Imaging Fourier Transform Spectrometer Research Articles

Properties of supernova remnants in SIGNALS galaxies – I. NGC 6822 and M33

Abstract We present a spatially resolved study of the kinematical properties of known supernova remnants (SNRs) in the nearest galaxies of the SIGNALS survey, namely NGC 6822 (one object) and M33 (163 objects), based on data obtained with the SITELLE Imaging Fourier Transform Spectrometer (iFTS) at the Canada-France-Hawaii Telescope. The purpose of this paper is to provide a better scheme of identification for extragalactic SNRs and, in particular, to distinguish between H ii regions and SNRs. For that we have used diagrams which involve both the [S ii]/Hα ratio and the velocity dispersion (σ). We also introduce a new parameter, $\xi = {[\rm S\, \small {II}] \over H\alpha } \times \sigma$, which enhances still the contrast between SNRs and the rest of the ionised gas. More than 90% of the SNRs in our entire sample show an integrated [S ii]/Hα ratio larger than the canonical value (0.4). 86% of the SNRs present in our field show a significant velocity dispersion. The spectral resolution of our observations allows us to observe the complex velocity structure of some SNRs.

More Frequent Spaceborne Sampling of XCO2 Improves Detectability of Carbon Cycle Seasonal Transitions in Arctic‐Boreal Ecosystems

AbstractSurface, aircraft, and satellite measurements indicate pervasive early cold season (Augut–September) CO2 emissions across Arctic regions, consistent with increased ecosystem metabolism in plants and soils. A key remaining question is whether cold season sources will become large enough to permanently shift the Arctic into a net carbon source. Polar orbiting GHG satellites provide robust estimation of regional carbon budgets but lack sufficient spatial coverage and repeat frequency to track sink‐to‐source transitions in the early cold season. Mission concepts such as the Arctic Observing Mission (AOM) advocate for flying imaging spectrometers in highly elliptical orbits (HEO) over the Arctic to address sampling limitations. We perform retrieval and flux inversion simulation experiments using the AURORA mission concept, leveraging a Panchromatic imaging Fourier Transform Spectrometer (PanFTS) in HEO. Our simulations demonstrate the potential benefits of increased CO2 sampling for detecting emissions during the early cold season.

Quantifying flare combustion efficiency using an imaging Fourier transform spectrometer

ABSTRACT Mid-wavelength infrared (MWIR) imaging Fourier transform spectrometers (IFTSs) are a promising technology for measuring flare combustion efficiency (CE) and destruction removal efficiency (DRE). These devices generate spectrally resolved intensity images of the flare plume, which may then be used to infer column densities of relevant species along each pixel line-of-sight. In parallel, a 2D projected velocity field may be inferred from the apparent motion of flow features between successive images. Finally, the column densities and velocity field are combined to estimate the mass flow rates for the species needed to calculate the CE or DRE. Since the MWIR IFTS can measure key carbon-containing species in the flare plume, it is possible to measure CE without knowing the fuel flow rate, which is important for fenceline measurements. This work demonstrates this approach on a laboratory heated vent, and then deploys the technique on two working flares: a combustor burning natural gas at a known rate, and a steam-assisted flare at a petrochemical refinery. Analysis of the IFTS data highlights the potential of this approach, but also areas for future development to transform this approach into a reliable technique for quantifying flare emissions. Implications: Our research is motivated by the need to assess hydrocarbon emissions from flaring, which is a critical problem of global significance. For example, recent studies have shown that methane destruction efficiency of flaring from upstream oil may be significantly lower than the commonly assumed figure of 98%; work by Plant et al., in particular, suggest that this discrepancy amounts to CO2 emissions from 2 to 8 million automobiles annually, considering the US alone. Similarly, the international energy agency (IEA) estimates a global flare efficiency of 92%,which translates in 8 million tons of CH4 emitted by flares in 2020. Highlighted by these studies and supported by the World Bankinitiatives toward zero routine flaring emissions, there is an urgent need for oil and gas industry to assess their flare methane emission, and overall hydrocarbon emissions. At the very least, it is critical to identify problematic flare operating conditions and means to mitigate flare emissions. Focusing on remote quantification of plume species, the measurement technique and quantification method presented in this paper is a considerable step forward in that direction by computing combustion efficiency and key components for destruction efficiency.

