The Third Gravitational Lensing Accuracy Testing (Great3) Challenge Handbook

Abstract

The GRavitational lEnsing Accuracy Testing 3 (GREAT3) challenge is the third in a series of image analysis challenges, with a goal of testing and facilitating the development of methods for analyzing astronomical images that will be used to measure weak gravitational lensing. This measurement requires extremely precise estimation of very small galaxy shape distortions, in the presence of far larger intrinsic galaxy shapes and distortions due to the blurring kernel caused by the atmosphere, telescope optics, and instrumental effects. The GREAT3 challenge is posed to the astronomy, machine learning, and statistics communities, and includes tests of three specific effects that are of immediate relevance to upcoming weak lensing surveys, two of which have never been tested in a community challenge before. These effects include many novel aspects including realistically complex galaxy models based on high-resolution imaging from space; spatially varying, physically-motivated blurring kernel; and combination of multiple different exposures. To facilitate entry by people new to the field, and for use as a diagnostic tool, the simulation software for the challenge is publicly available, though the exact parameters used for the challenge are blinded. Sample scripts to analyze the challenge data using existing methods will also be provided. See http://great3challenge.info and http: //great3.projects.phys.ucl.ac.uk/leaderboard/ for more information.

The alignment and shape of dark matter, stellar, and hot gas distributions in the EAGLE and cosmo-OWLS simulations

“…Many observational studies have measured the value of the e rms for populations of galaxies. Early results were reported in Hoekstra, Franx & Kuijken (2000), and more statistically robust results were obtained using SDSS data (Reyes et al 2012), COSMOS (Joachimi et al 2013;Mandelbaum et al 2014) and the CFHTLenS survey (Heymans et al 2013;Miller et al 2013). Unfortunately, despite the tremendous progress in the statistical power of the galaxy surveys employed in these studies, obtaining an accurate estimate of e rms remains challenging, especially because of the fact that the quantity that is accessible observationally always has a (often non-negligible) noise contribution (see e.g.…”

The impact of cosmic variance on simulating weak lensing surveys

“…We use the COSMOS survey to define a parent sample of galaxy images to be used for making image simulations, following the approach taken in Mandelbaum et al (2012Mandelbaum et al ( , 2014.…”

“…High resolution images taken through the wide F814W filter (broad I ) for all galaxies passing the above cuts were used to create a collection of postage stamp images for the GREAT3 challenge (Mandelbaum et al 2014), using the procedure described in Mandelbaum et al (2012). Each galaxy postage stamp image has a corresponding PSF image that can be used by GalSim or other software to remove the effects of the HST PSF before simulating the galaxy image as it would appear at lower resolution.…”

“…These were carried out using the method described in Lackner & Gunn (2012), and include Sérsic profile fits and 2 component bulge + disk fits described in detail in Mandelbaum et al (2014) and briefly in Sec. 3.3 of this work.…”

“…Software packages like GalSim 2 (Rowe et al 2014) can generate images of galaxies from the HST as they would appear with an additional lensing shear and viewed by some lower resolution telescope. Examples of training samples from the HST include the COSMOS survey (used by the GREAT3 challenge, Mandelbaum et al 2014) or the Ultra Deep Field (UDF, used by Jee et al 2013). These two examples serve as the extremes in the HST samples used as the basis for image simulation, with COSMOS being the widest contiguous area surveyed by the HST currently and hence representive, and the latter being extremely deep but narrow.…”