PERSIST – Prototype Earth observing System using Image Slicer Mirrors D. Lee*a, J. Barlowb, A. Vicka, P. Hastingsa, D. Atkinsona, M. Blacka, S. Wilsona, and P. I. Palmerb Science & Technology Facilities Council, UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, United Kingdom. b School of GeoSciences, University of Edinburgh, King’s Buildings, West Mains Road, Edinburgh, EH9 3JW, United Kingdom. a
ABSTRACT The measurement of the atmospheric concentration of greenhouse gases such as carbon dioxide (CO2) requires the simultaneous observation of a number of wavelength channels. Current and planned CO2 missions typically measure three wavebands using a hyperspectral sensor containing three spectrometers fed by an optical relay system to separate the wavelength channels. The use of one spectrometer per wavelength channel is inefficient in terms of number of detectors required and the mass and volume. This paper describes the development of an alternative solution which uses two key technologies to enable a more compact design; an image slicer mirror placed at the focal plane, and a multiple slit spectrometer operating in multiple diffraction orders. Both of these technologies are in common use in advanced astronomical spectrometers on large telescopes. The imager slicer mirror technology, as used on the James Webb Space Telescope instrument MIRI, enables the spectrometer to be illuminated with three input slits, each at a different wavelength. The spectrometer then disperses the light into multiple diffraction orders, via an echelle grating, to simultaneously capture spectra for three wavelength channels. The United Kingdom Astronomy Technology Centre, in collaboration with the Department of GeoSciences at the University of Edinburgh, has built a prototype system to demonstrate the image slicer and multiple order spectrometer technology. This paper will describe the design and performance results obtained from PERSIST – the Prototype Earth obseRving System using Image Slicer Technology. The paper will also present a summary of the performance benefits this technology can provide. Keywords: Image Slicer, Hyperspectral Imager, Spectrometer, Integral Field Spectroscopy
1. INTRODUCTION The atmospheric mole fraction of carbon dioxide (CO2), a prominent greenhouse gas attributed to contemporary climate change, has increased steadily over the past century with a recent growth rate of ~2 ppm/year over the past decade; the global annual mean CO2 mole fraction for 2011 was ~390 ppmv. Superimposed on this secular upward trend are substantial seasonal variations (mainly determined by the northern hemisphere growing season) and year-to-year variability largely driven by changes in natural and anthropogenic emission and uptake processes. Despite the obvious importance of CO2 in understanding past and future climate we do not have a complete understanding of its mass balance. Simply put, we know to a reasonable degree how much is emitted by fossil fuel combustion and other anthropogenic activities and how much resides in the atmosphere. Based on our current understanding of carbon dynamics associated with terrestrial vegetation and global oceans we are still missing a substantial (suspected biological) uptake term; it is “missing” in the sense we know that it exists but do not know its location or its underlying nature and therefore we know nothing about its robustness in the face of anticipated future warming. Up until now we have relied solely on sparse but accurate surface mole fraction measurements to observed variations in CO2, with inverted transport models utilizing advanced Bayesian techniques to infer spatial distributions of CO2 fluxes (emissions minus uptake) that are consistent with these data. Space-borne column observations of CO2 offer a new global perspective that could *
Contact e-mail:
[email protected]
potentially result in a step-wise change in our current understanding of CO2 but the relatively noisy measurements of weak column gradients introduce new technological and scientific challenges that are being met by the community. The importance of CO2 observation has resulted in the design a number of hyperspectral imaging systems for its measurement. These hyperspectral imaging systems typically use either push-broom spectrometers or Fourier transform spectrometers. Push-broom systems, e.g. the Orbiting Carbon Observatory (OCO) [1, 2, 3], provide a spectrum from one spatial dimension, building up a second spatial dimension temporally using the satellites own motion or a scanning system. These systems use a dispersive spectrograph to sample the spectral dimension. The limitation of this technique is in the utilisation of light from the telescope focal plane; only a small amount of the available light is transmitted through the slit leading to reduced spatial or spectral sampling