Download or read book CALCLENS Weak Lensing Simulations for Large area Sky Surveys and Second order Effects in Cosmic Shear Power Spectra written by Matthew Rand Becker and published by . This book was released on 2013 with total page 64 pages. Available in PDF, EPUB and Kindle. Book excerpt: I present a new algorithm, CALCLENS, for efficiently computing weak gravitational lensing shear signals from large N-body light cone simulations over a curved sky. This new algorithm properly accounts for the sky curvature and boundary conditions, is able to produce redshift- dependent shear signals including corrections to the Born approximation by using multiple- plane ray tracing, and properly computes the lensed images of source galaxies in the light cone. The key feature of this algorithm is a new, computationally efficient Poisson solver for the sphere that combines spherical harmonic transform and multigrid methods. As a result, large areas of sky (~10,000 square degrees) can be ray traced efficiently at high-resolution using only a few hundred cores. Using this new algorithm and curved-sky calculations that only use a slower but more accurate spherical harmonic transform Poisson solver, I study the convergence, shear E-mode, shear B-mode and rotation mode power spectra. Employing full-sky E/B-mode decompositions, I confirm that the numerically computed shear B-mode and rotation mode power spectra are equal at high accuracy ( ≲ 1%) as expected from perturbation theory up to second order. Coupled with realistic galaxy populations placed in large N-body light cone simulations, this new algorithm is ideally suited for the construction of synthetic weak lensing shear catalogs to be used to test for systematic effects in data analysis procedures for upcoming large-area sky surveys. The implementation presented in this work, written in C and employing widely available software libraries to maintain portability, is publicly available at http://code.google.com/p/calclens.
Download or read book Seeing in the Dark written by Eric Michael Huff and published by . This book was released on 2012 with total page 308 pages. Available in PDF, EPUB and Kindle. Book excerpt: Statistical weak lensing by large-scale structure { cosmic shear { is a promising cosmological tool, which has motivated the design of several large upcoming astronomical surveys. This Thesis presents a measurement of cosmic shear using coadded Sloan Digital Sky Survey (SDSS) imaging in 168 square degrees of the equatorial region, with r 2 and median redshift of zmed = 0.52. These coadds were generated using a new rounding kernel method that was intended to minimize systematic errors in the lensing measurement due to coherent PSF anisotropies that are otherwise prevalent in the SDSS imaging data. Measurements of cosmic shear out to angular separations of 2 degrees are presented, along with systematics tests of the catalog generation and shear measurement steps that demonstrate that these results are dominated by statistical rather than systematic errors. Assuming a cosmological model corresponding to WMAP7 (Komatsu et al., 2011) and allowing only the amplitude of matter fluctuations [sigma]8 to vary, the best-t value of the amplitude of matter fluctuations is [sigma]8=0.636+0.109-0.154 (1[sigma]); without systematic errors this would be [sigma]8=0.636+0.099-0.137 (1[sigma]). Assuming a flat & LambdaCDM model, the combined constraints with WMAP7 are [sigma]8=0.784super>+0.028-0.026 (1[sigma]). The 2[sigma] error range is 14 percent smaller than WMAP7 alone. Aside from the intrinsic value of such cosmological constraints from the growth of structure, some important lessons are identied for upcoming surveys that may face similar issues when combining multi-epoch data to measure cosmic shear. Motivated by the challenges faced in the cosmic shear measurement, two new lensing probes are suggested for increasing the available weak lensing signal. Both use galaxy scaling relations to control for scatter in lensing observables. The first employs a version of the well-known fundamental plane relation for early type galaxies. This modified "photometric fundamental plane" replaces velocity dispersions with photometric galaxy properties, thus obviating the need for spectroscopic data. We present the first detection of magnication using this method by applying it to photometric catalogs from the Sloan Digital Sky Survey. This analysis shows that the derived magnication signal is comparable to that available from conventional methods using gravitational shear. We suppress the dominant sources of systematic error and discuss modest improvements that may allow this method to equal or even surpass the signal-to-noise achievable with shear. Moreover, some of the dominant sources of systematic error are substantially different from those of shear-based techniques. The second outlines an idea for using the optical Tully-Fisher relation to dramatically improve the signal-to-noise and systematic error control for shear measurements. The expected error properties and potential advantages of such a measurement are proposed, and a pilot study is suggested in order to test the viability of Tully-Fisher weak lensing in the context of the forthcoming generation of large spectroscopic surveys.