Stray Light Analysis and Suppression for an Infrared Fourier Imaging Spectrometer

To improve the accuracy of infrared radiation characteristics measurement in the aviation field, an infrared Fourier transform imaging spectrometer based on a double-swing solid angle reflector was designed. This imaging spectrometer operates in the 3–5 μm wavelength range and has a field of view of 1.7° × 1.7°. This article presents a comprehensive analysis of the system’s stray light and also studies the impact of external stray light on the imaging quality, along with the influence of internal stray light on the interference effects and the spectral resolution. It also present the design of a hood that suppresses the point source transmittance of the external stray light down to the order of 10−4. Based on this, we propose a method that incorporates the introduction of wedge and inclination angles. Additionally, a numerical range is provided for the addition of these angles on the beam splitter mirror and compensation plate. This ensures the effective suppression of any internal stray light. This study fills the gap in the knowledge about Fourier transform imaging spectrometers operating in the mid-infrared band for aviation applications, and proposes a suppression method suitable for interference systems, which is also suitable for Fourier transform imaging spectrometers based on other types of interferometers. This study broadens the application field of Fourier transform imaging spectrometers in stray light, and has great significance to promote the development of Fourier transform imaging spectrometer.

Dual-Interference Channels Static Fourier Transform Imaging Spectrometer Based on Stepped Micro-Mirror: Data Processing and Experiment Research

The use of a dual-interference channels static Fourier transform imaging spectrometer based on stepped micro-mirror (D-SIFTS) for environmental gas monitoring has the advantages of high throughput, a compact structure, and a stable performance. It also has the characteristics of both a broad spectral range and high spectral resolution. However, its unique structural features also bring many problems for subsequent data processing, mainly including the complex distribution of the interference data, the low signal-to-noise ratio (SNR) of infrared scene images, and a unique inversion process of material information. To this end, this paper proposes a method of image and spectra information processing and gas concentration inversion. A multiscale enhancement algorithm for infrared images incorporating wavelet denoising is used to obtain high-quality remote sensing scene images, and spectral reconstruction optimization algorithms, such as interference intensity sequence resampling, are used to obtain accurate spectral information; the quantitative calibration model of the detected gas concentration is established to achieve high-precision inversion of gas concentration, and its distribution is visualized in combination with the scene image. Finally, the effectiveness and accuracy of the data processing algorithm are verified through the use of several experiments, which provide essential theoretical guidance and technical support for the practical applications of D-SIFTS.

Intelligent pointing increases the fraction of cloud-free CO2 and CH4 observations from space

For most CO2 and CH4 satellites, only a small percentage (∼10%) of observations yield successful retrievals, with the remaining ∼90% rejected, primarily due to the effects of clouds. Discarding this large fraction of data is an inefficient strategy worth reconsidering due to the costs involved in developing, launching and operating the satellites to make these observations. However, if real-time cloud data are available together with pointing capability, cloud data can guide the instrument pointing in an “intelligent pointing” strategy for cloud avoidance. In this work, multiple intelligent pointing simulations were conducted, demonstrating the significant advantages of this approach for satellites in a highly elliptical orbit (HEO), from which nearly the whole Earth disk can be observed. Multiple factors are shown to contribute to intelligent pointing efficiency such as the size and shape (or aspect ratio) of the field of view (FOV). For the current baseline orbit and Imaging Fourier Transform Spectrometer (IFTS) observing characteristics for the proposed Arctic Observing Mission (AOM), the monthly fraction of cloud-free observations is roughly a factor of 2 (ranging from ∼1.5–2.5) more than obtained with standard pointing (in which cloud information is not used). A similar efficiency is expected in a geostationary orbit (GEO) with an IFTS, however, for a dispersive instrument in HEO or GEO, the gain is more modest. This result is primarily attributed to the ∼1:1 aspect ratio of the IFTS FOV, since it is more efficient for cloud avoidance and scanning irregularly-shaped land masses than the long and narrow slit projection of a typical dispersive spectrometer. These results have implications for the design of future CO2 or CH4 monitoring satellites and constellation architectures, as well as other fields of satellite earth observation in which clouds significantly impact observations.

