Durham extremely large telescope adaptive optics simulation platform

Adaptive optics systems are essential on all large telescopes for which image quality is important. These are complex systems with many design parameters requiring optimization before good performance can be achieved. The simulation of adaptive optics systems is therefore necessary to categorize the expected performance. We describe an adaptive optics simulation platform, developed at Durham University, which can be used to simulate adaptive optics systems on the largest proposed future extremely large telescopes as well as on current systems. This platform is modular, object oriented, and has the benefit of hardware application acceleration that can be used to improve the simulation performance, essential for ensuring that the run time of a given simulation is acceptable. The simulation platform described here can be highly parallelized using parallelization techniques suited for adaptive optics simulation, while still offering the user complete control while the simulation is running. The results from the simulation of a ground layer adaptive optics system are provided as an example to demonstrate the flexibility of this simulation platform

Atmospheric scintillation in astronomical photometry

Scintillation noise due to the Earth's turbulent atmosphere can be a dominant noise source in high-precision astronomical photometry when observing bright targets from the ground. Here we describe the phenomenon of scintillation from its physical origins to its effect on photometry. We show that Young's scintillation-noise approximation used by many astronomers tends to underestimate the median scintillation noise at several major observatories around the world. We show that using median atmospheric optical turbulence profiles, which are now available for most sites, provides a better estimate of the expected scintillation noise and that real-time turbulence profiles can be used to precisely characterize the scintillation-noise component of contemporaneous photometric measurements. This will enable a better understanding and calibration of photometric noise sources and the effectiveness of scintillation correction techniques. We also provide new equations for calculating scintillation noise, including for extremely large telescopes where the scintillation noise will actually be lower than previously thought. These equations highlight the fact that scintillation noise and shot noise have the same dependence on exposure time and so if an observation is scintillation limited, it will be scintillation limited for all exposure times. The ratio of scintillation noise to shot noise is also only weakly dependent on telescope diameter and so a bigger telescope may not yield a reduction in fractional scintillation noise

Defect configurations and dynamical behavior in a Gay-Berne nematic emulsion

To model a nematic emulsion consisting of a surfactant-coated water droplet dispersed in a nematic host, we performed a molecular dynamics simulation of a droplet immersed in a system of 2048 Gay-Berne ellipsoids in a nematic phase. Strong radial anchoring at the surface of the droplet induced a Saturn ring defect configuration, consistent with theoretical predictions for very small droplets. A surface ring configuration was observed for lower radial anchoring strengths, and a pair of point defects was found near the poles of the droplet for tangential anchoring. We also simulated the falling ball experiment and measured the drag force anisotropy, in the presence of strong radial anchoring as well as zero anchoring strength.Comment: 17 pages, 15 figure

Health utility after emergency medical admission: a cross-sectional survey

Objectives: Health utility combines health related quality of life and mortality to produce a generic outcome measure reflecting both morbidity and mortality. It has not been widely used as an outcome measure in evaluations of emergency care and little is known about the feasibility of measurement, typical values obtained or baseline factors that predict health utility. We aimed to measure health utility after emergency medical admission, to compare health utility to age, gender and regional population norms, and identify independent predictors of health utility. Methods: We selected 5760 patients across three hospitals who were admitted to hospital by ambulance as a medical emergency. The EQ-5D questionnaire was mailed to all who were still alive 30 days after admission. Health utility was estimated by applying tariff values to the EQ-5D responses or imputing a value of zero for those who had died. Multivariable analysis was used to identify independent predictors of health utility at 30 days. Results: Responses were received from 2488 (47.7%) patients, while 541 (9.4%) had died. Most respondents reported some or severe problems with each aspect of health. Mean health utility was 0.49 (standard deviation 0.35) in survivors and 0.45 (0.36) including non-survivors. Some 75% had health utility below their expected value (mean loss 0.32, 95% confidence interval 0.31 to 0.33) and 11% had health utility below zero (worse than death). On multivariable modelling, reduced health utility was associated with increased age and lower GCS, varied according to ICD10 code and was lower among females, patients with recent hospital admission, steroid therapy, or history of chronic respiratory disease, malignancy, diabetes or epilepsy. Conclusions: Health utility can be measured after emergency medical admission, although responder bias may be significant. Health utility after emergency medical admission is poor compared to population norms. We have identified independent predictors or health utility that need to be measured and taken into account in nonrandomized evaluations of emergency care

A Shack-Hartmann wavefront sensor projected on to the sky with reduced focal anisoplanatism

A method for producing a laser guide star wavefront sensor for adaptive optics with reduced focal anisoplanatism is presented. A theoretical analysis and numerical simulations have been carried out and the results are presented. The technique, named Sky-Projected Laser Array Shack–Hartmann (SPLASH), is shown to suffer considerably less from focal anisoplanatism than a conventional laser guide star system. The method is potentially suitable for large telescope apertures (8 m), and possibly for extremely large telescopes

