Back to Terapix T0002 main page.
|Stacked Images|| Median of all input images listed in ".lst" file (FILENAME: CFHTLS_[D/W]_[filter]_[RA][DEC]_T0002.lst) , weighted accordingly, and combined using SWarp. Equatorial Coordinate system, J2000 (FK5). Projection: TAN. All images are aligned to the I-band data. All images have 19354x19354 pixels, 0.186 arc-second/pixel. Astrometric reference catalog used: USNO-B1. Each image is composed of one single FITS file (no more MEF with 36 FITS files; CCDs are no longer singled out after stacking). FILENAME: CFHTLS_[D/W]_[filter]_[RA][DEC]_T0002.fits .
The image are compressed using the RICE algorithm. It produces .fz files that can be uncompressed using the imcopy CFITS/IO tool. See the CADC compression page for details.
Note that the D4-z is now part of the release (it was not included in T0001).
|Seeing||The TERAPIX seeing is defined as twice the median FLUX_RADIUS of a selection of point sources on each CCD. FLUX_RADIUS, as measured by SExtractor, is the radius of the disk that contains 50% of the total flux (it is sometimes called rh in the literature). For a Gaussian profile, the TERAPIX seeing is equal to the Full Width at Half Maximum (FWHM). For a typical MEGACAM Point Spread Function (PSF), the TERAPIX seeing is slightly larger ( 10%) than the true PSF FWHM.|
|Mag. ZP|| Photometric calibration is carried out in two steps. In the first step, the photometric solution provided in the header of each image by CFHT (see CFHT page ) is used to re-scale the zero-point of each image to the values given in the Magnitude ZP columns of the summary table (for a one second exposure at zero airmass). In the second step, overlapping sources on all exposures in a given field for a given filter are compared, and the image which has the sources with the highest relative flux (and is therefore the least extincted) is identified and used as a reference image. All other images are then scaled to this image. The second step is done by the new SCAMP tool developped at Terapix and installed in the Terapix pipeline. This absolute calibration is accurate to 0.05 magnitudes, stack-to-stack, based on consideration of stellar colour-colour plots and number counts (see below).
The magnitudes of objects in the final stacks is computed as follows: m=(30+Corr)-2.5log(counts). The (30+Corr) values are given in the "Magnitude ZP" columns of the summary table. The magnitude system is instrumental AB
|Effective FOV||Effective area is derived by subtracting from the MEGACAM fields all the masked areas defined using the .reg mask file. The effective area is same for all filters of a given field.|
|Chi2 Image||Constructed by combining images noted "Y" in the table using "SWarp" (hereafter=filterlist)- Pixel values are actually sqrt(chi2) , not chi2 . There is one Chi2 image per field (joined multi-band image). Chi2 image is generated using the swarp.conf of the co-added images, but with the 'COMBINE_TYPE' chi2' option. FILENAME: CFHTLS_[D/W]_chi2[filterlist]_[RA][DEC]_T0002.fits .|
|Dust map Images||E(B-V) FITS images constructed from the E(B-V) images from Schlegel et al , using SWarp, centred and sized according to the I-band images of each deep field. Native Schlegel et al. images have been oversampled, interpolated and transformed into the CFHTLS Equatorial coordinate system using bi-linear interpolation. There is one FITS "dust image" per field, but we DID NOT apply correction to the data. FILENAME: CFHTLS_[D/W]_dust_[RA][DEC]_T0002.fits .|
|Weight map images||FITS weight map image. There is one weight map image for each FITS image. Input weight maps are generated during the QualityFITS-in process and output weight maps are produced during the image stacking using SWarp. Weight is defined as the local sky background flux variance. The input weight-maps are generated using the master flat field and mask FITS images delivered by CFHT. When used during co-addition, the weighting scheme takes into account the image-to-image flux rescaling, as described in the SWarp User's manual. FILENAME: CFHTLS_[D/W]_[filter]_[RA][DEC]_T0002_weight.fits .|
|Completeness||Derived by adding simulated stellar objects with a Moffat profile to sub-sections extracted near the center of each stack. Simulated stars have an equivalent FHWM of 1". The x-axis shows the magnitude of the simulated stars, the y-axis shows the fraction of stars which are recovered for each 0.5 magnitude slice. There is one plot per field, and all filters are plotted together. Magnitudes are SExtractor's MAG_AUTO. MAG_AUTO are Kron-like "quasi-total" magnitudes. The typical shift with respect to true "total" magnitudes is 0.06 mag (the flux being slightly underestimated). The magnitude system is instrumental AB. FILENAME: CFHTLS_[D/W]_[ugriz/gri]_[RA][DEC]_T0002_comp.(png/ps) .|
|Stellar colour-colour|| Star were selected by automatically locating the stellar branch in the compactness(rh)-magnitude diagram in a serie of 10 arcminute cells distributed over each MEGACAM I-stack; only stars with MAG_AUTO magnitudes in the range [16.5-21.0] mag. have been selected. Magnitude system is instrumental AB.
