Instrument Spectral Resolution Research Articles

Use of the GOES-R Split-Window Difference to Diagnose Deepening Low-Level Water Vapor

AbstractThe depth of boundary layer water vapor plays a critical role in convective cloud formation in the warm season, but numerical models often struggle with accurate predictions of above-surface moisture. Satellite retrievals of water vapor have been developed, but they are limited by the use of a model’s first guess, instrument spectral resolution, horizontal footprint size, and vertical resolution. In 2016, Geostationary Operational Environmental Satellite-R (GOES-R), the first in a series of new-generation geostationary satellites, will be launched. Its Advanced Baseline Imager will provide unprecedented spectral, spatial, and temporal resolution. Among the bands are two centered at 10.35 and 12.3 μm. The brightness temperature difference between these bands is referred to as the split-window difference, and has been shown to provide information about atmospheric column water vapor. In this paper, the split-window difference is reexamined from the perspective of GOES-R and radiative transfer model simulations are used to better understand the factors controlling its value. It is shown that the simple split-window difference can provide useful information for forecasters about deepening low-level water vapor in a cloud-free environment.

Direct detection of SDSS J0926+3624 orbital expansion with ARCONS

AM Canum Venaticorum (AM CVn) stars belong to a class of ultra-compact, short period binaries with spectra dominated largely by helium. SDSS J0926+3624 is of particular interest as it is the first observed eclipsing AM CVn system. We observed SDSS J0926+3624 with the \textbf{Ar}ray \textbf{C}amera for \textbf{O}ptical to \textbf{N}ear-IR \textbf{S}pectrophotometry (ARCONS) at the Palomar 200" telescope. ARCONS uses a relatively new type of energy-resolved photon counters called Microwave Kinetic Inductance Detectors (MKIDs). ARCONS, sensitive to radiation from 350 to 1100 nm, has a time resolution of several microseconds and can measure the energy of a photon to $\sim10%$. We present the light curves for these observations and examine changes in orbital period from prior observations. Using a quadratic ephemeris model, we measure a period rate of change $\dot{P} = (3.07 \pm 0.56)\times 10^{-13}$. In addition, we use the high timing resolution of ARCONS to examine the system's high frequency variations and search for possible quasi-periodic oscillations (QPOs). Finally, we use the instrument's spectral resolution to examine the light curves in various wavelength bands. We do not find any high frequency QPOs or significant spectral variability throughout an eclipse.

Sparse sampling for fast hyperspectral coherent anti-Stokes Raman scattering imaging

We demonstrate a method to increase the acquisition speed in coherent anti-Stokes Raman scattering (CARS) hyperspectral imaging while retaining the relevant spectral information. The method first determines the important spectral components of a sample from a hyper-spectral image over a small number of spatial points but a large number of spectral points covering the accessible spectral range and sampling the instrument spectral resolution at the Nyquist limit. From these components we determine a small set of frequencies needed to retrieve the weights of the components with minimum error for a given measurement noise. Hyperspectral images with a large number of spatial points, for example covering a large spatial region, are then measured at this small set of frequencies, and a reconstruction algorithm is applied to generate the full spectral range and resolution. The resulting spectra are suited to retrieve from the CARS intensity the CARS susceptibility which is linear in the concentration, and apply unsupervised quantitative analysis methods such as FSC3. We demonstrate the method on CARS hyperspectral images of human osteosarcoma U2OS cell, with a reduction in the acquisition time by a factor of 25. This method is suited also for other coherent vibrational microscopy techniques such as stimulated Raman scattering, and in general for hyperspectral imaging techniques with sequential spectral acquisition.

Automated image curvature assessment and correction for high-throughput Raman spectroscopy and microscopy

Raman spectroscopy and microscopy can provide molecular information for complex materials such as biological tissue and cells. In these applications, light-collection throughput is essential for speedy acquisition of high-quality data. To im- prove throughput, two-dimensional detectors and high numerical aperture (NA) optical systems have been employed. However, owing to the out-of-plane diffraction in grating-based dispersive spectrograph, the entrance slit image formed at the detector plane is curved along the vertical direction. Direct vertical binning of individual detector rows without correcting the curvature results in degraded spectral resolution and peak misalignment. We evaluate two software approaches to remove the image cur- vature after high-throughput data acquisition, with the objective to retain instrument spectral resolution and peak accuracy as if a linear-array detector were used. Curvature correction and detection are achieved in two steps: calibration of the image curva- ture using a Raman active material and application of the correction to future curved images. This method has been employed for a high-NA, large CCD Raman spectroscopic system deigned for non-invasive glucose sensing, a medium-NA, medium-size CCD line-scan Raman microscope designed for high-throughput tissue and cellular imaging, and an active-illumination Raman microscope. We show that remarkable improvement in data fidelity can be obtained as assessed by peak misalignment, distri- bution of data variance, and the waveform of principal component spectra. High quality curvature correction is essential for quantitative analysis such as the multivariate calibration, spectral pattern recognition, and peak shift detection based techniques. The software approach is highly flexible for instrument modification.