in order to achieve the required signal level on the detector. Many future missions are also proposing to use the conventional push-broom format, such as CarbonSat [4], OCO-2 [5], and TROPOMI [6]. Fourier transform spectrographs, e.g. the Japanese Greenhouse gases Observing Satellite GOSAT [7], have also been designed for space missions in order to improve the spectral sampling. Whilst these can offer very high spectral resolution, the complexity of these instruments leads to a corresponding increase in cost and weight, and the effective scanning of the instruments means that significant inefficiencies in light collection still exist. They also typically provide sparse spatial sampling compared with push-broom systems. In order to provide increased efficiency in light collection using a dispersive spectrometer it is advantageous to be able to increase the number, N, of input slits. It is then necessary to reformat the light so that the N input slits can efficiently illuminate the spectrometer. This function is provided by an optical device called an Integral Field Unit (IFU) which reformats the entrance field of view into a format suitable for the spectrometer. The input image is manipulated and reformatted using an image slicer mirror (see Figure 5) in combination with re-imaging optics, to form a series of N slit images at the spectrometer entrance aperture. The addition of an IFU offers two benefits: the N slits can be used to observe the same wavelength and hence increase sensitivity by scanning the scene N times, or alternatively the N slits can be used to simultaneously observe N wavelength channels with a single spectrometer. The use of IFUs to increase the efficiency of hyperspectral imaging has become widespread in astronomy, for various scientific applications, over a range of wavelengths. The James Webb Space Telescope’s instruments MIRI [8] and NIRSPEC [9] both contain IFUs using image slicer technology [10] and other astronomy space missions such as SNAP are also in progress [11]. The use of image slicer technology to enable hyperspectral imaging of biological samples is also being developed [12]. The simultaneous measurement of N wavelength channels with a single spectrometer requires the use of another innovation – the use of a grating operating with multiple diffraction orders. The spectrometer is designed such that wavelength λ1 is dispersed using diffraction order m1, wavelength λ2 in order m2, and so on. For all of the spectra to appear aligned on the detector requires m1λ1 = m2λ2 = m3λ3 etc so that the angle of diffraction determined by the grating equation is the same for all wavelengths. The grating equation is: mρλ = sin(α) + sin(β), where m is the diffraction order, ρ is the ruling frequency of the grating, λ is the wavelengths, α is the angle of incidence, and β is the angle of diffraction. The simultaneous observation of multiple diffraction orders is also the principle of operation of an echelle spectrometer, commonly used for spectroscopic applications requiring very high spectral resolution. This paper describes the development of the Prototype Earth obseRvation System using Image Slicer Technology, referred to by the acronym PERSIST. The aim of this project is to build a small laboratory prototype Earth observation spectrometer that will demonstrate the advantages of using IFUs, image slicer technology, and multiple order spectrometers for the measurement of atmospheric carbon dioxide concentration. The main goals of the PERSIST prototype are as follows:
Build a bench mounted prototype IFU spectrometer with a short wavelength infrared detector system.
The system will simultaneously capture three wavelength ranges (via three slices) on one detector.
The prototype will demonstrate the feasibility of the technology for use in an Earth Observation instrument to measure atmospheric carbon dioxide.
Increase the technology readiness level of the multiple-order spectrometer concept.
The principle of operation of the PERSIST hyperspectral imaging system is illustrated schematically in Figure 1. PERSIST operates as a push-broom spectrometer with along track scanning. The field of view on the ground is sampled by the image slicer mirror, illustrated in Figure 1 with three slices. Next, re-imaging optics are used to reformat the three slices into three slits, labelled s1, s2, and s3, which form the entrance slit at the focal plane of the spectrometer. Each slit forms a spectrum with a different wavelength range. Finally the he spectrometer disperses the light from each slit to form three spectra. There is a small gap between each of the slices and this causes a short time delay between the observation for slices 1, 2, and 3 but this will not affect the accuracy of the observation.
Figure 1. Schematic of the principle of operation of the PERSIST hyperspectral imager.