Stellar tracks from Pickles overplotted for comparison. The Pickles data have not been corrected from atmospheric transmission, so offest between data and stellar tracks are expected. One plot per field and per filter-triplet (hereafter filtertriplet). FILENAME: CFHTLS_[D/W]_[filtertriplet]_[RA][DEC]_T0002_diagcc.(png/ps) .
|Galaxy counts||Galaxy number counts per 0.5 mag., per deg2, computed for the effective area of each field, i.e., after the application of the masks, and derived from the deep catalog produced from the chi2 image. Magnitudes are SExtractor MAG_AUTO. No extinction correction has been applied. The magnitude system is instrumental AB ; there is one plot per filter. FILENAME: CFHTLS_[D/W]_[filter]_galcount_T0002.(png/ps) .|
|Astrometric quality assessment plots|| Astrometric calibration is the most important change of T0002 with respect to T0001. Instead of the astrometric software Astrometrix and Photometrix used for T0001, T0002 astrometry and field to field photometric rescaling were calibrated with the new Terapix tool SCAMP (Bertin al 2005) that turns out to be 100 times faster than Astrometrix. As SWarp and SExtractor, SCAMP uses plain text configuration files and can be installed on any POSIX-compliant platform.
SCAMP takes as input SExtractor catalogues and produces as output WCS images headers (.head or .ahead files) that can be read by SWarp to re-scale, re-sample and co-add images. The first task for SCAMP is to examine the image headers and group exposures by sky area, "astrometric context" and "photometric context". An "astrometric context" defines a set of exposures that have been observed with a common, mechanically stable configuration. For the MegaPrime/Megacam CFHTLS data, these are exposures which share the same filter and QRunID. There is one astrometric solution per astrometric context. Likewise, a "photometric context" characterises a set of exposures with the same spectral response, that is, with the same filter. Before proceeding further, the CCDs is each available astrometric context are re-positionned to a common "median" focal plane, to correct to strong CCD-to-CCD jittering introduced by the Elixir astrometric solutions. A subset of the reference astrometric catalogue (USNO-B1 for T0002) surrounding each identified sky area is automatically downloaded from CDS. Cross-correlation is used to match each exposure with respect to the reference catalogue: pixel scales, frame position angles, and shifts in RA-DEC are determined within around 1 arc-second. Pattern matching is followed by a cross-identification with a search radius of 2" between all overlapping detections. In the context of the CFHTLS, SCAMP computes a polynomial model of Megacam image distortions by minimising a weighted, quadratic sum of differences in position between both overlapping detections and matched sources from the USNO-B1 astrometric reference catalogue. Since the catalogue has a typical accuracy of 0.3 arc-second, which is insufficient, combining the constraints given by the relative positions of overlapping detections and those from the reference catalogue provides both a very accurate adjustement of exposures on small scales, and a correct positionning on the sky on larger scales. Residual position errors provide an approximate quality of the final astrometric solution. Roughly, on large scale (larger than a typical dithering step) errors are dominated by those of the reference catalogue which is typically a few tenth of an arc-second. At the smallest scale, the relative astrometric accuracy is typically 10 to 30 mas RMS.
Note that for T0002, all positions are expressed in the J2000 system and ignore proper motions. Also, the *.head files used for each Megacam image used in co-additions are provided with a generic name CFHTLS_[W/D]_[FILTER]_[RA]-[DEC]_T0002_[#image-number]_head.txt .
A photometric calibration correction is applied to images, prior to image co-addition, in a similar way to the astrometric distortion coefficient determination, by minimizing the weighted quadratic sum of MAG_AUTO magnitude differences from overlapping detections. MAG_AUTO was chosen as it is fairly robust and insensitive to the seeing or the size of sources. Adding these photometric constraints the residual Zero-Point discrepancy seen amongst overlapping "photometric" (that is labelled as photometric "P" from CFHT analysis) fields is about 1-2%. The resulting residual on bright objects (S/N > 100) are typically 0.02-0.05 magnitudes RMS, but the relative ZP uncertainties are much smaller. These ZP uncertainties are dominated by the effects of CCD-to-CCD variations, seeing changes and residual gradients within a Megacam field. The SCAMP output headers contain astrometric and photometric calibration data and a series of quality assessment output files. The .head and .ahead calibration data are ASCII files that contain a multi-FITS header with keywords providing updated WCS astrometric data and photometric rescaling coefficients attached to each CCD. As for the astrometric information, the photometric rescaling values are in CFHTLS_[W/D]_[FILTER]_[RA]-[DEC]_T0002_[#image-number]_head.txt .