Evolution of Heat Flow, Hydrothermal Circulation and Permeability on the Young Southern Flank of the Costa Rica Rift

We analyze 67 new conductive heat flow measurements on the southern flank of the Costa Rica Rift (CRR). Heat flow measurements cover five sites ranging in oceanic crustal age between approximately 1.6 and 5.7 Ma, and are co-located with a high-resolution multi-channel seismic line that extends from slightly north of the first heat flow site (1.6 Ma) to beyond ODP Hole 504B in 6.9 Ma crust. For the five heat flow sites, the mean observed conductive heat flow is ≈ 85 mWm−2. This value is approximately 30 per cent of the mean lithospheric heat flux expected from a half-space conductive cooling model, indicating that hydrothermal processes account for about 70 per cent of the heat loss. The advective heat loss fraction varies from site to site and is explained by a combination of outcrop to outcrop circulation through exposed basement outcrops and discharge through faults. Super-critical convection in Layer 2A extrusives occurs between 1.6 and 3.5 Ma, and flow through a thinly-sedimented basement high occurs at 4.6 Ma. Advective heat loss diminishes rapidly between ≈ 4.5 and ≈ 5.7 Ma, which contrasts with plate cooling reference models that predict a significant deficit in conductive heat flow up to ages ≈ 65 ± 10 Ma. At ≈ 5.7 Ma the CRR topography is buried under sediment with an average thickness ≈ 150 m, and hydrothermal circulation in the basement becomes sub-critical or perhaps marginally critical. The absence of significant advective heat loss at ≈ 5.7 Ma at the CRR is thus a function of both burial of basement exposure under the sediment load and a reduction in basement permeability that possibly occurs as result of mineral precipitation and original permeability at the time of formation. Permeability is a non-monotonic function of age along the southern flank of the CRR, in general agreement with seismic velocity tomography interpretations that reflect variations in the degree of ridge-axis magma supply and tectonic extension. Hydrothermal circulation in the young oceanic crust at southern flank of CRR is affected by the interplay and complex interconnectedness of variations in permeability, sediment thickness, topographical structure, and tectonic and magmatic activities with age

Stereo-SCIDAR: optical turbulence profiling with high sensitivity using a modified SCIDAR instrument

The next generation of adaptive optics systems will require tomographic reconstruction techniques to map the optical refractive index fluctuations, generated by the atmospheric turbulence, along the line of sight to the astronomical target. These systems can be enhanced with data from an external atmospheric profiler. This is important for Extremely Large Telescope scale tomography. Here we propose a new instrument which utilizes the generalized Scintillation Detection And Ranging (SCIDAR) technique to allow high sensitivity vertical profiles of the atmospheric optical turbulence and wind velocity profile above astronomical observatories. The new approach, which we refer to as ‘stereo-SCIDAR’, uses a stereoscopic system with the scintillation pattern from each star of a double-star target incident on a separate detector. Separating the pupil images for each star has several advantages including increased magnitude difference tolerance for the target stars; negating the need for re-calibration due to the normalization errors usually associated with SCIDAR; an increase of at least a factor of 2 in the signal-to-noise ratio of the cross-covariance function and hence the profile for equal magnitude target stars and up to a factor of 16 improvement for targets of 3 mag difference and easier real-time reconstruction of the wind-velocity profile. Theoretical response functions are calculated for the instrument, and the performance is investigated using a Monte Carlo simulation. The technique is demonstrated using data recorded at the 2.5-m Nordic Optical Telescope and the 1.0-m Jacobus Kapteyn Telescope, both on La Palma

Path Integral Variational Methods for Strongly Correlated Systems

We introduce a new approach to highly correlated systems which generalizes the Fermi Hypernetted Chain and Correlated Basis Function techniques. While the latter approaches can only be applied to systems for which a nonrelativistic wave function can be defined, the new approach is based on the variation of a trial hamiltonian within a path integral framework and thus can also be applied to relativistic and field theoretical problems. We derive a diagrammatic scheme for the new approach and show how a particular choice of the trial hamiltonian corresponds exactly to the use of a Jastrow correlated ansatz for the wave function in the Fermi Hypernetted Chain approach. We show how our new approach can be used to find upper bounds to ground state energies in systems which the FHNC cannot handle, including those described by an energy-dependent effective hamiltonian. We demonstrate our approach by applying it to a quantum field theoretical system of interacting pions and nucleons.Comment: 35 RevTeX pages, 7 separated ps figures available on reques

On the Red-Green-Blue Model

We experimentally study the red-green-blue model, which is a sytem of loops obtained by superimposing three dimer coverings on offset hexagonal lattices. We find that when the boundary conditions are ``flat'', the red-green-blue loops are closely related to SLE_4 and double-dimer loops, which are the loops formed by superimposing two dimer coverings of the cartesian lattice. But we also find that the red-green-blue loops are more tightly nested than the double-dimer loops. We also investigate the 2D minimum spanning tree, and find that it is not conformally invariant.Comment: 4 pages, 7 figure