RCSLenS CATALOGUE DATA DESCRIPTION ================================== Hendrik Hildebrandt, 2016-03-15 (email: hendrik@astro.uni-bonn.de) ACKNOWLEDGEMENT =============== Acknowledging the use of RCSLenS data products in publications: The use of the RCSLenS imaging, photometry or shear catalogues should be acknowledged in publications by including the paragraph listed below, in addition to citing the RCSLenS technical paper within the text. Here we provide an example paragraph that includes all the relevant references: "We use data from the Red Cluster Sequence Lensing Survey (Hildebrandt et al. 2016), hereafter referred to as RCSLenS which is based on the Red Sequence Cluster Survey 2 (Gilbank et al. 2011). The RCSLenS team 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 Hildebrandt et al. (2016), with additional error analyses of the photometric redshift measurements presented in Choi et al. (2016)." 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. RCSLenS data processing was made possible thanks to significant computing support from the NSERC Research Tools and Instruments grant program." We are very keen for the community to make full use of this data product, so do not hesitate to contact the RCSLenS PI, Hendrik Hildebrandt (hendrik@astro.uni-bonn.de) if you have any questions about the survey and the catalogues. CATALOGUE CREATION ================== Summary of the catalogue creation: The catalogues were created from the RCSLenS images released at http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/community/rcslens/csky.html The following steps were performed for the photometric analysis. Details can be found in Hildebrandt et al. (2016) 1. Star selection from a high-threshold, single-band SExtractor catalogue. 2. After selecting stars from the detection image (r-band) these star catalogues are used to Gaussianise the images. 3. Then SExtractor is run in dual image mode five times (for RCSLenS fields with griz-band data). The detection image is the unconvolved e-band image and the measurement images are the Gaussianised images in the four bands and - in the fifth run - the unconvolved r-band image. This last run is performed to get total magnitudes in one band, whereas the first four runs yield accurate colours based on isophotal magnitudes. All magnitudes are in the AB system. 4. Limiting magnitudes are added for each object based on SExtractor RMS check images. 5. Extinction values are added based on the Schlegel maps (Schlegel et al. 1998). 6. The total magnitude in the r-band is combined with the colour estimates, the limiting magnitudes (to decide whether an object is detected in a given band), and the extinction values to yield estimates of the total magnitudes in the other bands (MAG_[griyz]). IMPORTANT: This assumes that there are no colour gradients in the objects. For galaxies with colour gradients the total magnitudes in the giyz-bands might be biased and only the total r-band magnitude is reliable. Note that the isophotal magnitudes do not appear in the released catalogues anymore. MAG_[griyz] are the only magnitude estimates and can be used to calculate accurate colours as well. 7. A mask column based on the final, eye-balled and modified masks is added. 8. BPZ is run to estimate the photo-z's. 9. Absolute rest-frame magnitudes in the MEGACAM filters as well as stellar masses (see Velander et al. 2012 for details) are added based on the BPZ photo-z estimate and a best-fit template from the Bruzual & Charlot (2003) library. This step is performed keeping the redshift fixed using the LePhare (Arnouts et al. 2002) code and the Ilbert et al. (2010) technique. The shear measurements are described in Miller et al (2012) and Hildebrandt et al. (2016), to which reference should be given when using the shear catalogue. All calibrated, individual exposures (typically just one exposure for RCSLenS) were passed to the shear measurement stage, which comprised joint fitting of models to individual galaxies, taking into account the varying PSF on each individual exposure. Corrections to resulting shear estimates should be made following Miller et al. (2012) and Hildebrandt et al. (2012). COLUMN DESCRIPTION ================== All quantities in the catalogue refer to the detection band if not otherwise specified, either by an _[griyz] or in obvious cases (photo-z, stellar mass, etc.). Additional information on certain columns in the catalogue: - MASK: If MASK>0 the object lies within a mask. However, objects with MASK<=1 can safely be used for most scientific purposes. - T_B: BPZ spectral type. 1=CWW-Ell, 2=CWW-Sbc, 3=CWW-Scd, 4=CWW-Im, 5=KIN-SB3, 6=KIN-SB2. Note that we use a recalibrated template set described in Capak et al. (2004) and that the templates are