Download or read book Simulations of Weak Gravitational Lensing written by Christopher Vale and published by . This book was released on 2006 with total page 342 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Weak Lensing Techniques on the Curved Sky written by Zoe M. Vallis and published by . This book was released on 2019 with total page 204 pages. Available in PDF, EPUB and Kindle. Book excerpt: Future weak lensing surveys will cover larger areas of the sky that necessitate analysis on the curved sky geometry instead of projections to the plane. This thesis focuses on reconstructing convergence maps from cosmic shear on the curved sky geometry. These convergence maps are useful tools for cosmological analysis, including probing non-Gaussian properties. The first chapter focuses on evaluating the performance of different methods of projecting simulated shear data from the curved sky to the plane, to subsequently undergo Kaiser-Squires reconstruction and analysis, and drawing comparisons to reconstruction and analysis directly on the sphere, using peak counts and Minkowski Functionals as the statistics selected for comparison. It is found that projections to the plane are only effective for small areas and it is preferable to perform analysis directly on the sphere when possible. Under ideal circumstances, peak counts derived from data projected using the sine and orthographic projections are most accurate to the spherical case. For the Minkowski Functionals there are significant differences that persist even when attempting to mitigate the projection effects. While certain projections allow reasonable approximations of the spherical sky geometry, it is impractical to use such projections on data covering large areas of the sky and performing analysis on the spherical setting is preferable. The second and third chapters focus on the separation of E-modes and B-modes through wavelet pure mode estimators on the sphere, which cancel mode mixing caused by masking of the shear data. The aim is to remove ambiguous modes to produce pure E-B modes, providing greater accuracy for studying cosmology from them. An evaluation of the accuracy of this method is performed using simulated data to compare the Kaiser-Squires, harmonic pure estimator and wavelet pure estimator methods. This finds a significant improvement in the accuracy of recovering simulated E-modes and B-modes when using the wavelet pure estimator, over the Kaiser-Squires method and harmonic pure estimator. This wavelet pure estimator method is applied to DES Y1 data and statistics, including the Minkowski Functionals, are derived and discussed. The wavelet pure estimator successfully reconstructs the E-mode and B-mode maps accurate to previous studies of the data. The Minkowski Functionals of the E-modes and B-modes display distinct differences to the analytic form for a 2D Gaussian random field. A new problem is discovered in the apodisation of the data near the mask boundary, and a potential solution is attempted through identifying and removing apodised pixels with a new mask.
Download or read book The Application of Weak Lensing written by Yong-Seon Song and published by . This book was released on 2004 with total page 382 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Cosmological Constraints from Surveys of the Weak Lensing of Galaxies written by Michael David Schneider and published by . This book was released on 2008 with total page 186 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Spurious Shear in Weak Lensing with LSST written by and published by . This book was released on 2012 with total page 20 pages. Available in PDF, EPUB and Kindle. Book excerpt: The complete 10-year survey from the Large Synoptic Survey Telescope (LSST) will image ≈ 20,000 square degrees of sky in six filter bands every few nights, bringing the final survey depth to r ≈ 27.5, with over 4 billion well measured galaxies. To take full advantage of this unprecedented statistical power, the systematic errors associated with weak lensing measurements need to be controlled to a level similar to the statistical errors. This work is the first attempt to quantitatively estimate the absolute level and statistical properties of the systematic errors on weak lensing shear measurements due to the most important physical effects in the LSST system via high fidelity ray-tracing simulations. We identify and isolate the different sources of algorithm-independent, additive systematic errors on shear measurements for LSST and predict their impact on the final cosmic shear measurements using conventional weak lensing analysis techniques. We find that the main source of the errors comes from an inability to adequately characterise the atmospheric point spread function (PSF) due to its high frequency spatial variation on angular scales smaller than ≈ 10′ in the single short exposures, which propagates into a spurious shear correlation function at the 10−4-10−3 level on these scales. With the large multi-epoch dataset that will be acquired by LSST, the stochastic errors average out, bringing the final spurious shear correlation function to a level very close to the statistical errors. Our results imply that the cosmological constraints from LSST will not be severely limited by these algorithm-independent, additive systematic effects.