To demonstrate the potential advantages of the PERSIST concept consider the example of using PERSIST to measure atmospheric carbon dioxide concentration using the same three wavelength channels as OCO. These are: the O2 A-band at 765 nm, the weak CO2 band at 1610 nm, and the strong CO2 band at 2060 nm. In the OCO spectrometer design [1, 2, 3] there are three separate spectrometers, each with a single slit illuminating a region of the detector of ~ 190 pixels by 1024 pixels. It would clearly be more efficient to fill as much of the detector array as possible with valuable spectral data. By using an IFU containing an image slicer with three slices the light can be reformatted to form three entrance slits to a single spectrometer. The three wavelength channels are captured on a single detector by using multiple diffraction order operation, as shown on the right of Figure 1. The IFU therefore enables the detector to be completely and efficiently filled with data. The use of an IFU and multiple order spectrometer system would result in OCO only needing one spectrometer and one detector (assuming the detector is sensitive to all three channels). This represents a considerable volume, cost and mass saving. It is important to note that the IFU design is not limited to three slices. If space is available on the detector many slices, or slits, can be defined, each one carrying different wavelength information. In astronomy IFU systems with ~20 slices [10] and 40 slices [11] have been manufactured whilst 250 slices are used for biomedical imaging [12]. The PERSIST breadboard prototype demonstrates the layout of the image slicer, wavelength sorting, and multiple order spectrometer to combine, for example, all three OCO wavelength channels onto one detector. This principle of operation can also be adapted to many other scientific wavelength regions of interest, not just CO2 measurement.
2. PERSIST DESIGN 2.1 Design Overview A schematic of the optical layout of the PERSIST system is shown in Figure 2. PERSIST is broken down into five subsystems: 1) the light source, 2) a telescope simulator, 3) an integral field unit, 4) a bench spectrometer, and 5) the detector. A summary of the specification of these sub-systems is given in Table 1. Where possible many of the components were sourced as Commercial Off The Shelf (COTS) items to reduce cost and complexity. The PERSIST instrument can be illuminated by a choice of light source. The first source operates within the laboratory and consists of a white light source and monochromator, indicated by the grey box at the top left of Figure 2. Alternatively, the system can be illuminated by sunlight, received via an optical fibre illuminated by a small telescope located outside the laboratory, as indicated in the blue box. Light from the source is reimaged into the integral field unit by the telescope simulator lens which is used to provide the correct image magnification at the image slicer mirror. The schematic shows light entering the IFU through an aperture
Atmospheric illumination
Laboratory illumination
at the side. However, in the final design of the IFU the light enters the IFU through an aperture at the front, as illustrated in the opto-mechanical layout shown in Figure 3 and Figure 4.
Figure 2. Schematic of the image slicing multiple-order spectrometer system PERSIST.
Table 1. Overview of the PERSIST hardware. Sub-system Light Source Telescope simulator
Image slicer system
Bench spectrometer
Components
Description
White light source Monochromator Image relay lens Aperture stop Input fold mirror Image slicer mirror Re-imaging lens Band-pass filter Pupil lens Output slit mask Collimator lens Diffraction grating Camera lens Cryostat window Thermal blocking filter
Oriel Apex illuminator Oriel 1/8m grating monochromator Provides ×2 magnification Used to define the input focal ratio Diamond machined flat mirror Diamond machined, 3 off-axis spherical mirrors Plano-convex COTS filter Plano-convex
SWIR detector Focal plane array
Achromatic doublet lens, f=300 mm 150 lines per mm, gold coating Achromatic doublet lens, f=250 mm Fused Silica. Prevents radiation at wavelengths >2.1 μm from reaching the detector and saturating the image. Raytheon VIRGO detector, 2048 × 2048 pixels, 20 μm pitch, operational wavelength range 0.8 – 2.5 μm.
A complex and expensive high resolution spectrometer, as would normally be required for measuring greenhouse gases, was beyond the scope of this initial design study. High resolution spectrometers for Earth observation applications are also of a high technology readiness level, e.g. OCO, and do not require further investigation. For the purposes of testing the PERSIST concept a simple bench mounted spectrometer using COTS components was designed. This consists of a collimator lens, a diffraction grating, a fold mirror, and a camera lens, with parameters as listed in Table 1. The main requirement of the spectrometer is to demonstrate the simultaneous observation of multiple wavelengths with multiple diffraction orders. It is believed the use of multiple diffraction orders in a single spectrometer has not been previously used in Earth observation instrumentation. High resolution echelle spectrometers, which operate with tens of diffraction orders, are commonly used in astronomical instrumentation and in various COTS products. For the PERSIST instrument the wavelength range is limited by the sensitivity of the VIRGO detector and the choice of COTS band-pass filters. The VIRGO detector has an operational wavelength range of 0.8 – 2.5 μm preventing PERSIST from measuring the O2 A-band at 765 nm. The wavelength range over which PERSIST can operate was therefore defined to be 1000 nm, 1600 nm, and 2000 nm corresponding to readily available COTS filters. In terms of the multiple diffraction order selection the wavelengths 1000 nm, 1600 nm, and 2000 nm correspond exactly to orders 8, 5, and 4 respectively. 2.2 Integral Field Unit Design The opto-mechnical design of the IFU is shown in Figure 3 and the optical layout in Figure 4. The opto-mechanical design of the IFU is based on that previously developed by the UK Astronomy Technology Centre for the JWST MIRI instrument [10]. The assembly consists of predominantly aluminium components including the mirrors. The IFU optical layout consists of an input fold mirror, an image slicer component, a set of lenses, band-pass filters, a pupil aperture mask, and an output slit mask. All of the optical components are housed within a light tight enclosure with an entrance aperture located at one end and three output slits at the opposite end. The input and output optical axes are parallel providing a more compact layout of the IFU and make the system easier to align. The IFU contains three band-pass filters that define the operational wavelength range of each slice and reject out of band radiation. The magnification of the IFU is 0.333 with the 24 mm input image (on the slice) is converted to an 8 mm image at the slit by the re-imaging lens.