Hyperspectral Channel-Modulated Static Birefringent Fourier Transform Imaging Spectropolarimeter with Zoomable Spectral Resolution

A novel channel-modulated static birefringent Fourier transform imaging spectropolarimeter (CSBFTIS) is introduced, which is based on a double Wollaston prism (DWP). With an adjustable air gap (AG), the spectral resolution can be adjusted by changing the AG. The CSBFTIS combines the channel-modulated imaging spectropolarimeter and the slit-free static birefringent Fourier transform imaging spectrometer technology with adjustable spectral resolution. The device is compact and robust, with a wide spectral range and a large luminous flux. Compared with various previous spectropolarimeters, it can greatly reduce the size of the spectral image data to adapt to different application requirements. A prototype is built, and simulation and experiments are carried out, and the results prove the effectiveness of the method.

Stratospheric-trace-gas-profile retrievals from balloon-borne limb imaging of mid-infrared emission spectra

Abstract. The Limb Imaging Fourier Transform Spectrometer Experiment (LIFE) instrument is a balloon-borne prototype of a satellite instrument designed to take vertical images of atmospheric limb emission spectra in the 700–1400 cm−1 wavenumber range from the upper-troposphere–lower-stratosphere (UTLS) altitude region of the atmosphere. The prototype builds on the success of past and existing instruments while reducing the complexity of the imaging design. This paper details the results of a demonstration flight on a stabilized stratospheric balloon gondola from Timmins, Canada, in August 2019. Retrievals of vertical trace gas profiles for the important greenhouse gases H2O, O3, CH4, and N2O, as well as HNO3, are performed using an optimal estimation approach and the SASKTRAN radiative transfer model. The retrieved profiles are compared to approximately coincident observations made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) solar occultation and Microwave Limb Sounder (MLS) instruments. An evaluation of the LIFE measurements is performed, and areas of improvement are identified. This work increases the overall technical readiness of the approach for future balloon, aircraft, and space applications.

Luminous Flux Utilization of Static Birefringent Fourier Transform Imaging Spectrometer with Zoomable Spectral Resolution

The optical displacement calculation formula for the dual Wollaston prism is derived for any incident plane and angle using the wave normal tracing method. The validity of the calculation is confirmed by comparing it with captured photographs. The relationship between the maximum incident aperture, incident angle, and incident surface angle is given when plane and spherical waves incident on the prism group. The aperture utilization of a novel static birefringent Fourier transform imaging spectrometer based on dual Wollaston prisms is also analyzed. The relationship of aperture utilization with the incident surface angle, incident angle, and prisms’ air gap thickness is given. The results provide a theoretical foundation for fully describing the optical transmission characteristics of the dual Wollaston prism group and developing a high-performance spatio-tempo hybrid modulated birefringent spectral zoom Fourier transform imaging spectrometer.

Reducing the Influence of Systematic Errors in Interference Core of Stepped Micro-Mirror Imaging Fourier Transform Spectrometer: A Novel Calibration Method

The stepped micro-mirror imaging Fourier transform spectrometer (SIFTS) has the advantages of high throughput, compactness, and stability. However, the systematic errors in the interference core of the SIFTS have a significant impact on the interferogram and the reconstructed spectrum. In order to reduce the influence of systematic errors, a transfer error model of the systematic errors in the interference core of the SIFTS is established, and an interferogram and spectrum calibration method is presented, which combines the least squares fitting calibration and the row-by-row fast Fourier transform-inverse fast Fourier transform (FFT-IFFT) flat-field calibration. The experimental results show that the methods can sufficiently reduce the influence of systematic errors in the interference core of the SIFTS, such as the interferogram fringe tilt, the peak position shift of the reconstructed spectrum, and the error of spectral response.

Exact optical path difference and complete performance analysis of a spectral zooming imaging spectrometer.