Download or read book Impact of Spurious Shear on Cosmological Parameter Estimates from Weak Lensing Observables written by and published by . This book was released on 2014 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: We research, residual errors in shear measurements, after corrections for instrument systematics and atmospheric effects, can impact cosmological parameters derived from weak lensing observations. Here we combine convergence maps from our suite of ray-tracing simulations with random realizations of spurious shear. This allows us to quantify the errors and biases of the triplet ([Omega]m, w, [sigma]8) derived from the power spectrum (PS), as well as from three different sets of non-Gaussian statistics of the lensing convergence field: Minkowski functionals (MFs), low-order moments (LMs), and peak counts (PKs). Our main results are as follows: (i) We find an order of magnitude smaller biases from the PS than in previous work. (ii) The PS and LM yield biases much smaller than the morphological statistics (MF, PK). (iii) For strictly Gaussian spurious shear with integrated amplitude as low as its current estimate of [sigma]sys2 H"10-7, biases from the PS and LM would be unimportant even for a survey with the statistical power of Large Synoptic Survey Telescope. However, we find that for surveys larger than H"100 deg2, non-Gaussianity in the noise (not included in our analysis) will likely be important and must be quantified to assess the biases. (iv) The morphological statistics (MF, PK) introduce important biases even for Gaussian noise, which must be corrected in large surveys. The biases are in different directions in ([Omega]m, w, [sigma]8) parameter space, allowing self-calibration by combining multiple statistics. Our results warrant follow-up studies with more extensive lensing simulations and more accurate spurious shear estimates.
Download or read book Lossy Compression of Weak Lensing Data written by and published by . This book was released on 2011 with total page 11 pages. Available in PDF, EPUB and Kindle. Book excerpt: Future orbiting observatories will survey large areas of sky in order to constrain the physics of dark matter and dark energy using weak gravitational lensing and other methods. Lossy compression of the resultant data will improve the cost and feasibility of transmitting the images through the space communication network. We evaluate the consequences of the lossy compression algorithm of Bernstein et al. (2010) for the high-precision measurement of weak-lensing galaxy ellipticities. This square-root algorithm compresses each pixel independently, and the information discarded is by construction less than the Poisson error from photon shot noise. For simulated space-based images (without cosmic rays) digitized to the typical 16 bits per pixel, application of the lossy compression followed by image-wise lossless compression yields images with only 2.4 bits per pixel, a factor of 6.7 compression. We demonstrate that this compression introduces no bias in the sky background. The compression introduces a small amount of additional digitization noise to the images, and we demonstrate a corresponding small increase in ellipticity measurement noise. The ellipticity measurement method is biased by the addition of noise, so the additional digitization noise is expected to induce a multiplicative bias on the galaxies measured ellipticities. After correcting for this known noise-induced bias, we find a residual multiplicative ellipticity bias of m ≈ -4 x 10−4. This bias is small when compared to the many other issues that precision weak lensing surveys must confront, and furthermore we expect it to be reduced further with better calibration of ellipticity measurement methods.
Download or read book Atmospheric Dispersion Effects in Weak Lensing Measurements written by and published by . This book was released on 2012 with total page 11 pages. Available in PDF, EPUB and Kindle. Book excerpt: The wavelength dependence of atmospheric refraction causes elongation of finite-bandwidth images along the elevation vector, which produces spurious signals in weak gravitational lensing shear measurements unless this atmospheric dispersion is calibrated and removed to high precision. Because astrometric solutions and PSF characteristics are typically calibrated from stellar images, differences between the reference stars' spectra and the galaxies' spectra will leave residual errors in both the astrometric positions (dr) and in the second moment (width) of the wavelength-averaged PSF (dv) for galaxies. We estimate the level of dv that will induce spurious weak lensing signals in PSF-corrected galaxy shapes that exceed the statistical errors of the DES and the LSST cosmic-shear experiments. We also estimate the dr signals that will produce unacceptable spurious distortions after stacking of exposures taken at different airmasses and hour angles. We also calculate the errors in the griz bands, and find that dispersion systematics, uncorrected, are up to 6 and 2 times larger in g and r bands, respectively, than the requirements for the DES error budget, but can be safely ignored in i and z bands. For the LSST requirements, the factors are about 30, 10, and 3 in g, r, and i bands, respectively. We find that a simple correction linear in galaxy color is accurate enough to reduce dispersion shear systematics to insignificant levels in the r band for DES and i band for LSST, but still as much as 5 times than the requirements for LSST r-band observations. More complex corrections will likely be able to reduce the systematic cosmic-shear errors below statistical errors for LSST r band. But g-band effects remain large enough that it seems likely that induced systematics will dominate the statistical errors of both surveys, and cosmic-shear measurements should rely on the redder bands.
