We present the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS) that accurately determines a weak gravitational lensing signal from the full 154 deg2 of deep multicolour data obtained by the CFHT Legacy Survey. Weak gravitational lensing by large‐scale structure is widely recognized as one of the most powerful but technically challenging probes of cosmology. We outline the CFHTLenS analysis pipeline, describing how and why every step of the chain from the raw pixel data to the lensing shear and photometric redshift measurement has been revised and improved compared to previous analyses of a subset of the same data. We present a novel method to identify data which contributes a non‐negligible contamination to our sample and quantify the required level of calibration for the survey. Through a series of cosmology‐insensitive tests we demonstrate the robustness of the resulting cosmic shear signal, presenting a science‐ready shear and photometric redshift catalogue for future exploitation.

We present cosmological constraints from 2D weak gravitational lensing by the largescale structure in the Canada-France Hawaii Telescope Lensing Survey (CFHTLenS) which spans 154 square degrees in five optical bands. Using accurate photometric redshifts and measured shapes for 4.2 million galaxies between redshifts of 0.2 and 1.3, we compute the 2D cosmic shear correlation function over angular scales ranging between 0.8 and 350 arcmin. Using non-linear models of the dark-matter power spectrum, we constrain cosmological parameters by exploring the parameter space with Population Monte Carlo sampling. The best constraints from lensing alone are obtained for the small-scale density-fluctuations amplitude σ 8 scaled with the total matter density Ω m . For a flat ΛCDM model we obtain σ 8 (Ω m /0.27) 0.6 = 0.79 ± 0.03.We combine the CFHTLenS data with WMAP7, BOSS and an HST distance-ladder prior on the Hubble constant to get joint constraints. For a flat ΛCDM model, we find Ω m = 0.283 ± 0.010 and σ 8 = 0.813 ± 0.014. In the case of a curved wCDM universe, we obtain Ω m = 0.27 ± 0.03, σ 8 = 0.83 ± 0.04, w 0 = −1.10 ± 0.15 and Ω K = 0.006 +0.006 −0.004 . We calculate the Bayesian evidence to compare flat and curved ΛCDM and dark-energy CDM models. From the combination of all four probes, we find models with curvature to be at moderately disfavoured with respect to the flat case. A simple dark-energy model is indistinguishable from ΛCDM. Our results therefore do not necessitate any deviations from the standard cosmological model.

We present data products from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS). CFHTLenS is based on the Wide component of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). It encompasses 154 deg 2 of deep, optical, high-quality, sub-arcsecond imaging data in the five optical filters u * g r i z . The scientific aims of the CFHTLenS team are weak gravitational lensing studies supported by photometric redshift estimates for the galaxies. The article presents our data processing of the complete CFHTLenS data set. We were able to obtain a data set with very good image quality and high-quality astrometric and photometric calibration. Our external astrometric accuracy is between 60-70 mas with respect to SDSS data and the internal alignment in all filters is around 30 mas. Our average photometric calibration shows a dispersion on the order of 0.01 to 0.03 mag for g r i z and about 0.04 mag for u * with respect to SDSS sources down to i SDSS 21. We demonstrate in accompanying articles that our data meet necessary requirements to fully exploit the survey for weak gravitational lensing analyses in connection with photometric redshift studies. In the spirit of the CFHTLS all our data products are released to the astronomical community via the Canadian Astronomy Data Centre at http://www.cadc-ccda.hia-iha. nrc-cnrc.gc.ca/community/CFHTLens/query.html. We give a description and how-to manuals of the public products which include image pixel data, source catalogues with photometric redshift estimates and all relevant quantities to perform weak lensing studies.

Here we present the results of various approaches to measure accurate colours and photometric redshifts (photo‐z) from wide‐field imaging data. We use data from the Canada–France–Hawaii Telescope Legacy Survey which have been re‐processed by the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS) team in order to carry out a number of weak gravitational lensing studies. An emphasis is put on the correction of systematic effects in the photo‐z arising from the different point spread functions (PSFs) in the five optical bands. Different ways of correcting these effects are discussed and the resulting photo‐z accuracies are quantified by comparing the photo‐z to large spectroscopic redshift (spec‐z) data sets. Careful homogenization of the PSF between bands leads to increased overall accuracy of photo‐z. The gain is particularly pronounced at fainter magnitudes where galaxies are smaller and flux measurements are affected more by PSF effects. We discuss ways of defining more secure subsamples of galaxies as well as a shape‐ and colour‐based star–galaxy separation method, and we present redshift distributions for different magnitude limits. We also study possible re‐calibrations of the photometric zero‐points (ZPs) with the help of galaxies with known spec‐z. We find that if PSF effects are properly taken into account, a re‐calibration of the ZPs becomes much less important suggesting that previous such re‐calibrations described in the literature could in fact be mostly corrections for PSF effects rather than corrections for real inaccuracies in the ZPs. The implications of this finding for future surveys like the Kilo Degree Survey (KiDS), Dark Energy Survey (DES), Large Synoptic Survey Telescope or Euclid are mixed. On the one hand, ZP re‐calibrations with spec‐z values might not be as accurate as previously thought. On the other hand, careful PSF homogenization might provide a way out and yield accurate, homogeneous photometry without the need for full spectroscopic coverage. This is the first paper in a series describing the technical aspects of CFHTLenS.