CFTHTLenS IMAGING DATA DESCRIPTION ================================== Thomas Erben, 23.10.2012 (e-mail: terben@astro.uni-bonn.de) INTRODUCTION ============ This file describes Imaging data from the CFHTLS Lensing Survey (CFHTLenS; see also http://www.cfhtlens.org/). The survey offers co-added images and catalogues from 154 sq. degrees of the Wide part from the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). The main documentation for the products, its creation and its content are described in Erben et al. (2012) (in preparation). The algorithms and procedures to process our data are largely based on the CFHTLS-Archive Research Survey (CARS; see Erben et al. 2009, A&A 493, 1197). IMPORTANT NOTE ============== Please read especially the section 'Influence of our cosmic ray removal on stellar sources' in Erben et al. (2012) when extracting source lists from the provided imaging data. The section contains important information on the incompleteness of stellar counts at faint magnitudes. AVAILABLE DATA PRODUCTS ======================= For each 1 sq. deg. CFHTLenS pointing we offer FITS image pixel data for the five SDSS-like broad-band filters ugriz. We discuss them with the CFHTLenS pointing W1m0p1 as an example. Our FITS files are named like W1m0p1_g.V2.2A.swarp.cut.(weight., flag., sum.)fits The name contains the following information: W1m0p1: name of CFHTLenS field g: filter of the observation (possible are the five MegaPrime broad-band filters u, g, r, i (or y) and z) !! IMPORTANT NOTE !! During 2008 the CFHT MEGAPRIME i-band filter broke and was replaced by a new one. Fields with the 'y' filter identifier were observed with the 'new' MEGAPRIME i-band filter. (NOTE that we use 'y' here just as an identifier for the 'new' i-band filter. It has nothing in common with a Y-band which is located red-ward of SDSS-z) V2.2A: internal CFHTLenS version number swarp: the image was co-added with swarp cut: all images of a given pointing are cut to the same size (21k x 21k) and all filters are projected to exactly the same pixel grid! (weight.,flag.,sum.): for each pointing/colour we offer the co-added science image, a corresponding weight map, a flag mask and a so-called sum image. See Erben et al. 2005, AN 326, 432 and Erben et al. 2009 for a more detailed description of the image weights. The flag image has a '0' where the weight is unequal to zero and a '1' where the weight is zero, i.e. a '1' indicates a 'bad pixel' of the co-added science frame. The sum image gives, for each pixel, how many individual images contributed to that pixel. !! IMPORTANT: !! The SExtractor WEIGHT_TYPE for all provided weight images is MAP_WEIGHT The science images are offered in uncompressed form and have a size of about 1.7GB. Weight, Flag and Sum images take about 40MB in gzipped form. The weights uncompress to the same size as corresponding science frames, the flag and sum images to about a quarter of this (integer pixel values). FIELD FILTER COVERAGE ===================== The released CFHTLenS survey data covers individual 1sq. degrees pointings in five broad-band filters (ugriz). Some of those fields have observations in the 'old' and 'new' i-band filter (see 'AVAILABLE DATA PRODUCTS' above). IMAGE HEADERS ============= CFHTLenS Image headers contain all necessary astrometric and photometric information to extract object catalogues; see Erben et al. 2012 and Hildebrandt et al. 2012 (MNRAS 421, 2355) for a discussion on the quality of our astrometric and especially of our photometric calibration. Astrometric header keywords follow standard WCS descriptions (see e.g. Greisen & Calabretta A&A 395, 1061). We discuss here some important header keywords to obtain photometric quantities: - TEXPTIME: total exposure time in seconds - EXPTIME: 'effective' exposure time in seconds (always 1s for CARS; the pixel unit of all CARS images is ADU/s) - MAGZP: magnitude zeropoint; apparent object AB magnitudes need to be estimated via: mag = MAGZP - 2.5 * log10(object counts); If using SExtractor 'correct' apparent AB magnitudes are obtained by putting the MAGZP value into the MAG_ZEROPOINT configuration parameter. - GAIN: The effective median GAIN of the exposure. To obtain meaningful magnitude error estimates within SExtractor the GAIN configuration parameter needs to be set the the GAIN header value! - SEEING: measures 'mean' image seeing for the exposure. Put this value into the SEEING_FWHM SExtractor parameter to obatin a meaningful SExtractor star/galaxy separation. IMPORTANT: Note that MAGZP, GAIN and SEEING are in general different for each pointing/filter! SUGGESTIONS, QUESTIONS, REQUESTS etc. ===================================== For questions concerning the described products please contact: Thomas Erben (terben@astro.uni-bonn.de) For general questions about CFHTLenS do not hesitate to contact the CFHTLenS PIs, Catherine Heymans (heymans@roe.ac.uk) and Ludovic Van Waerbeke (waerbeke@phas.ubc.ca) ACKNOWLEDGEMENTS ================ The use of the CFHTLenS imaging, photometry or shear catalogues should be acknowledged in publications by including the paragraph listed below, in addition to citing specific CFHTLenS articles within the text. For publications that use the shear and photometric catalogues we provide an example paragraph that includes all the relevant references: We use data from the Canada-France-Hawaii Lensing Survey (Heymans et al. 2012), hereafter referred to as CFHTLenS. The CFHTLenS survey analysis combined weak lensing data processing with THELI (Erben et al. 2012), shear measurement with lensfit (Miller et al. 2012), and photometric redshift measurement with PSF-matched photometry (Hildebrandt et al. 2012). A full systematic error analysis of the shear measurements in combination with the photometric redshifts is presented in Heymans et al. (2012), with additional error analyses of the photometric redshift measurements presented in Benjamin et al. (2012). For publications which use the imaging data alone, cite Erben et al. (2012). For publications which use the photometry alone, cite Erben et al. (2012) and Hildebrandt et al. (2012). Please also include the following paragraph in your acknowledgements section: This work is based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. CFHTLenS data processing was made possible thanks to significant computing support from the NSERC Research Tools and Instruments grant program.