Download or read book Cosmology Constraints from the Weak Lensing Peak Counts and the Power Spectrum in CFHTLenS written by and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Lensing peaks have been proposed as a useful statistic, containing cosmological information from non-Gaussianities that is inaccessible from traditional two-point statistics such as the power spectrum or two-point correlation functions. Here we examine constraints on cosmological parameters from weak lensing peak counts, using the publicly available data from the 154 deg2 CFHTLenS survey. We utilize a new suite of ray-tracing N-body simulations on a grid of 91 cosmological models, covering broad ranges of the three parameters [Omega]m, [sigma]8, and w, and replicating the galaxy sky positions, redshifts, and shape noise in the CFHTLenS observations. We then build an emulator that interpolates the power spectrum and the peak counts to an accuracy of ≤ 5%, and compute the likelihood in the three-dimensional parameter space ([Omega]m, [sigma]8, w) from both observables. We find that constraints from peak counts are comparable to those from the power spectrum, and somewhat tighter when different smoothing scales are combined. Neither observable can constrain w without external data. When the power spectrum and peak counts are combined, the area of the error "banana" in the ([Omega]m, [sigma]8) plane reduces by a factor of ≈ two, compared to using the power spectrum alone. For a flat [Lambda] cold dark matter model, combining both statistics, we obtain the constraint [sigma]8([Omega]m/0.27)0.63 = 0.85+0.03-0.03.
Download or read book Atmospheric PSF Interpolation for Weak Lensing in Short Exposure Imaging Data written by and published by . This book was released on 2012 with total page 15 pages. Available in PDF, EPUB and Kindle. Book excerpt: A main science goal for the Large Synoptic Survey Telescope (LSST) is to measure the cosmic shear signal from weak lensing to extreme accuracy. One difficulty, however, is that with the short exposure time (≃15 seconds) proposed, the spatial variation of the Point Spread Function (PSF) shapes may be dominated by the atmosphere, in addition to optics errors. While optics errors mainly cause the PSF to vary on angular scales similar or larger than a single CCD sensor, the atmosphere generates stochastic structures on a wide range of angular scales. It thus becomes a challenge to infer the multi-scale, complex atmospheric PSF patterns by interpolating the sparsely sampled stars in the field. In this paper we present a new method, psfent, for interpolating the PSF shape parameters, based on reconstructing underlying shape parameter maps with a multi-scale maximum entropy algorithm. We demonstrate, using images from the LSST Photon Simulator, the performance of our approach relative to a 5th-order polynomial fit (representing the current standard) and a simple boxcar smoothing technique. Quantitatively, psfent predicts more accurate PSF models in all scenarios and the residual PSF errors are spatially less correlated. This improvement in PSF interpolation leads to a factor of 3.5 lower systematic errors in the shear power spectrum on scales smaller than ≈ 13, compared to polynomial fitting. We estimate that with psfent and for stellar densities greater than ≃1/arcmin2, the spurious shear correlation from PSF interpolation, after combining a complete 10-year dataset from LSST, is lower than the corresponding statistical uncertainties on the cosmic shear power spectrum, even under a conservative scenario.