entrance aperture
slicing mirror
Integral-Field Module
mirror unit central housing
cross-section of hardware
fold mirror lens unit
collimating lens field-imaging lens order-sorting filter pupil-imaging lens
slit mask
Figure 3. Opto-mechanical layout of the integral field unit.
The PERSIST optical system is well baffled with numerous aperture stops. There is a field stop located at the slicer mirror, a pupil stop immediately following the collimator lens, and an output slit mask. The bare aluminium areas surrounding each of the mirrors are masked by a series of black baffles. The system is black anodised internally to reduce unwanted reflections.
The ray-trace diagram in Figure 4 shows the optical path for each of the three slices with the three wavelengths indicated by the three colours. The telescope field of view is brought to a focus on the image slicer mirror where each slice directs the light towards an output slit. The collimator lens is used to ensure the output beam is telecentric. The re-imaging lens forms an image of the slice at the output slit. The pupil lenses place the IFU output pupil at the correct location relative to the spectrometer.
Entrance Aperture Fold Mirror
Re-imaging Lenses
Image Slicer
Collimator Lens Band-pass Filters Pupil Lenses Output slits Figure 4. Zemax shaded model of the PERSIST Integral Field Unit.
The image slicer component is shown in Figure 5. The component has three slicer mirrors, one for each waveband, and each slice is an off-axis spherical mirror with a radius of curvature of 200 mm. Each slice is 1 mm wide by 26 mm long. The component is diamond machined from a single piece of aluminium. The optical design of the slicer mirror is based on that previously developed for JWST MIRI [10]. The image slicer was manufactured at the Centre for Astronomical Instrumentation at the University of Durham.
(a)
(b)
Figure 5. Picture of the slicer mirror component (a) and the slicer CAD model (b).
The entrance aperture to the IFU is shown in Figure 6 (a) and the three output slits in Figure 6 (b). For demonstration purposes the IFU has temporarily been fitted with red, green, and blue colour filters. The image of the three slices can be seen through the entrance aperture and the three coloured output slits demonstrates the three wavelengths of operation.
(a)
(b)
Figure 6. Picture of the IFU input aperture (a) and output slits (b).
The opto-mechanical design of the IFU results in the central slit being offset with respect to the top and bottom slits and this will cause a shift of the spectrum on the detector. To compensate for this the central wavelength of the central slit was defined to be 1020 nm instead of the nominal 1000 nm. This results in all three spectra appearing in a line, as shown in Figure 8. 2.3 PERSIST bench layout A picture of the fully assembled PERSIST hardware is shown in Figure 7. The light path, from the white light source to the cryostat, is indicated by the orange lines. Not shown in Figure 7 are the various baffles and light tight box which are fitted prior to performing any exposures. The small red box, labelled ‘calibration spectrometer’, is used to measure and confirm the output wavelength of the monochromator system. The large purple container at the bottom right of Figure 7 is the detector cryostat. This used to cool the VIRGO detector to its operating temperature of approximately 70 K.
Diffraction Grating
Calibration spectrometer White light source
Collimator Lens
IFU
Monochromator Telescope simulator
Cryostat Cooling system Figure 7. Plan view of the PERSIST instrument. The optical path is indicated by the orange arrows.