The optical path difference (OPD) equations of the dual Wollaston prisms (DWP) with an adjustable air gap (AG) are derived by the wave normal tracing method, which is suitable for arbitrary incidence plane and angle. The spatial distribution of the OPD for various AG is presented. The validity of the OPD equation is verified by comparing the calculated interferograms with experimentally observed one. The performance of a novel static birefringent Fourier transform imaging spectrometer (SBFTIS) based on the DWP is investigated. The spectral resolution can be adjusted by changing the AG and the field of view can reach 10.0°, which is much larger than that predicted by our previous work. The results obtained in this article provide a theoretical basis for completely describing the optical transmission characteristic of the DWP and developing the high-performance birefringent spectral zooming imaging spectrometer.

Quantification and mitigation of the instrument effects and uncertainties of the airborne limb imaging FTIR GLORIA

Abstract. The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) is an infrared imaging FTS (Fourier transform spectrometer) with a 2-D infrared detector that is operated on two high-flying research aircraft. It has flown on eight campaigns and measured along more than 300 000 km of flight track. This paper details our instrument calibration and characterization efforts, which, in particular, almost exclusively leverage in-flight data. First, we present the framework of our new calibration scheme, which uses information from all three available calibration sources (two blackbodies and upward-pointing “deep space” measurements). Part of this scheme is a new algorithm for correcting the erratically changing nonlinearity of a subset of detector pixels and the identification of the remaining bad pixels. Using this new calibration, we derive a 1σ bound of 1 % on the instrument gain error and a bound of 30 nW cm−2 sr−1 cm on the instrument offset error. We show how we can examine the noise and spectral accuracy for all measured atmospheric spectra and derive a spectral accuracy of 5 ppm on average. All these errors are compliant with the initial instrument requirements. We also discuss, for the first time, the pointing system of the GLORIA instrument. Combining laboratory calibration efforts with the measurement of astronomical bodies during the flight, we can achieve a pointing accuracy of 0.032∘, which corresponds to one detector pixel. The paper concludes with a brief study of how these newly characterized instrument parameters affect temperature and ozone retrievals. We find that the pointing uncertainty and, to a lesser extent, the instrument gain uncertainty are the main contributors to the error in the result.

Simulated multispectral temperature and atmospheric composition retrievals for the JPL GEO-IR Sounder

Abstract. Satellite measurements enable quantification of atmospheric temperature, humidity, wind fields, and trace gas vertical profiles. The majority of current instruments operate on polar orbiting satellites and either in the thermal and mid-wave or in the shortwave infrared spectral regions. We present a new multispectral instrument concept for improved measurements from geostationary orbit (GEO) with sensitivity to the boundary layer. The JPL GEO-IR Sounder, which is an imaging Fourier transform spectrometer, uses a wide spectral range (1–15.4 µm) encompassing both reflected solar and thermal emission bands to improve sensitivity to the lower troposphere and boundary layer. We perform retrieval simulations for both clean and polluted scenarios that also encompass different temperature and humidity profiles. The results illustrate the benefits of combining shortwave and thermal infrared measurements. In particular, the former adds information in the boundary layer, while the latter helps to separate near-surface and mid-tropospheric variability. The performance of the JPL GEO-IR Sounder is similar to or better than currently operational instruments. The proposed concept is expected to improve weather forecasting as well as severe storm tracking and forecasting and also benefit local and global air quality and climate research.

Learning a Fully Connected U-Net for Spectrum Reconstruction of Fourier Transform Imaging Spectrometers

Fourier transform imaging spectrometers (FTISs) are widely used in global hyperspectral remote sensing due to the advantages of high stability, high throughput, and high spectral resolution. Spectrum reconstruction (SpecR) is a classic problem of FTISs determining the acquired data quality and application potential. However, the state-of-the-art SpecR algorithms were restricted by the length of maximum optical path difference (MOPD) of FTISs and apodization processing, resulting in a decrease in spectral resolution; thus, the applications of FTISs were limited. In this study, a deep learning SpecR method, which directly learned an end-to-end mapping between the interference/spectrum information with limited MOPD and without apodization processing, was proposed. The mapping was represented as a fully connected U-Net (FCUN) that takes the interference fringes as the input and outputs the highly precise spectral curves. We trained the proposed FCUN model using the real spectra and simulated pulse spectra, as well as the corresponding simulated interference curves, and achieved good results. Additionally, the performance of the proposed FCUN on real interference and spectral datasets was explored. The FCUN could obtain similar spectral values compared with the state-of-the-art fast Fourier transform (FFT)-based method with only 150 and 200 points in the interferograms. The proposed method could be able to enhance the resolution of the reconstructed spectra in the case of insufficient MOPD. Moreover, the FCUN performed well in visual quality using noisy interferograms and gained nearly 70% to 80% relative improvement over FFT for the coefficient of mean relative error (MRE). All the results based on simulated and real satellite datasets showed that the reconstructed spectra of the FCUN were more consistent with the ideal spectrum compared with that of the traditional method, with higher PSNR and lower values of spectral angle (SA) and relative spectral quadratic error (RQE).

