Design and implementation of sparse aperture imaging systems

In order to better understand the technological difficulties involved in designing and building a sparse aperture array, the challenge of building a white light Golay-3 telescope was undertaken. The MIT Adaptive Reconnaissance Golay-3 Optical Satellite (ARGOS) project exploits wide-angle Fizeau interferometer technology with an emphasis on modularity in the optics and spacecraft subsystems. Unique design procedures encompassing the nature of coherent wavefront sensing, control and combining as well as various system engineering aspects to achieve cost effectiveness, are developed. To demonstrate a complete spacecraft in a 1-g environment, the ARGOS system is mounted on a frictionless air-bearing, and has the ability to track fast orbiting satellites like the ISS or the planets. Wavefront sensing techniques are explored to mitigate initial misalignment and to feed back real-time aberrations into the optical control loop. This paper presents the results and the lessons learned from the conceive, design and implementation phases of ARGOS. A preliminary assess-ment shows that the beam combining problem is the most challenging aspect of sparse optical arrays. The need for optical control is paramount due to tight beam combining tolerances. The wavefront sensing/control requirements appear to be a major technology and cost driver

ARGOS testbed: study of multidisciplinary challenges of future spaceborne interferometric arrays

Future spaceborne interferometric arrays must meet stringent optical performance and tolerance requirements while exhibiting modularity and acceptable manufacture and integration cost levels. The Massachusetts Institute of Technology (MIT) Adaptive Reconnaissance Golay-3 Optical Satellite (ARGOS) is a wide-angle Fizeau interferometer spacecraft testbed designed to address these research challenges. Designing a space-based stellar interferometer, which requires tight tolerances on pointing and alignment for its apertures, presents unique multidisciplinary challenges in the areas of structural dynamics, controls, and multiaperture phasing active optics. In meeting these challenges, emphasis is placed on modularity in spacecraft subsystems and optics as a means of enabling expandability and upgradeability. A rigorous theory of beam-combining errors for sparse optical arrays is derived and flown down to the design of various subsystems. A detailed elaboration on the optics system and control system is presented based on the performance requirements and beam-combining error tolerances. The space environment is simulated by floating ARGOS on a frictionless air-bearing that enables it to track both fast and slow moving targets

A Framework for Orbital Performance Evaluation in Distributed Space Missions for Earth Observation

Distributed Space Missions (DSMs) are gaining momentum in their application to earth science missions owing to their unique ability to increase observation sampling in spatial, spectral and temporal dimensions simultaneously. DSM architectures have a large number of design variables and since they are expected to increase mission flexibility, scalability, evolvability and robustness, their design is a complex problem with many variables and objectives affecting performance. There are very few open-access tools available to explore the tradespace of variables which allow performance assessment and are easy to plug into science goals, and therefore select the most optimal design. This paper presents a software tool developed on the MATLAB engine interfacing with STK, for DSM orbit design and selection. It is capable of generating thousands of homogeneous constellation or formation flight architectures based on pre-defined design variable ranges and sizing those architectures in terms of predefined performance metrics. The metrics can be input into observing system simulation experiments, as available from the science teams, allowing dynamic coupling of science and engineering designs. Design variables include but are not restricted to constellation type, formation flight type, FOV of instrument, altitude and inclination of chief orbits, differential orbital elements, leader satellites, latitudes or regions of interest, planes and satellite numbers. Intermediate performance metrics include angular coverage, number of accesses, revisit coverage, access deterioration over time at every point of the Earth's grid. The orbit design process can be streamlined and variables more bounded along the way, owing to the availability of low fidelity and low complexity models such as corrected HCW equations up to high precision STK models with J2 and drag. The tool can thus help any scientist or program manager select pre-Phase A, Pareto optimal DSM designs for a variety of science goals without having to delve into the details of the engineering design process

Characterization of a CCD array for Bragg spectroscopy

The average pixel distance as well as the relative orientation of an array of 6 CCD detectors have been measured with accuracies of about 0.5 nm and 50 $\mu$rad, respectively. Such a precision satisfies the needs of modern crystal spectroscopy experiments in the field of exotic atoms and highly charged ions. Two different measurements have been performed by illuminating masks in front of the detector array by remote sources of radiation. In one case, an aluminum mask was irradiated with X-rays and in a second attempt, a nanometric quartz wafer was illuminated by a light bulb. Both methods gave consistent results with a smaller error for the optical method. In addition, the thermal expansion of the CCD detectors was characterized between -105 C and -40 C.Comment: Submitted to Review of Scientific Instrument

Rapid magma generation or shared magmatic reservoir? Petrology and geochronology of the Rat Creek and Nelson Mountain Tuffs, CO, USA

