CFTHTLenS IMAGING DATA DESCRIPTION ================================== Thomas Erben, 23.04.2012 (e-mail: terben@astro.uni-bonn.de) INTRODUCTION ============ This file describes Imaging data from the CFHTLS Lensing Survey (CFHTLenS). The survey offers single frames, co-added images and catalogues from 179 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 will be described in a forthcoming CFHTLenS publication (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). AVAILABLE DATA PRODUCTS - coadded data ====================================== For each 1 sq. deg. CFHTLenS pointing we offer FITS image pixel data for the five SDSS-like broad-band filters ugriz. Please see the accompanying plots CFHTLenS_W[1-4]_field_layout.eps for a field-layout of the four CFHTLenS patches. 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 - coadded data ==================================== The CFHTLS-Wide Survey covers 171 sq. degrees in five broad-band colours (ugriz). Some of those fields have observations in the 'old' and 'new' i-band filter (see 'AVAILABLE DATA PRODUCTS' above). There are eight fields in the W2 patch that were observed only in a sub-set of these filters. Those are: W2m0m2 (i and r), W2m1m2 (i and r), W2m2m0 (g, r and i), W2m2m1 (g, r and i), W2m2m2 (r and i), W2m2p1 (g, r and i), W2m2p2 (g, r and i) and W2m2p3 (g, r and i) IMAGE HEADERS - coadded data ============================ CFHTLenS Image headers contain all necessary astrometric and photometric information to extract object catalogues; see Erben et al. 2009 for a discussion on the quality of our astrometric and especially of our photometric calibration. Important modifications with respect to this work are described in Hildebrandt et al. 2011 (arXiv:1111.4434). 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 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. The given estimates for magnitude ZP errors (keyword MAGZPERR) for the different filters are discussed in Erben et al. 2009. IMPORTANT: Note that MAGZP, GAIN and SEEING are in general different for each pointing/filter! AVAILABLE DATA PRODUCTS - single image data =========================================== Besids co-added imaging data we make available all sky-subtracted single-image data from our primary lensing analysis band, the Sloan i-band. This encompasses all i- and y-band data (see above). Note that we do not have available single-image data from the complementary filters u, g, r and z! The provided data form the basis of our complete shape analysis with the lensfit tool (see Miller et al., MNRAS 382, 315). The single-image data have names such as: 871705p_5C.sub.fits The meaning of the different parts of that name are: 871705p: original CFHT naming string for the image _5: The THELI chip number. It gives the position within the MEGAPRIME mosaic; see below C: THELI processing status; this is always a sime 'C' for MEGACAM@CFHT images .sub: sky-subtracted image Each sky-subtracted science image is accompanied by a corresponding weight map. The weight is an 8-bit image and contains a '1' for a good and a '0' for a bad pixel (cosmic ray, hot pixel, part of satellite track, etc.). The THELI chip labelling for the 1sq degree MEGACAM@CFHT camera is as follows (the camera is a 36-chip mosaic with 4 rows containing 9 chips each): 28 29 30 31 32 33 34 35 36 19 20 21 22 23 24 25 26 27 10 11 12 13 14 15 16 17 18 1 2 3 4 5 6 7 8 9 Sky Orientation: North is up and East to the left. Note that this scheme is different from the original CFHT MEGACAM chip naming convention! In addition to the images we provide scamp/swarp image headers for all single frames. These headers contain astrometric information from the calibration process with the 'scamp' tool. Please consult the 'scamp' manual (see http://astromatic.iap.fr/) for a description of the headers content. SUGGESTIONS, QUESTIONS, REQUESTS etc. ===================================== For questions concerning the described products please contact: Thomas Erben (terben@astro.uni-bonn.de) ACKNOWLEDGEMENTS ================ If you use the CFHTLenS products for your own research, please give proper acknowledgment to CADC (http://www4.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/), the CFHTLS project (http://www.cfht.hawaii.edu/Science/CFHLS/cfhtlspublitext.html) and cite us with the phrase: 'In this analysis we use data from the CFHT Lensing Survey (CFHTLenS) based on the data reduction method described in Erben et al 2009 (A&A 493, 1197) and the photometric redshift method described in Hildebrandt et al 2011 (arXiv:1111.4434).'