3. TEST RESULTS A series of optical tests were performed to characterize the performance of the PERSIST instrument. The tests were designed to demonstrate particular key performance areas such as: the effective multiple order operation of the spectrometer and the amount of scattered light between spectra. A summary of these tests and the results is listed in Table 2. Table 2. Overview of the PERSIST performance tests. Input Light Source
Tests Performed
Results
White light source and monochromator providing monochromatic illumination
Wavelength scan to determine bandwidth, dispersion, and check for light leaks
White light source providing broadband illumination White light source and an integrating sphere providing uniform white light
Simultaneous illumination of all three slits to measure scattered light performance Simultaneous illumination of all three slits to measure scattered light performance
Hg-Ar lamp and Optical fibre providing a wavelength calibration spectrum
Measure image quality of 1020 nm band
Spectral FWHM 7.3 pixels implying resolution ~2,700 consistent with optical models
External telescope and optical fibre providing solar illumination
Record example atmospheric spectrum.
Shown in Figure 9
No light leaks found at wavelengths 0.8 – 2.5 μm. Bandwidth and dispersion consistent with expectations Scattered light level in intra-channel gap measured to be < 2 % As above
In the first test PERSIST was illuminated with monochromatic light and the wavelength was progressively scanned from 0.8 μm to 2.5 μm. The sensitivity of the detector is such that a light leak with intensity >10-4 of the measured spectrum would be detected. No light leaks were found at wavelengths outside the band-pass defined by the filters confirming the excellent out of band rejection of the filters. In the next test the monochromator was removed from the system and the white light source was used to directly illuminate the PERSIST input field of view. This test illuminates all three slices simultaneously with white light over the full range of wavelengths to which the detector is sensitive. This effectively simulates the illumination of PERSIST with an Earth observation scene. The resulting image is shown in Figure 8 with the 1600 nm spectrum at the top, the 1020 nm spectrum in the centre, and the 2000 nm spectrum at the bottom. The direction of dispersion is indicated by the arrow. The image has been processed to remove the instrument thermal background emission. The horizontal stripes seen in each spectrum correspond to the spatial structure of the coils of the lamp filament in the light source. The colours in Figure 8 (b) have been scaled using a logarithmic intensity scale to highlight the regions of scattered light that occur above and below each spectrum. The amount of scattered light between the spectra was measured to be less than 2%. This test clearly demonstrates PERSIST’s ability to simultaneously capture three discrete spectra on one detector. The fringes occasionally seen in the spectra, e.g. in the 2000 nm spectrum at the bottom of Figure 8, are caused by an interference effect within the substrate of the detector. They are commonly seen when the detector is illuminated with monochromatic light and can be removed from the spectrum by appropriate flat-field calibration. An analysis of the scattered and stray light performance of the PERSIST optics was performed using the optical modelling software TracePro®. This analysis showed a potential issue with ghost images caused by the various diffraction orders that come from using the grating with multiple diffraction orders. Fortunately the ghost images can be moved out of the field of view of the detector by a small adjustment of the grating angle. The TracePro® model was also used to predict the amount of stray light generated by the diamond machined optics within the IFU. The predictions were consistent with the measured values of 2%. The main source of scattered light within the IFU is the surface roughness of the diamond machined optics. The surface roughness of the mirrors was measured at the University of Durham and found to be 8 nm RMS for the fold mirror and 9 nm RMS for the slicer mirror. The predicted total integrated scatter at 1020 nm is predicted to be ~2% for the two mirrors. This loss is predicted to increase to nearly 4% at 765 nm, which might be an unacceptable, meaning that mirrors with lower surface roughness may be necessary for CO2 measurements.
Wavelength (a)
(b)
Figure 8. PERSIST image recorded with direct illumination of the input field of view with a white light source. Image (a) uses a linear intensity scale whilst image (b) uses a logarithmic scale to highlight areas of scattered light.
To enable PERSIST to capture an atmospheric spectrum a small telescope was set-up outside the optical laboratory. The telescope was used to form an image of the sun onto an optical fibre. The fibre was routed through the laboratory window to the PERSIST optical bench where the fibre output was used to illuminate the IFU, as shown schematically in Figure 2. The telescope consisted of a one inch diameter 50 mm focal length lens, a lens tube, and an attachment plate for the optical fibre. A measured atmospheric spectrum for the 1020 nm band is shown in Figure 9. The optical fibre used for the solar observations has a core diameter of 50 μm so the corresponding image size at the detector should therefore be 28 μm or 1.4 pixels. The height of the spectrum formed by the fibre image was measured to be 3.5 pixels FHWM or 70 μm. This is a factor of 2.5 greater than the expected size and is an indication of the end to end aberrations in the PERSIST optical system.