FILENAME: CFHTLS_[D/W]_[filter]_[RA][DEC]_T0002*.ps .
|.head files||ASCII files containing the FITS keywords derived from the astrometric solution found by SCAMP and the FSCALE FITS keyword derived from the photometric rescaling using SCAMP as well (that has to be used in the swarp.conf file : FSCALE_KEYWORD FSCALE) . The .head files can be used to re-SWarp and re-stack FITS images, if needed, without need to run the astrometry software again. FILENAME: CFHTLS_[D/W]_[filter]_[RA][DEC]_T0002_head.txt for the final stack, and, as written in the previous section, CFHTLS_[W/D]_[FILTER]_[RA]-[DEC]_T0002_[#image-number]_head.txt , for individual images used for each stack .|
|Mask files||Built using the "polygon" option of the DS9. visualisation tool to create a "*.reg" file which describes, in world co-ordinates, the regions of the images which should not be used for scientific analysis. Can be used with DS9 (or SAOimage, after they have been transformed to pixel co-ordinates). They mask the MEGACAM field boundary, saturated stars, satellite tracks and image defects, if any. It is an ASCII file: polygons may be added or removed according to user wishes It is imporant that any users check the masks and modify them according to its scientific needs. Note that the masks are constructed using the I-band images which are the references for all filters. so they can be used with the u,g,r and z images. Users who require catalogues selected in other bands (for instance, u or z) many need to make some changes to these masks. FILENAME: CFHTLS_[D/W]_[filter]_[RA][DEC]_T0002.reg .|
|Catalogs-chi2||These catalogues were produced using SExtractor in dual-image mode, using the gri chisquared image as reference, as well as the weight-map for the final stack. Each catalogue id for each filter for a given field refers to the same object. Note that these catalogues have not been corrected for galactic extinction. For almost all purposes, the Kron-like pseudo-total magnitudes in column 39 gives the best estimate of the object's magnitude (in non-crowded regions). Users who require z- or u- limited catalogues should use these files with caution. FILENAME: CFHTLS_D_[filter]chisq_[RA][DEC]_T0002.cat and CFHTLS_[D/W]_[ugriz/gri]chisq_[RA][DEC]_T0002.cat .|
|Catalogs||These catalogues ('.cat' files) were extracted using only the stacked image and the weight map for each filter. Each catalogue id for each filter for a given field refers to a different object. Note that these catalogues have not been corrected for galactic extinction. For almost all purposes, the Kron-like pseudo-total magnitudes in column 39 gives the best estimate of the object's magnitude (in non-crowded regions). Columns of catalogs with '.cat' extentions are defined in the '.param' files. FILENAME: CFHTLS_[D/W]_[filter]_[RA][DEC]_T0002.cat . (please read the Notes on the catalogue names and contents for the deep fields for some useful details)|
|Three colour images||Can be produced on request by combining the stacked FITS images in 3 filters (hereafter: colorlist) using "STIFF". These 2 x 2 binned JPG (for the Deep only in the archive, or PNG/TIF/JPG for the Wide or Deep or Very Wide on request) colour images are not fully optimised for public outreach posters but only for quality assessment, to check whether colour gradients remain in objects between each filter, in particular far from the centre of the field. FILENAME: CFHTLS_[D/W]_[colorlist]_[RA][DEC]_bin22_T0002.jpg .|
|Qfits-in|| Provide quality assessments produced by QualityFITS on ALL images transferred on TERAPIX disk. Astronomical and technical meta-data regarding each image as well as quality control information are avalaible to all Canadian and French institutes at the restricted access URL Qfits . The list of images selected for the T0002 release and details on each image can then be recovered using the following selection criteria in the front panel: acquisition between June 1st, 2003 and November 22, 2004; seeing <1.1 (except u band <1.4); airmass < 1.5; exposure time >60 sec., TERAPIX quality class A or B. A summary table of selected targets can be saved using the option "Export the whole results in CSV" (ASCII file).
Qfits panels contain 500 x 550 pixels PNG images of each field and their attached weight and background maps, a PSF map image of the field (one PSF model per CCD), histograms of sky background and seeing over each CCD, a preliminary estimate of galaxy and stellar number counts (rough galaxy-star separation, no mask used to avoid defects and very noisy parts of the CCDs in order to use the effective Megacam area. Have a look to the plots of galaxy counts given above that have been done carefully.), with the selected objects shown on rh-mag. diagrams, a summary table (global or for each CCD) with a link to binned 4 x 4 PNG images of each CCD. The PNG field image and its attached weight map are clickable PNG files that provides a 1000 x 1000 resolution PNG image, if needed.
IMPORTANT NOTICE: all qualityFITS-in data products, including individual weight map images attached to each Megacam image, are sent to CADC and available to CFHTLS Users
|Qfits-out||QualityFITS quality assessments information on final stacked image. Provides similar panels with the same information as Qfits-in described above (except for the PSF map image of the field which has a 10' resolution. A summary FITS table provides information on stacked images . FILENAME: CFHTLS_[D/W]_[filter]_[RA][DEC]_T0002_diag.fits.|
|Files delivered to CADC||File description and names delivered to CADC are listed in the T0002 explanatory document .|
Back to Terapix T0002 main page.