop, as a test for doing the same from the diffuse hot interstellar medium (ISM). The reasons for wanting to look at the ISM this way are discussed, along with the properties of SHS. Important points about alignment (zero-path and zero-spatial-frequency setting), as well as shake testing and data analysis techniques (fringe correction, thermal drift tracking) are described. The SHS payload did not obtain a spectrum for th Cygnus Loop C IV emission line, because of reduced efficiency of the optical components. Results of post- flight efficiency tests to locate the source(s) of this loss are presented, showing contamination as the most likely problem. Ways to eliminate this contamination for future SHS sounding rocket and satellite missions conclude the presentation.
We are developing a field-widened spatial heterodyne spectrometer (SHS) for suborbital observations of the hot component of the diffuse interstellar medium. Our goal for these observations is to obtain the first velocity-resolved (20 km/s) line profiles of the C IV 154.8, 5.1 nm emission line from the Cygnus loop and from one direction at high galactic latitude. Long term, our interest is to develop an SHS instrument for a radial-velocity-resolved sky survey of the 105 K 'coronal gas' in the interstellar medium using a small satellite. This paper reviews the scientific motivation and SHS concept and then discusses in more detail the optical design of the sounding rocket payload and the data taking strategy for the observations.
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