(a)
(b)
Figure 9. Atmospheric Spectrum measured at 1020 nm. In the left plot (a) the blue line shows a plot of the atmospheric spectrum in the channel 2 waveband centred at 1020 nm. The red line shows a spectrum of a Tungsten-Halogen lamp used for calibration of the atmospheric spectrum. The right plot (b) shows the extracted spectrum after calibration with the lamp spectrum.
The spectral resolution of the 1020 nm channel was determined by observation of the spectral emission lines from a Mercury-Argon gas discharge lamp. The bandwidth of the 1020 nm filter meant that only one spectral line was observed, a Mercury line at 1013.975 nm. No spectral lines were seen in the 1600 nm or 2000 nm channels. From the image of the spectral line the instrument point spread function was determined to have a FWHM of 7.3 pixels in the spectral direction corresponding to a spectral resolution of 2,700. The FWHM in the spatial direction is 1.7 pixels indicating significantly higher spectral resolution could be obtained if the PSF was optimised for spectral image quality rather than spatial image quality. The measured spectral resolution of the bench spectrometer was not sufficiently high enough to allow accurate measurements of atmospheric CO2 concentration. However, the principle of operation of the multiple order spectrometers has been clearly demonstrated by the tests performed.
4. SUMMARY & CONCLUSIONS A number of Earth observation missions are currently being developed to enable scientists to monitor the concentration of various gases in the Earth’s atmosphere. Many of these mission use conventional push-broom hyperspectral imagers to capture data. This document described the design of PERSIST, a new type of hyperspectral imager that uses image slicer mirrors and a spectrometer operating with multiple diffraction orders. The main purpose of the PERSIST instrument was to demonstrate the performance of the image slicer system and the capability of the spectrometer to simultaneously record spectra from multiple wavelength channels. The science requirements for PERSIST were that it should be able to measure three wavelength ranges as used for the measurement of the concentration of atmospheric CO2. A complete bench top hyperspectral imager system was constructed to enable these performance verifications, which consisted of a light source, a telescope simulator, an integral field unit with an image slicer mirror, a spectrometer operating with three diffraction orders, and a cooled infrared detector system. The optical and opto-mechanical design of the integral field unit was described in detail. The design of the IFU resulted in a neat compact sub-system that was straightforward to align and gives excellent performance. To keep costs at an acceptable level the bench spectrometer was constructed using COTS components. This meant the spectrometer can only achieve a spectral resolution about one tenth of that needed for accurate measurement of atmospheric CO2. The low resolution was considered adequate for the purposes of this breadboard prototype. The performance of the IFU and bench spectrometer was verified during an intensive test campaign which was carried out in the optical laboratory at the UK ATC. The results of these tests showed that the spectrometer cleanly captures three spectra at wavelengths of 1020 nm, 1600 nm, and 2000 nm. The scattered light level between the spectra is typically below 2% and is consistent with the theoretical predictions. The low level of scattered light shows that the image slicer mirror is working well at the wavelengths needed for CO2 measurement. It also demonstrates that the bandpass filters are working well, and only the wavelength region of interest is being detected by the spectrometer, indicating there is no out of band light leak. Although the PERSIST instrument operates at wavelengths normally used for the measurement of atmospheric CO2 the design is compatible with other wavelengths. The wavelength of operation of the instrument can be changed simply by changing the filters within the IFU and adjusting the grating within the spectrometer. The use of the PERSIST technology in the context of a CO2 mission similar to OCO would result in an instrument requiring only a single spectrometer and detector instead of the three needed for OCO. Preliminary results of instrument system modelling for PERSIST [13] show that it can achieve the same performance targets as OCO, such as signal to noise ratio and spectral resolution, with an instrument approximately a factor of 2 – 3 smaller in terms of mass and volume.
ACKNOWLEDGEMENTS The authors wish to acknowledge the Centre for Earth Observation Instrumentation for funding this work and STFC’s Centre for Instrumentation for their financial support for the detector system. JB acknowledges the centre for Earth Observation Instrumentation and the National Environment Research Council for funding his studentship, number NE/1528818/1.