This study was supported by the ETH Research Grant ETH-34 15-2 (JS).Large-volume silicic volcanism poses global hazards in the form of proximal pyroclastic density currents, distal ash fall and short-term climate perturbations, which altogether warrant the study of how silicic magma bodies evolve and assemble. The southern rocky mountain volcanic field (SRMVF) is home to some of the largest super-eruptions in the geological record, and has been studied to help address the debate over how quickly eruptible magma batches can be assembled–whether in decades to centuries, or more slowly over 100’s of kyr. The present study focuses on the San Luis caldera complex within the SRMVF, and discusses the paradigms of rapid magma generation vs. rapid magma assembly. The caldera complex consists of three overlapping calderas that overlie the sources of three large-volume mid-Cenozoic ignimbrites: first, the Rat Creek Tuff (RCT; zoned dacite-rhyolite, 150 km3), followed by the Cebolla Creek Tuff (mafic dacite, 250 km3) and finally, the Nelson Mountain Tuff (NMT; zoned dacite-rhyolite, 500 km3), which are all indistinguishable in age by 40Ar/39Ar dating. We argue for a shared magmatic history for the three units on the basis of (1) similar mineral trace element compositions in the first and last eruptions (plagioclase, sanidine, biotite, pyroxene, amphibole, titanite, and zircon), (2) overlapping zircon U-Pb ages in all three units, and (3) similar thermal rejuvenation signatures visible in biotite (low-Mn, high-Ba) and zircon (low-Hf, low-U) geochemistry within the RCT and NMT. It is postulated that the NMT was sourced from a pre-existing magma reservoir to the northeast, which is corroborated by the formation of the nearby Cochetopa Caldera during the eruption of the NMT. The inferred lateral magma transport has two important implications: (1) it demonstrates long-distance transport of highly viscosity magmas at volumes (100’s of km3) not previously recorded, and (2) the sourcing of magma from a nearby pre-existing magma reservoir greatly reduces the rate of magma generation necessary to explain the close coincidence of three overlapping, large-volume magma systems. Additionally, the concept of magmatic “flux” (km3 kyr−1) is discussed in this context, and it is argued that an area-normalized flux (km3 kyr–1 km−2) provides a more useful number for measuring magma production rates: it is expected that magmatic volumes will scale with footprint of the thermal anomaly, and not taking this into account may lead to errant volumetric flux (km3 kyr−1) estimates. Meanwhile, area-normalized flux estimates in a given area are similar between units, consistent with evolution in a relatively constant thermal regime. Such estimates also demonstrate similar fluxes for ∼cogenetic volcanic and plutonic units.Publisher PDFPeer reviewe

Observation of quantum depletion in a nonequilibrium exciton-polariton condensate

The property of superfluidity, first discovered in liquid 4He, is closely related to Bose-Einstein condensation (BEC) of interacting bosons. However, even at zero temperature, when one would expect the whole bosonic quantum liquid to become condensed, a fraction of it is excited into higher momentum states via interparticle interactions and quantum fluctuations -- the phenomenon of quantum depletion. Quantum depletion of weakly interacting atomic BECs in thermal equilibrium is well understood theoretically but is difficult to measure. This is even more challenging in driven-dissipative systems such as exciton-polariton condensates(photons coupled to electron-hole pairs in a semiconductor), since their nonequilibrium nature is predicted to suppress quantum depletion. Here, we observe quantum depletion of an optically trapped high-density exciton-polariton condensate by directly detecting the spectral branch of elementary excitations populated by this process. Analysis of the population of this branch in momentum space shows that quantum depletion of an exciton-polariton condensate can closely follow or strongly deviate from the equilibrium Bogoliubov theory, depending on the fraction of matter (exciton) in an exciton-polariton. Our results reveal the effects of exciton-polariton interactions beyond the mean-field description and call for a deeper understanding of the relationship between equilibrium and nonequilibrium BECs.Comment: 18 pages, 5 figures, with supplementary informatio

Loss of Sunda clouded leopards and forest integrity drive potential impacts of mesopredator release on vulnerable avifauna

Amongst the unintended consequences of anthropogenic landscape conversion is declining apex predator abundance linked to loss of forest integrity, which can potentially re-order trophic networks. One such re-ordering, known as mesopredator release, occurs when medium-sized predators, also called mesopredators, rapidly increase in abundance following the decline in apex predator abundance, consequently reducing the abundance of mesopredator prey, notably including terrestrial avifauna. We examine the cascading impacts of declining Sunda clouded leopard abundance, itself consequent upon a reduction in forest integrity, on the mesopredator community of Sabah, Malaysia, to determine whether the phenomenon of mesopredator release is manifest and specifically whether it impacts the terrestrial avifauna community of pheasants and pittas. To explore this trophic interaction, we used a piecewise structural equation model to compare changes in the relative abundance of organisms. Our results suggest that loss of forest integrity may have broad impacts on the community and trigger mesopredator release, the two acting additively in their impact on already vulnerable species of terrestrial avifauna: a result not previously documented in tropical systems and rarely detected even on a global scale. The limiting effect that the Sunda clouded leopard has on the Sunda leopard cat could illuminate the mechanism whereby mesopredator release impacts this system. Both Bulwer’s pheasant and pittas appear to be significantly impacted by the increase in Sunda leopard cats, while the great argus pheasant shows similar compelling, although not statistically significant, declines as Sunda leopard cats increase. The inverse relationship between Sunda clouded leopards and Sunda leopard cats suggests that if a mesopredator release exists it could have downstream consequences for some terrestrial avifauna. These results suggest the under-studied interface between mammalian carnivores and avifauna, or more broadly species interactions in general, could offer important conservation tool for holistic ecosystem conservation efforts. Forest integrity Mesopredator release Bird Structural equation model Trophic cascade Oil palm plantationspublishedVersio