Download or read book Weak Lensing and the Sloan Digital Sky Survey written by and published by . This book was released on 1995 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Karhunen Lo ve Analysis for Weak Gravitational Lensing written by Jacob T. Vanderplas and published by . This book was released on 2012 with total page 151 pages. Available in PDF, EPUB and Kindle. Book excerpt: In the past decade, weak gravitational lensing has become an important tool in the study of the universe at the largest scale, giving insights into the distribution of dark matter, the expansion of the universe, and the nature of dark energy. This thesis research explores several applications of Karhunen-Loève (KL) analysis to speed and improve the comparison of weak lensing shear catalogs to theory in order to constrain cosmological parameters in current and future lensing surveys. This work addresses three related aspects of weak lensing analysis: Three-dimensional Tomographic Mapping: (Based on work published in Vanderplas et al 2011) We explore a new fast approach to three-dimensional mass mapping in weak lensing surveys. The KL approach uses a KL-based filtering of the shear signal to reconstruct mass structures on the line-of-sight, and provides a unified framework to evaluate the efficacy of linear reconstruction techniques. We find that the KL-based filtering leads to near-optimal angular resolution, and computation times which are faster than previous approaches. We also use the KL formalism to show that linear non-parametric reconstruction methods are fundamentally limited in their ability to resolve lens redshifts. Shear Peak Statistics with Incomplete Data: (Based on work published in Vanderplas et al 2012) We explore the use of KL eigenmodes for interpolation across masked regions in observed shear maps. Mass mapping is an inherently non-local calculation, meaning gaps in the data can have a significant effect on the properties of the derived mass map. Our KL mapping procedure leads to improvements in the recovery of detailed statistics of peaks in the mass map, which holds promise of improved cosmological constraints based on such studies. Two-point parameter estimation with KL modes: The power spectrum of the observed shear can yield powerful cosmological constraints. Incomplete survey sky coverage, however, can lead to mixing of power between Fourier modes, and obfuscate the cosmologically sensitive signal. We show that KL can be used to derive an alternate orthonormal basis for the problem which avoids mode-mixing and allows a convenient formalism for cosmological likelihood computations. Cosmological constraints derived using this method are shown to be competitive with those from the more conventional correlation function approach. We also discuss several aspects of the KL approach which will allow improved handling of correlated errors and redshift information in future surveys.
Download or read book The Weak Gravitational Lensing of Light in Cosmological N body Simulations written by Andrew J. Barber and published by . This book was released on 1999 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Non Gaussian Cosmology written by Matteo Rizzato (docteur en cosmologie).) and published by . This book was released on 2019 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: In this thesis, we address key points for an efficient implementation of likelihood codes for modern weak lensing large-scale structure surveys. Specifically, we will focus on the joint weak lensing convergence power spectrum-bispectrum probe and we will tackle the numerical challenges required by realistic analyses. In order to clearly convey the importance of our research, we first provide an in-depth review of the background material required for a comprehensive understanding of the final results. The cosmological context of the study is provided, followed by a description of the technical elements inherent to unbiased covariance matrix estimation for the probe considered. Under the assumption of multivariate Gaussian likelihood, we developed a high performance code that allows highly parallelised prediction of the binned tomographic observables and of their joint non-Gaussian covariance matrix accounting for terms up to the 6-point correlation function and super-sample effects. This performance allows us to qualitatively address several interesting scientific questions. We find that the bispectrum provides an improvement in terms of signal-to-noise ratio (S/N) of about 10% on top of the power spectrum alone, making it a non-negligible source of information for future surveys. Furthermore, we are capable to address the impact of theoretical uncertainties in the halo model used to build our observables; with presently allowed variations we conclude that the impact is negligible on the S/N. Finally, we consider data compression possibilities to optimise future analyses of the weak lensing bispectrum. We find that, ignoring systematics, 5 equipopulated redshift bins are enough to recover the information content of a Euclid-like survey, with negligible improvement when increasing to 10 bins. We also explore principal component analysis and dependence on the triangle shapes as ways to reduce the numerical complexity of the problem.
Download or read book Improving Accuracy in Gravitational Weak Lensing Measurements of Clusters written by Julia Young and published by . This book was released on 2013 with total page 106 pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract: Measuring the distribution of galaxy clusters provides a powerful constraint on cosmological parameters. Currently, the largest challenge for using the observed abundance of clusters to constrain cosmology is to measure their mass accurately. The best tool to measure the mass of clusters is weak gravitational lensing, which measures the baryonic and dark matter present in galaxy clusters by observing the distortion of the shape of sources behind the gravitational lens called shear. Weak lensing measurements are technically challenging to measure due to the distortion in the shape of sources from the atmosphere and telescope optics called the Point Spread Function or PSF. To measure shear, images are processed by various software programs called lensing pipelines which correct for the distortion due to the PSF. Using image simulations with sources of known characteristics and known shear the systematic error of different lensing pipelines can be compared. In this dissertation the results of the Cluster Shear TEsting Program (CSTEP), a test of lensing pipelines on simulated images, is presented. CSTEP was developed to accurately measure the systematic bias on weak lensing measurements of clusters expected by the Dark Energy Survey (DES). The systematic error from lensing pipelines is then used to predict the error on mass measurements of galaxy clusters observed by DES.
