33 research outputs found
An Imaging Fabry-Perot System for the Robert Stobie Spectrograph on the Southern African Large Telescope
We present the design of the Fabry-Perot system of the Robert Stobie
Spectrograph on the 10-meter class Southern African Large Telescope and its
characterization as measured in the laboratory. This system provides
spectroscopic imaging at any desired wavelength spanning a bandpass 430 - 860
nm, at four different spectral resolving powers ranging from 300 to 9000. Our
laboratory tests revealed a wavelength dependence of the etalon gap and
parallelism with a maximum variation between 600 - 720 nm that arises because
of the complex structure of the broadband multi-layer dielectric coatings. We
also report an unanticipated optical effect of this multi-layer coating
structure that produces a significant, and wavelength dependent, change in the
apparent shape of the etalon plates. This change is caused by two effects: the
physical non-uniformities or thickness variations in the coating layers, and
the wavelength dependence of the phase change upon refection that can amplify
these non-uniformities. We discuss the impact of these coating effects on the
resolving power, finesse, and throughput of the system. This Fabry-Perot system
will provide a powerful tool for imaging spectroscopy on one of the world's
largest telescopes.Comment: 17 pages, 14 figures, accepted for publication in The Astronomical
Journa
Rayleigh Imaging of Graphene and Graphene Layers
We investigate graphene and graphene layers on different substrates by
monochromatic and white-light confocal Rayleigh scattering microscopy. The
image contrast depends sensitively on the dielectric properties of the sample
as well as the substrate geometry and can be described quantitatively using the
complex refractive index of bulk graphite. For few layers (<6) the
monochromatic contrast increases linearly with thickness: the samples behave as
a superposition of single sheets which act as independent two dimensional
electron gases. Thus, Rayleigh imaging is a general, simple and quick tool to
identify graphene layers, that is readily combined with Raman scattering, which
provides structural identification.Comment: 8 pages, 9 figure
Refractive Index of Humid Air in the Infrared: Model Fits
The theory of summation of electromagnetic line transitions is used to
tabulate the Taylor expansion of the refractive index of humid air over the
basic independent parameters (temperature, pressure, humidity, wavelength) in
five separate infrared regions from the H to the Q band at a fixed percentage
of Carbon Dioxide. These are least-squares fits to raw, highly resolved spectra
for a set of temperatures from 10 to 25 C, a set of pressures from 500 to 1023
hPa, and a set of relative humidities from 5 to 60%. These choices reflect the
prospective application to characterize ambient air at mountain altitudes of
astronomical telescopes.Comment: Corrected exponents of c0ref, c1ref and c1p in Table
\emph{In-situ} determination of astro-comb calibrator lines to better than 10 cm s
Improved wavelength calibrators for high-resolution astrophysical
spectrographs will be essential for precision radial velocity (RV) detection of
Earth-like exoplanets and direct observation of cosmological deceleration. The
astro-comb is a combination of an octave-spanning femtosecond laser frequency
comb and a Fabry-P\'erot cavity used to achieve calibrator line spacings that
can be resolved by an astrophysical spectrograph. Systematic spectral shifts
associated with the cavity can be 0.1-1 MHz, corresponding to RV errors of
10-100 cm/s, due to the dispersive properties of the cavity mirrors over broad
spectral widths. Although these systematic shifts are very stable, their
correction is crucial to high accuracy astrophysical spectroscopy. Here, we
demonstrate an \emph{in-situ} technique to determine the systematic shifts of
astro-comb lines due to finite Fabry-P\'erot cavity dispersion. The technique
is practical for implementation at a telescope-based spectrograph to enable
wavelength calibration accuracy better than 10 cm/s.Comment: 11 pages, 7 figure
On the influence of Si:Al ratio and hierarchical porosity of FAU zeolites in solid acid catalysed esterification pretreatment of bio-oil
A family of faujasite (FAU) zeolites with different Si:Al ratio, and/or hierarchical porosity introduced via post-synthetic alkaline desilication treatment, have been evaluated as solid acid catalysts for esterification pretreatments of pyrolysis bio-oil components. Acetic acid esterification with aliphatic and aromatic alcohols including methanol, anisyl alcohol, benzyl alcohol, p-cresol and n-butanol was first selected as a model reaction to identify the optimum zeolite properties. Materials were fully characterised using N2 porosimetry, ICP, XRD, XPS, FT-IR, pyridine adsorption, NH3 TPD, In-situ ATR and inverse gas chromatography (IGC). IGC demonstrates that the surface polarity and hence hydrophobicity of FAU decreases with increased Si:Al ratio. Despite possessing a higher acid site loading and acetic acid adsorption capacity, high Al-content FAU possess weaker acidity than more siliceous catalysts. Esterification activity increases with acid strength and decreasing surface polarity following the order FAU30>FAU6>FAU2.6. The introduction of mesoporosity through synthesis of a hierarchical HFAU30 material further enhances esterification activity through improved acid site accessibility and hydrophobicity. Methanol was the most reactive alcohol for esterification, and evaluated with HFAU30 for the pretreatment of a real pyrolysis bio-oil, reducing the acid content by 76% under mild conditions
Description of Atmospheric Conditions at the Pierre Auger Observatory using the Global Data Assimilation System (GDAS)
Atmospheric conditions at the site of a cosmic ray observatory must be known
for reconstructing observed extensive air showers. The Global Data Assimilation
System (GDAS) is a global atmospheric model predicated on meteorological
measurements and numerical weather predictions. GDAS provides
altitude-dependent profiles of the main state variables of the atmosphere like
temperature, pressure, and humidity. The original data and their application to
the air shower reconstruction of the Pierre Auger Observatory are described. By
comparisons with radiosonde and weather station measurements obtained on-site
in Malarg\"ue and averaged monthly models, the utility of the GDAS data is
shown
Hydrogen storage capacity of selected activated carbon electrodes made from brown coal
Electrochemical storage of hydrogen in activated carbon (aC) electrodes as part of a reversible fuel cell offers a potentially attractive option for storing surplus electrical energy from inherently variable solar and wind energy resources. Such a system - which we have called a proton flow battery - promises to have a roundtrip energy efficiency comparable to lithium ion batteries, while having higher gravimetric and volumetric energy densities. Activated carbons with high internal surface area, high pore volume, light weight and easy availability have attracted considerable research interest as a solid-state hydrogen storage medium. This paper compares the physical characteristics and hydrogen storage capacities of four activated carbon (aC) electrodes made from brown coal. The fabrication methods for these samples are explained. Their proton conductivity was measured using electrochemical impedance spectroscopy and their hydrogen storage capacity by galvanostatic charging and discharging in a three-electrode electrolytic cell with 1 mol sulphuric acid as electrolyte at atmospheric pressure and room temperature. The highest hydrogen storage capacity obtained was 1.29 wt%, which compares favourably with metal hydrides used in commercially available solid-state hydrogen storages. Finally, the relation between the hydrogen storage capacity of the samples and their Dubinin-Radushkevich surface area (calculated by the CO2 adsorption method) was investigated. The results point the way towards selecting high-performing electrodes for proton flow batteries and signal the potential competitiveness of this energy storage technology