Influence of aerosol and surface reflectance variability on hyperspectral observed radiance

Abstract. Current aerosol retrievals based on visible and near infrared remote-sensing, are prone to loss of accuracy, where the assumptions of the applied algorithm are violated. This happens mostly over land and it is related to misrepresentation of specific aerosol conditions or surface properties. New satellite missions, based on high spectral resolution instruments, such as PRISMA (Hyperspectral Precursor of the Application Mission), represent a valuable opportunity to improve the accuracy of τa550 retrievable from a remote-sensing system developing new atmospheric measurement techniques. This paper aims to address the potential of these new observing systems in more accurate retrieving τa550, specifically over land in heterogeneous and/or homogeneous areas composed by dark and bright targets. The study shows how the variation of the hyperspectral observed radiance can be addressed to recognise a variation of Δτa550 = 0.02. The goal has been achieved by using simulated radiances by combining two aerosol models (urban and continental) and two reflecting surfaces: dark (represented by water) and bright (represented by sand) for the PRISMA instrument, considering the environmental contribution of the observed radiance, i.e., the adjacency effect. Results showed that, in the continental regime, the expected instrument sensitivity would allow for retrieval accuracy of the aerosol optical thickness at 550 nm of 0.02 or better, with a dark surface surrounded by dark areas. The study also showed that for the urban regime, the surface plays a more significant role, with a bright surface surrounded by dark areas providing favourable conditions for the aerosol load retrievals, and dark surfaces representing less suitable situations for inversion independently of the surroundings. However, over all, the results obtained provide evidence that high resolution observations of Earth spectrum between 400 and 1000 nm would allow for a significant improvement of the accuracy of the τa550 for anthropogenic/natural aerosols over land.

Flux-calibrated stellar population models of Lick absorption-line indices with variable element abundance ratios

We present new stellar population models of Lick absorption-line indices with variable element abundance ratios. The models are based on our new calibrations of absorption-line indices with stellar parameters derived from the MILES stellar library. The key novelty compared to our previous models is that they are now available at the higher spectral resolution of MILES (~2.7A FWHM) and flux-calibrated, hence not tied anymore to the Lick/IDS system. This is essential for the interpretation of galaxy spectra where calibration stars are not available, such as large galaxy redshift surveys or other high-redshift observations. We note that the MILES resolution appears to be comparable to SDSS resolution, so that our models can be applied to SDSS data without any corrections for instrumental spectral resolution. For the first time we provide random errors for the model predictions based on the uncertainties in the calibration functions and the underlying stellar parameter estimates. We show that random errors are small except at the edges of the parameter space (high/low metallicities and young ages <1 Gyr) where the stellar library is under-sampled. We calibrate the base model for the parameters age, metallicity and alpha/Fe ratio with galactic globular cluster and galaxy gradient data. We discuss two model flavours with different input stellar evolutionary tracks from the Frascati and Padova groups. The new model release now includes abundance variations of the elements C, N, Mg, Na, Si, Ca, Ti, Cr, and Fe. The individual elements that are best accessible with these models and the standard set of Lick absorption features are C, N, Mg, Ca, Ti, and Fe. The model data is available at www.icg.port.ac.uk/~thomasd.