Lenslet-based snapshot full-Stokes imaging spectropolarimeter

A lenslet-based snapshot channeled imaging spectropolarimeter (SCISP) system is presented. By introducing the multiple-image Fourier transform imaging spectrometer into the archetype of channeled spectropolarimetry, SCISP can simultaneously acquire the three-dimensional spatial and spectral (x,y,λ) data-cube for each of the Stokes parameters S0, S1, S2 and S3 in a snapshot using a single detector. The validity and accuracy of SCISP are showcased by attaining the outdoor spectral-polarimetric video and different laboratory accuracy experiments. Unlike conventional channeled spectropolarimeters, SCISP maintains the real snapshot feature (2D spatial imaging) and it is compact, robust. These capacities of the camera open new prospects for many applications, such as remote sensing, space exploration, and biological analysis.

Optimized ultraviolet grayscale process for high vertical resolution applied to spectral imagers

NanoCarb is a miniature Fourier transform imaging spectrometer dedicated to the measurement of atmospheric CO2 and CH4. The key element of NanoCarb is an array of Fabry–Perot microinterferometers having a stepcase shape. Lateral dimensions and height of each step depend on the used material, the focusing lenses, and the targeted optical path difference to be measured. In this paper, we developed a grayscale lithography process for a large surface patterning with high vertical resolution. This process is combined with plasma etching to transfer the as-obtained resist patterns into the silicon substrate. This method is an efficient and quick way for the realization of such arrays into silicon. A low contrast resist (ma-P1225G) was used for better control of the step height, and we investigated the effect of two annealing processes on the contrast curve slope: the soft bake and the postexposure bake. Therefore, combining the two processes leads to a 20 nm step height resolution in resist and 50 nm in silicon.

Non-circular flows in HIghMass galaxies in a test of the late accretion hypothesis

ABSTRACT We present H α velocity maps for the HIghMass galaxies UGC 7899, UGC 8475, UGC 9037, and UGC 9334, obtained with the SITELLE Imaging Fourier Transform Spectrometer on the Canada–France–Hawaii Telescope, to search for kinematic signatures of late gas accretion to explain their large atomic gas reservoirs. The maps for UGC 7899, UGC 9037, and UGC 9334 are amenable to disc-wide radial flow searches with the DiskFit algorithm, and those for UGC 7899 and UGC 9037 are also amenable to inner-disc kinematic analyses. We find no evidence for outer disc radial flows down to $\bar{V}_{\mathrm r} \sim 20 \ \mathrm{km\, s}^{-1}$ in UGC 9037 and UGC 9334, but hints of such flows in UGC 7899. Conversely, we find clear signatures of inner (r ≲ 5 kpc) non-circularities in UGC 7899 and UGC 9037 that can be modelled as either bisymmetric (which could be produced by a bar) or radial flows. Comparing these models to the structure implied by photometric disc–bulge–bar decompositions, we favour inner radial flows in UGC 7899 and an inner bar in UGC 9037. With hints of outer disc radial flows and an outer disc warp, UGC 7899 is the best candidate for late accretion among the galaxies examined, but additional modelling is required to disentangle potential degeneracies between these signatures in H i and H α velocity maps. Our search provides only weak constraints on hot-mode accretion models that could explain the unusually high H i content of HIghMass galaxies.

A balloon-borne imaging Fourier transform spectrometer for atmospheric trace gas profiling.

