arXiv subjects

James Osborn

Publications and source records attributed to James Osborn.

27 records · Page 2Linked to original sources

arXiv2014

Scintillation correction for astronomical photometry on large and extremely large telescopes with tomographic atmospheric reconstruction

We describe a new concept to correct for scintillation noise on high-precision photometry in large and extremely large telescopes using telemetry data from adaptive optics (AO) systems. Most wide-field AO systems designed for the current era of very large telescopes and the next generation of extremely large telescopes require several guide stars to probe the turbulent atmosphere in the volume above the telescope. These data can be used to tomographically reconstruct the atmospheric turbulence profile and phase aberrations of the wavefront in order to assist wide-field AO correction. If the wavefront aberrations and altitude of the atmospheric turbulent layers are known from this tomographic model, then the effect of the scintillation can be calculated numerically and used to normalize the photometric light curve. We show through detailed Monte Carlo simulation that for an 8 m telescope with a 16x16 AO system we can reduce the scintillation noise by an order of magnitude.

astro-ph.IM↗

arXiv2014

Tensor renormalization group study of the 2d O(3) model

We present our progress on a study of the $O(3)$ model in two-dimensions using the Tensor Renormalization Group method. We first construct the theory in terms of tensors, and show how to construct $n$-point correlation functions. We then give results for thermodynamic quantities at finite and infinite volume, as well as 2-point correlation function data. We discuss some of the advantages and challenges of tensor renormalization and future directions in which to work.

hep-lat↗

arXiv2012

Atmospheric turbulence profiling using multiple laser star wavefront sensors

This paper describes the data preprocessing and reduction methods together with SLODAR analysis and wind profiling techniques for GeMS: the Gemini MCAO System. The wavefront gradient measurements of the five GeMS's Shack-Hartmann sensors, each one pointing to a laser guide star, are combined with the DM commands sent to three deformable mirrors optically conjugated at 0, 4.5 and 9 km in order to reconstruct pseudo-open loop slopes. These pseudo-open loop slopes are then used to reconstruct atmospheric turbulence profiles, based on the SLODAR and wind-profiling methods. We introduce the SLODAR method, and how it has been adapted to work in a close-loop, multi Laser Guide Star system. We show that our method allows characterizing the turbulence of up to 16 layers for altitudes spanning from 0 to 19 km. The data preprocessing and reduction methods are described, and results obtained from observations made in 2011 are presented. The wind profiling analysis is shown to be a powerful technique not only for characterizing the turbulence intensity, wind direction and speed, but also as it can provide a verification tool for SLODAR results. Finally, problems such as fratricide effect in multiple laser system due to Rayleigh scattering, centroid gain variations, and limitations of the method are also addressed.

astro-ph.IM↗

arXiv2012

Adaptive pupil masking for quasi-static speckle suppression

Quasi-static speckles are a current limitation to faint companion imaging of bright stars. Here we show through simulation and theory that an adaptive pupil mask can be used to reduce these speckles and increase the visibility of faint companions. This is achieved by placing an adaptive mask in the conjugate pupil plane of the telescope. The mask consists of a number of independently controllable elements which can either allow the light in the subaperture to pass or block it. This actively changes the shape of the telescope pupil and hence the diffraction pattern in the focal plane. By randomly blocking subapertures we force the quasi-static speckles to become dynamic. The long exposure PSF is then smooth, absent of quasi-static speckles. However, as the PSF will now contain a larger halo due to the blocking, the signal to noise ratio (SNR) is reduced requiring longer exposure times to detect the companion. For example, in the specific case of a faint companion at 5xlambda/D the exposure time to achieve the same SNR will be increased by a factor of 1.35. In addition, we show that the visibility of companions can be greatly enhanced in comparison to long-exposures, when the dark speckle method is applied to short exposure images taken with the adaptive pupil mask. We show that the contrast ratio between PSF peak and the halo is then increased by a factor of approximately 100 (5 magnitudes), and we detect companions 11 magnitudes fainter than the star at 5xlambda/D and up to 18 magnitudes fainter at 22.5xlambda/D.

astro-ph.IM↗

arXiv2011

Using artificial neural networks for open-loop tomography

Modern adaptive optics (AO) systems for large telescopes require tomographic techniques to reconstruct the phase aberrations induced by the turbulent atmosphere along a line of sight to a target which is angularly separated from the guide sources that are used to sample the atmosphere. Multi-object adaptive optics (MOAO) is one such technique. Here, we present a method which uses an artificial neural network (ANN) to reconstruct the target phase given off-axis references sources. We compare our ANN method with a standard least squares type matrix multiplication method and to the learn and apply method developed for the CANARY MOAO instrument. The ANN is trained with a large range of possible turbulent layer positions and therefore does not require any input of the optical turbulence profile. It is therefore less susceptible to changing conditions than some existing methods. We also exploit the non-linear response of the ANN to make it more robust to noisy centroid measurements than other linear techniques.

astro-ph.IM↗

arXiv2010

Conjugate-plane photometry: Reducing scintillation in ground-based photometry

High precision fast photometry from ground-based observatories is a challenge due to intensity fluctuations (scintillation) produced by the Earth's atmosphere. Here we describe a method to reduce the effects of scintillation by a combination of pupil reconjugation and calibration using a comparison star. Because scintillation is produced by high altitude turbulence, the range of angles over which the scintillation is correlated is small and therefore simple correction by a comparison star is normally impossible. We propose reconjugating the telescope pupil to a high dominant layer of turbulence, then apodizing it before calibration with a comparison star. We find by simulation that given a simple atmosphere with a single high altitude turbulent layer and a strong surface layer a reduction in the intensity variance by a factor of ~30 is possible. Given a more realistic atmosphere as measured by SCIDAR at San Pedro Mártir we find that on a night with a strong high altitude layer we can expect the median variance to be reduced by a factor of 11. By reducing the scintillation noise we will be able to detect much smaller changes in brightness. If we assume a 2 m telescope and an exposure time of 30 seconds a reduction in the scintillation noise from 0.78 mmag to 0.21 mmag is possible, which will enable the routine detection of, for example, the secondary transits of extrasolar planets from the ground.

astro-ph.IM↗

arXiv2009

Strange quark content of the nucleon

We discuss the calculation of disconnected diagrams needed for determining the strange quark content of the nucleon on the lattice. We present results for the strange scalar form factor and the related parameter f_Ts, which enters into the cross-section for the scattering of dark matter off nuclei in supersymmetric extensions of the standard model. In addition, we present results for the strange contribution to the nucleon's axial and electromagnetic form factors. The calculations were performed with two dynamical flavors of Wilson fermions on a 24^3 x 64 anisotropic lattice with a_s = 3a_t = 0.11 fm and M_pi ~ 400 MeV.

hep-lat↗

arXiv2007

Strange quark contribution to nucleon form factors

We discuss methods for the calculation of disconnected diagrams and their application to various form factors of the nucleon. In particular, we present preliminary results for the strange contribution to the scalar and axial form factors, calculated with N_f=2 dynamical flavors of Wilson fermions on an anisotropic lattice.

hep-lat↗

arXiv2004

Topological susceptibility in staggered fermion chiral perturbation theory

The topological susceptibility of the vacuum in quantum chromodynamics has been simulated numerically using the Asqtad improved staggered fermion formalism. At nonzero lattice spacing the residual fermion doublers (fermion ``tastes'') in the staggered fermion formalism give contributions to the susceptibility that deviate from conventional continuum chiral perturbation theory. In this brief report we estimate the taste-breaking artifact and compare it with results of recent simulations, finding that it accounts for roughly half of the scaling violation.

hep-lat↗