Investigating the influence of inversion methods in extrapolated magnetic fields of the solar corona

The solar magnetic field at coronal level is responsible for phenomena such as solar flares and coronal mass ejections. It is thus of interest to investigate the topological features that create the conditions for these events to occur. At the corona, the magnetic field is considered to be a force free field: r × E B = α E B . If α is a constant the force free field is linear and the differential equation has known solutions, such as the Fourier transform solution [1]. The boundary conditions are given by magnetograms, maps of the magnetic field at photospheric level. The magnetograms are produced from measurements of the Stokes parameters in spectral lines and application of inversion methods to derive the three components of the magnetic field. Apart from intrinsic issues (e.g. Zeeman saturation effect), the production of these maps must deal with other issues, such as noise and the instrument spectral and spatial resolution, and assumptions in the inversion method. We investigate what are the consequences for the extrapolated magnetic field due to boundary conditions determined from different sets of magnetograms. We aim to estimate the inaccuracies induced by the inversion codes in our method of extrapolation, comparing extrapolations from sets of data of two instruments: the Michelson Doppler Imager (MDI) on board SOHO observatory and the Spectro-Polarimeter (SP) of the Solar Optical Telescope (SOT) on board Hinode spacecraft. Thus, we evaluate the correlations and compare the topological features of the MDI and SP-SOT extrapolations.

Optical fiber-based setup for in vivo measurement of the delayed fluorescence lifetime of oxygen sensors

A new optical-fiber-based spectrofluorometer for in vivo or in vitro detection of delayed fluorescence is presented and characterized. This compact setup is designed so that it can be readily adapted for future clinical use. Optical excitation is done with a nitrogen laser-pumped, tunable dye laser, emitting in the UV-vis part of the spectrum. Excitation and luminescence signals are carried to and from the biological tissues under investigation, located out of the setup enclosure, by a single optical fiber. These measurements, as well as measurements performed without a fiber on in vitro samples in a thermostable quartz cell, in a controlled-atmosphere enclosure, are possible due to the efficient collection of the laser-induced luminescence light which is collected and focused on the detector with a high aperture parabolic mirror. The detection is based on a gated photomultiplier which allows for time-resolved measurements of the delayed fluorescence intensity. Thus, relevant luminescence lifetimes, typically in the sub-microsecond-to-millisecond range, can be measured with near total rejection of the sample's prompt fluorescence. The instrument spectral and temporal resolution, as well as its sensitivity, is characterized and measurement examples are presented. The primary application foreseen for this setup is the monitoring and adjustment of the light dose delivered during photodynamic therapy.

Spectrographs in amateur astronomy

This paper reviews the overall principles of astronomical spectrographs and the design rules that cover low, medium, and high spectral resolution instruments. Several examples of spectrograph designs are described that are easy to build and are optimised for amateur telescopes. Results are given for each of these instruments. Despite the modest size of amateur telescopes, we show that high performance spectrographs in the hands of amateur astronomers provide access to some specialised areas where the amateur could contribute to useful astrophysical work.

Infrared Spectroscopy of Calcium‐Aluminium‐rich Inclusions: Analog Material for Protoplanetary Dust?

We have performed infrared spectroscopy of meteoritic calcium-aluminum-rich inclusions (CAIs) from the CV chondrites Leoville and Allende. While some previous studies have focused on the wavelength range λ < 25 μm, we aimed at a detection and identification of CAI bands at wavelengths from 6 to 75 μm. The resulting spectra have been compared with laboratory mineral spectra of typical CAI constituents such as melilite (a solid solution of gehlenite [Ca2Al2SiO7] and akermanite [Ca2MgSi2O7]), spinel (MgAl2O4), diopside (CaMgSi2O6), and its Al- and Ti-rich variety fassaite. Sixteen bands in a type-B1 Leoville CAI transmittance spectrum could be assigned to melilite and spinel, 13 bands in the spectrum of a B3 Allende CAI were assigned to Al-Ti-diopside (fassaite), spinel, and nepheline; both cases were in accordance with the sample's respective mineralogical and chemical composition. CaO (lime) is not detected spectroscopically in the CAI spectra, as it is (contrary to previous notions) not even a trace component in CAIs, at least in their present (partially reprocessed) state. The astrophysical relevance of our study lies in the fact that the main CAI minerals such as melilite, Al-Ti-diopside (fassaite), spinel, and anorthite are expected to be present in protoplanetary disks. Hence, the detection of Ca,Al-minerals in protoplanetary disks should only be a matter of increased instrumental sensitivity, or spatial and spectral resolution, and quality (e.g., wavelength coverage) of comparative laboratory spectra.