REFERENCES [1] Crisp, D., Atlas, R. M., Breon, F.-M., Brown, L. R., Burrows, J. P., Ciais, P., Connor, B. J., Doney, S. C., Fung, I. Y., Jacob, D .J., Miller, C. E., O'Brien, D., Pawson, S., Randerson, J. T., Rayner, P., Salawitch, R. J., Sander, S. P., Sen, B., Stephens, G. L., Tans, P. P., Toon, G. C., Wennberg, P. O., Wofsy, S. C., Yung, Y. L., Kuang, Z., Chudasama, B., Sprague, G., Weiss, B., Pollock, R., Kenyon, D., and Schroll, S., “The Orbiting Carbon Observatory (OCO) Mission,” Advances in Space Research, Vol. 34, 4, 700-709 (2004). [2] Crisp, D., "The Orbiting Carbon Observatory: NASA's first dedicated carbon dioxide mission," Proc. SPIE 7106, 710604 (2008). [3] Crisp, D., Miller, C. E., and DeCola, P. L., "NASA Orbiting Carbon Observatory: measuring the column averaged carbon dioxide mole fraction from space," J. Appl. Remote Sens. 2, 023508 (2008). [4] Bovensmann, H., Buchwitz, M., Burrows, J. P., Reuter, M., Gerilowski, O., Schneising, O., Heymann, J., Tretner, A., and Erzinger, J., "A remote sensing technique for global monitoring of power plant CO2 emissions from space and related applications," Atmos. Meas. Tech., 3, 781-811 (2010). [5] Pollock, R., Haring, R. E., Holden, J. R., Johnson, D. L., Kapitanoff, A., Mohlman, D., Phillips, C., Randall, D., Rechsteiner, D., Rivera, J., Rodriguez, J. I., Schwochert, M. A., and Sutin, B. M., "The Orbiting Carbon Observatory nstrument: performance of the OCO instrument and plans for the OCO-2 instrument,” Proc. SPIE 7826, 78260W (2010). [6] Veefkind, J. P., Aben, I., McMullan, K., Förster, H., de Vries, J., Otter, G., Claas, J., Eskes, H. J., de Haan, J. F., Kleipool, Q., van Weele, M., Hasekamp, O., Hoogeveen, R., Landgraf, J., Snel, R., Tol, P., Ingmann, P., Voors, R., Kruizinga, B., Vink, R., Visser, H., Levelt, P.F., “TROPOMI on the ESA Sentinel-5 Precursor: A GMES mission for global observations of the atmospheric composition for climate, air quality and ozone layer applications,” Remote Sensing of Environment, Vol. 120, 70-83 (2012). [7] Hamazaki, T., Kuze, A., and Kondo, K., "Sensor system for Greenhouse Gas Observing Satellite (GOSAT),” Proc. SPIE 5543, 275 (2004). [8] Wells, M., Lee, D., Oudenhuysen, A., Hastings, P., Pel, J., and Glasse, A., "The MIRI medium resolution spectrometer for the James Webb Space Telescope,” Proc. SPIE 6265, 626514 (2006). [9] Closs, M. F., Ferruit, P., Lobb, D. R., Preuss, W. R., Rolt, S., and Talbot, R. G., "The Integral Field Unit on the James Webb Space Telescope's Near-Infrared Spectrograph,” Proc. SPIE 7010, 701011 (2008). [10] Lee, D., Wells, M., Dickson, C. J., Shore P., and Morantz, P., "Development of diamond machined mirror arrays for integral field spectroscopy,” Proc. SPIE 6273, 62731Y (2006). [11] Prieto, E., Ealet, A., Milliard, B., Aumeunier, M., Bonissent, A., Cerna, C., Crouzet, P., Karst, P., Kneib, J., Malina, R., Pamplona, T., Rossin, C., Smadja C., and Vives, S., "An integral field spectrograph for SNAP,” Proc. SPIE 7010, 701019 (2008). [12] Kester, R. T., Gao, L., and Tkaczyk, T. S., "Development of image mappers for hyperspectral biomedical imaging applications," Appl. Opt. 49, 1886-1899 (2010). [13] Lee, D., Vick, A. J. A., and Palmer, P. I., “CEOI IFU Final Report,” report submitted to the Centre for Earth Observation Instrumentation (2011).