Shadow of a Colossus: A z=2.45 Galaxy Protocluster Detected in 3D Ly-a Forest Tomographic Mapping of the COSMOS Field

Using moderate-resolution optical spectra from 58 background Lyman-break galaxies and quasars at $z\sim 2.3-3$ within a $11.5'\times13.5'$ area of the COSMOS field ($\sim 1200\,\mathrm{deg}^2$ projected area density or $\sim 2.4\,h^{-1}\,\mathrm{Mpc}$ mean transverse separation), we reconstruct a 3D tomographic map of the foreground Ly$\alpha$ forest absorption at $2.2<z<2.5$ with an effective smoothing scale of $\sigma_{3d}\approx3.5\,h^{-1}\,\mathrm{Mpc}$ comoving. Comparing with 61 coeval galaxies with spectroscopic redshifts in the same volume, we find that the galaxy positions are clearly biased towards regions with enhanced IGM absorption in the tomographic map. We find an extended IGM overdensity with deep absorption troughs at $z=2.45$ associated with a recently-discovered galaxy protocluster at the same redshift. Based on simulations matched to our data, we estimate the enclosed dark matter mass within this IGM overdensity to be $M_{\rm dm} (z=2.45) = (9\pm4)\times 10^{13}\,h^{-1}\,\mathrm{M_\odot}$, and argue based on this mass and absorption strength that it will form at least one $z\sim0$ galaxy cluster with $M(z=0) = (3\pm 2) \times 10^{14}\,h^{-1}\mathrm{M_\odot}$, although its elongated nature suggests that it will likely collapse into two separate clusters. We also point out a compact overdensity of six MOSDEF galaxies at $z=2.30$ within a $r\sim 1\,h^{-1}\,\mathrm{Mpc}$ radius and $\Delta z\sim 0.006$, which does not appear to have a large associated IGM overdensity. These results demonstrate the potential of Ly$\alpha$ forest tomography on larger volumes to study galaxy properties as a function of environment, as well as revealing the large-scale IGM overdensities associated with protoclusters and other features of large-scale structure.Comment: To be submitted to ApJ. Figure 3 can be viewed on Youtube: https://youtu.be/KeW1UJOPMY

Spiro-based thermally activated delayed fluorescence emitters with reduced nonradiative decay for high-quantum-efficiency, low-roll-off, organic light-emitting diodes

E.Z.-C. thanks the Leverhulme Trust (No. RPG-2016-047) and the University of St Andrews for support. The authors are grateful to the EPSRC for financial support (grants EP/ P007805/1, EP/P010482/1, EP/L017008/1, EP/J01771X, and EP/J00916). M.M. thanks the Innovation Programme H2020-MSCA-IF-2014-659237 for financial support. W.L. thanks the China Scholarship Council (grant number 201708060003). V.L. thanks the F.R.S.-FNRS for his Research Associate position. Computational resources have been provided by the Consortium des É quipements de Calcul Intensif (CÉ CI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.FNRS) under Grant No. 2.5020.11, GEQ U.G006.15, 1610468, and RW/GEQ. (2016). D.B. is an FNRS Research Director.Herein, we report the use of spiro-configured fluorene-xanthene scaffolds as a novel, promising, and effective strategy in thermally activated delayed fluorescence (TADF) emitter design to attain high photoluminescence quantum yields (ΦPL), short delayed luminescence lifetime, high external quantum efficiency (EQE), and minimum efficiency roll-off characteristics in organic light-emitting diodes (OLEDs). The optoelectronic and electroluminescence properties of SFX (spiro-(fluorene-9,9′-xanthene))-based emitters (SFX-PO-DPA, SFX-PO-DPA-Me, and SFX-PO-DPA-OMe) were investigated both theoretically and experimentally. All three emitters exhibited sky blue to green emission enabled by a Herzberg–Teller mechanism in the excited state. They possess short excited-state delayed lifetimes (<10 μs), high photoluminescence quantum yields (ΦPL ∼ 70%), and small singlet–triplet splitting energies (ΔEST < 0.10 eV) in the doped films in an mCP host matrix. The OLEDs showed some of the highest EQEs using spiro-containing emitters where maximum external quantum efficiencies (EQEmax) of 11 and 16% were obtained for devices using SFX-PO-DPA and SFX-PO-DPA-OMe, respectively. Further, a record EQEmax of 23% for a spiro-based emitter coupled with a low efficiency roll-off (19% at 100 cd m–2) was attained with SFX-PO-DPA-Me.PostprintPostprintPeer reviewe

Preladenant as an Adjunctive Therapy With Levodopa in Parkinson Disease

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