The upper troposphere and lower stratosphere (UTLS) region is a highly variable region of the atmosphere and critical for understanding climate. Yet, it remains undersampled in the observational satellite record. Due to recent advances in interferometer and infrared detection technologies, imaging Fourier transform spectrometer (FTS) technology has been identified as a feasible remote sensing approach to obtain the required precision and spatial resolution of atmospheric trace gas composition in the UTLS. Building on the success of instruments such as the Michelson Interferometer for Passive Atmospheric Sounding and gimbaled limb observer for radiance imaging of the atmosphere, the limb imaging Fourier transform spectrometer experiment (LIFE) instrument, of which this paper details the designand performance, is a balloon-borne infrared imaging FTS developed as an early prototype of a low earth orbit satellite instrument. LIFE is constructed primarily with commercially available off-the-shelf components, with a designemphasis on greatly reducing the complexity of the instrument, particularly the cooling requirements, with a minimal reduction in information gain on the target atmospheric greenhouse gases of water vapor, methane, ozone, and nitrous oxide. The developed instrument was characterized through a series of thermal and vacuum tests and validated through a successful demonstration balloon flight during the 2019 Strato-Science campaign in Canada. In the calibration of the data from the balloon flight, an issue was identified regarding a lack of knowledge in the emissivity of the on-board blackbody calibration sources. These systematic effects were minimized through the application of an emissivity ratio determined from the characterization tests where a wider range of known blackbody temperatures were available. Despite this identified calibration issue, the results demonstrate that the instrument is capable of meeting primary performance requirements for trace gas retrievals of the target atmospheric species.

Assessing Flare Combustion Efficiency using Imaging Fourier Transform Spectroscopy

Flaring plays a critical role in reducing the environmental impact of upstream oil and gas processing by converting methane and other gaseous hydrocarbons into CO2, which has a lower global warming potential. This process is highly efficient under ideal conditions but efficiency may be significantly lower under certain scenarios such as fuel stripping under crosswind and emission of volatile organic compounds and unburned fuels due to over-aeration or over-steaming in assisted flares. This study assesses the potential of using imaging Fourier transform spectrometers (IFTSs) to directly measure combustion efficiency by combining species column densities estimated from a spectroscopic model with intensity-weighted velocities found using an optical flow model. Simulated measurements using a computational fluid dynamics (CFD)-large eddy simulation of a flare in a crosswind are used to establish the technique's viability, followed by experimental measurements on a heated gas vent to validate the optical flow model. Finally, preliminary measurements are carried out on a laboratory-scale steam- and air-assisted flare. While the simulated measurements and heated vent experiments support the feasibility of this approach, experimentally-derived spectra from the lab-scale flare were contaminated with artifacts attributed to turbulent fluctuations, which complicates the quantitative interpretation of the IFTS data.

Data reduction and calibration accuracy of the imaging Fourier transform spectrometer SITELLE

ABSTRACT SITELLE, an imaging Fourier Transform Spectrometer, is part of the Canada–France–Hawaii instrument suite. It delivers spectral cubes covering an 11 arcmin × 11 arcmin field of view with a seeing-limited spatial resolution and a tunable spectral resolution (R ∼ 1–10 000) in selected bands of the visible range (350–900 nm). We present a complete picture of the calibration accuracy obtained with the SITELLE processing pipeline ORBS. We put a particular emphasis on the description of our phase correction method and on the assessment of the flux calibration precision. We show that the absolute flux calibration uncertainty is to be considered between −15 per cent and 5 per cent. Flexure in the instrument is likely responsible for a wavelength calibration error gradient across the field of view, with an amplitude corresponding to 15 to 25 km s−1; measurements of the night-sky emission lines when present in a science cube reduces this error to ∼2 km s−1. The astrometric calibration is limited to ∼1 arcsec by the optical distortions. Considering that imaging Fourier transform spectrometers are not as widely used as dispersive spectrometers and because SITELLE and its prototype are the first instruments of their kind to provide data in the near-UV at a high spectral resolution and over a very large field of view, we took great care in explaining most of the core concepts behind this technique as well as exploring all the practical limitations that affect the precision of our calibrations. As such, this paper aims at providing a solid ground for subsequent developments of imaging Fourier transform spectrometers in astronomy.