High‐resolution terrestrial nightglow emission line atlas from UVES/VLT: Positions, intensities, and identifications for 2808 lines at 314–1043 nm

The atlas of terrestrial nightglow emission lines from spectra of the night sky obtained from the Ultraviolet and Visual Echelle Spectrograph (UVES) on the 8.2‐m UT2 telescope at the Very Large Telescope (VLT), European Southern Observatory, Cerro Paranal, Chile, consists of 2808 line positions, line widths, and intensities over the 314–1043 nm spectral range (Hanuschik, 2003). These lines have been absolute intensity calibrated and measured at a spectral resolution (λ/Δλ) of ∼43,000–45,000. Presented here are spectroscopic identifications for 98% of the lines in the atlas, made primarily through comparisons with synthetic spectra of prominent OH and O2 nightglow emission systems. The ability to simulate these systems successfully has shown that there are many additional lines that could be added to the atlas. We believe that all the O2 and OH lines in the measured region can now be successfully modeled with an accuracy better than the instrumental spectral resolution.

On-ground characterization of Rosetta/VIRTIS-M. I. Spectral and geometrical calibrations

The complete characterization of complex imaging spectrometers, such as VIRTIS-M (visual infrared thermal imaging spectrometer) aboard the Rosetta mission, requires a detailed and prolonged activity starting with the instrument integration and continuing during the entire operational life of the experiment. In this article we report the main experimental activities realized during the on-ground characterizations to evaluate the spectral and geometric performances in order to check the conformance with the technical requirements derived from the scientific goals of the experiment. Spectral calibrations allow to confirm instrumental spectral range, resolution, and sampling; geometric calibrations are necessary to estimate the pixel and slit functions, field of view extension, and possible optical aberrations. Two separate sections are dedicated to each one of these subjects, including the strategy followed to prepare measurements, a basic description of the on-ground experimental setups, and the analysis of the collected data.

Improving the Numerical Modeling of the OiResonance Triplet in the Solar Spectrum

We present a slight modification to an existing multilevel radiative transfer code that allows us to include the effects of frequency cross redistribution (XRD) between lines sharing the same upper level. With this improved code, we calculate theoretical profiles of the O I resonance triplet lines at 130 nm through a hydrostatic model of the average quiet Sun. The width of the calculated XRD profiles show good agreement with an observed, spatially averaged disk-center spectrum obtained with the high-resolution telescope spectrometer (HRTS) spectrograph. This is in stark contrast to profiles calculated with complete frequency redistribution (CRD) and ordinary partial frequency redistribution (PRD), which have Lorentzian wings that are too broad. We find deep central reversals, contrary to most observed profiles, but we note that this discrepancy is to a large degree the result of limited instrumental spectral resolution.

Mesures spectroscopiques de constituants et de polluants atmosphériques par techniques in situ et à distance, au sol ou embarquées

Infrared spectroscopy is a powerful tool for precise measurements of atmospheric trace species concentrations through the use of characteristic spectral signatures of the different molecular species and their associated vibration–rotation bands in the mid- or near-infrared. Different methods based on quantitative spectroscopy permit tropospheric or stratospheric measurements: in situ long path absorption, atmospheric absorption/emission by Fourier transform spectroscopy with high spectral resolution instruments on the ground, airborne, balloon-borne or satellite-borne.

Design considerations for fiber-coupled streaked optical spectroscopy

Optical fibers are used to couple light into fast time-resolved spectrometers for many applications. Here several issues arise that can seriously limit the temporal resolution, such as the wavelength dependent group delay, and intermodal dispersion. These issues are investigated in 10 m length fibers using either a 400 μm core step index or graded index fiber. A unique broadband (400–700 nm) short pulse (∼1 ps) light source is used to measure the fiber group delay with a time-resolved spectrometer. Intermodal dispersion is also studied in these fibers using a narrow-band 1 ps pulse that is injected into the fiber with various coupling schemes, and a novel technique is employed to record the time-dependent angular mode structure. Finally, a conceptual design using ∼100 μm core graded-index fibers is proposed for multichannel streaked optical spectroscopy on the National Ignition Facility. Designs appropriate for a low spectral resolution instrument (stimulated Raman backscattering) and a high spectral resolution instrument (stimulated Brillouin backscattering) are presented. The fiber dispersion issues are discussed in the design of these diagnostics.

Non-LTE radiative transfer in the 4.7 and 2.3 μm bands of CO: Vibration-rotational non-LTE and its effects on limb radiance

Vibrational temperatures and rotational distributions for the two lowest vibrational levels of CO molecules are calculated for day and night conditions for the altitude region between 0 and 150 km. The effects of vibrational and rotational non-LTE on the limb radiance distribution in the 4.7 μm fundamental and hot bands as well as in the 2.3 μm first overtone band are analyzed and implications for remote sensing with high spectral resolution instruments are discussed.