interpolated, hence fractional types occur. - NBPZ_FILT, NBPZ_FLAGFILT, NBPZ_NONDETFILT: The number of filters in which an object has 'reliable photometry' (NBPZ_FILT, magnitude errors < 1 mag and object brighter than the limiting magnitude); number of filters in which an object has formal magnitude errors of 1 mag or larger (NBPZ_FLAGFILT); number of filters in which an object is fainter than the formal limiting magnitude (NBPZ_NONDETFILT). If an object would fall into FLAGFILT AND NONDETFILT it is listed under FLAGFILT! Magnitude errors refer to MAG_ISO. - BPZ_FILT, BPZ_FLAGFILT, BPZ_NONDETFILT: These keys contain a 'binary' encoding which filters fulfil the respective conditions. Filter u* is assigned a '1', "g'" = '2', "r'" = '4', "i'" = '8' and "z'" = '16'. The keys BPZ_FILT etc. represent the sums of the filters fulfilling the corresponding criteria. - PZ_full: This is the full P(z) to z=3.5. There are 70 columns sampling P(z) at intervals of dz=0.05. The first bin is centered at z=0.025. Note these 70 columns do not always sum to 1. There is a final bin not included in the catalogues with z>3.5 that, in a small number of cases, has non-zero probability. In RCSLenS analysis we set a hard prior that there is zero probability past z>3.5, which corresponds to normalising each P(z) to one. For future flexibility however we do not impose this normalisation on the catalogue, leaving it to the user to apply. - SG_flag: Stars and galaxies are separated using second order brightness moments as described in Hildebrandt et al. (2016). - MAG_LIM_[griyz]: These are 1-sigma limiting magnitude measured in a circular aperture with a diameter of 2 * FWHM, where FWHM is the seeing in this band (see image header). - weight: The lensfit weight to be used in shear measurement for each galaxy as given by equation 8 of Miller et al. (2012). The weight is effectively an inverse-variance weight, including the variance of the intrinsic galaxy ellipticity distribution, and thus can be used to estimate the measurement uncertainty in the shear for an individual galaxy. - fitclass: Object classification as returned by lensfit. Classification values are 0: galaxy 1: star -1: bad fit: no useable data -2: bad fit: blended object -3: bad fit: miscellaneous reason -4: bad fit: chi-squared exceeds a critical value - size: Lensfit galaxy model scalelength, marginalised over other parameters, as defined by Miller et al (2012). Used for the calculation of the multiplicative correction as in Miller et al (2012), equations 15-17. - model_flux: Lensfit galaxy model total flux, in calibrated CCD data units. - SNratio: Signal-to-noise ratio of the object, measured on a stack of the supplied exposures within a limiting isophote 2$\sigma$ above the noise. Used for the calculation of the multiplicative correction as in Miller et al (2012), equations 15-17. - PSF_e1, PSF_e2: Mean of the PSF model ellipticity components measured on each exposure. PSF ellipticities are derived from the PSF model at the location of each galaxy and are top-hat weighted with radius 8\,pixels - PSF_Strehl_ratio: Mean of the PSF model "pseudo-Strehl ratio" values measured on each exposure. The psuedo-Strehl ratio is defined as the fraction of light falling in the central pixel, and is inversely related to the PSF size. - e1, e2: Expectation values of galaxy ellipticity, from the marginalised galaxy ellipticity likelihood surface, to be used for shear measurement. Ellipticity is defined as e=(a-b)/(a+b) where a,b are the semi-major and semi-minor axes. The ellipticity coordinate system is aligned with the celestial sphere at the location of each galaxy. Corrections should be applied to these values as described by Miller et al (2012) and Hildebrandt et al (2012). We strongly urge the user not to use these raw uncalibrated ellipticity values blindly in a lensing analysis. First, any shear measurement must measure weighted averages using the lensfit weight. Secondly an additive c-correction must be applied to the ellipticity components (see below). Thirdly a multiplicative shear calibration correction must be applied following equations 15-17 of Miller et al (2012). Note that it is incorrect to apply this multiplicative correction on an object by object basis. Instead this calibration correction must be applied as as an ensemble average (see section 4.1 of Heymans et al 2012 for a summary of the required calibration corrections). - m: Multiplicative shear bias correction. - c1_DP, c2_DP: Additive shear bias correction for the first pass. Should be subtracted from e1/e2. - c1_NB, c2_NB: Additive shear bias correction for the second pass. Should be subtracted from e1/e2. - n_exposures_used: the number of individual exposures used by lensfit for this galaxy.