Rotational Raman scattering (Ring effect) in satellite backscatter ultraviolet measurements

A detailed radiative transfer calculation has been carried out to estimate the effects of rotational Raman scattering (RRS) on satellite measurements of backscattered ultraviolet radiation. Raman-scattered light is shifted in frequency from the incident light, which causes filling in of solar Fraunhofer lines in the observed backscattered spectrum (also known as the Ring effect). The magnitude of the rotational Raman scattering filling in is a function of wavelength, solar zenith angle, surface reflectance, surface pressure, and instrument spectral resolution. The filling in predicted by our model is found to be in agreement with observations from the Shuttle Solar Backscatter Ultraviolet Radiometer and the Nimbus-7 Solar Backscatter Ultraviolet Radiometer.

Revised, fast, flexible algorithms for determination of electron number densities in plasma discharges

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hard copy text is accompanied by a disk with two electron number density ( n e) calculation programs written in Turbo Basic (NNE.EXE) and C language (NE.EXE), data mes ( ∗.DAT), a text file (NE.WP) with the hard copy paper in WordPerfect 5.1 and a text file (README) for the introduction of the data files and programs. The theoretical aspects and applications of NNE.EXE and NE.EXE are described in the main article for three plasmas having a wide range of electron number density (4 × 10 13 to 1 × 10 17 cm −3). The descriptions of the procedure, data format, and hardware requirements can be found in the Appendix and the README file. The determination of n e. is accomplished by least-squares fitting of the entire emission profile or the wing portions of the emission profile of the H β line (486.13 nm) to the theoretical Stark broadened profiles, compiled at electron temperature 2500, 5000, 10000 and 20000 K. This approach can reduce errors in the determination of n e owing to the structural dip in the center of the Stark profiles depending on n e value. The curve-fitting routine employs an interval-halving algorithm to produce new interpolated Stark profiles, reduces number of iterations and time required for calculation, and allows the calculation of n e to the limits set by instrument spectral resolution and the availability of theoretical spectral data. Additionally, each program provides graphic displays allowing the reader to observe the interpolated Stark profiles, the fitting process, and magnitude of residuals during the course of calculation, and the effects of plasma temperatures and spectral resolution on the estimation of n e. In addition to H β, theoretical profiles also are included for H α and H γ to illustrate the suitability and drawbacks of these lines for n e estimation. These programs are extended versions of a program reported earlier in Spectrochimica Acta 44B, 175 (1989).

Quantitative Analysis of Mixed Volatile Fluids by Raman Microprobe Spectroscopy: A Cautionary Note on Spectral Resolution and Peak Shape

Raman analyses of fluid inclusions can yield quantitative information on composition (from peak areas and heights) and density (from peak position and width). In this study, we examine the effect of instrumental spectral resolution on the ratios of these spectral parameters, and the selection of appropriate integration limits for the determination of peak areas in the CO2-CH4-N2 system. Spectral resolution was varied from about 1 to 9 cm−1 by co-varying the widths of all spectrometer slits. Changes in resolution produced a modest effect on peak-area ratios and a significant effect on peak-height ratios. Measured peak-width ratios varied strongly as a function of the spectral resolution. In addition, we observed a moderate shift in the measured peak position of N2, which can be related to the asymmetry of the band. These results indicate that accurate analysis requires careful attention to the selection of quantification factors, especially if the selected values were derived from studies at different spectral resolutions. Another factor that can have a significant effect on the calculated compositions of CH4- and H2-bearing fluid mixtures is the band broadening that occurs with increasing pressure.

Non-linear Raman spectroscopy in gases

Recent progress in high-resolution non-linear coherent Raman spectroscopy in the gas phase is reviewed. An instrumental spectral resolution of less than 100 MHz can be routinely achieved. It is limited by the convoluted linewidths of the lasers used for excitation and is of particular advantage in the gas phase, where the rotational structure of vibrational bands is to be resolved. It also allows the measurement of linewidths and line positions with an accuracy of 60 MHz when an appropriate wavemeter is used for calibration. After a short overview of the various techniques being used, examples of recent results of the evaluation of measurements on nitrogen, oxygen, carbon dioxide and methane will be presented.