Back to Terapix T0005 page.
This description of data products is valid for the T0005 release only. It does not apply to any previous release.
NEW feature: Deep stacks with the new i filter: the old i filter, i.MP9701, was broken last year and replaced by a new one, i.MP9702. The transmissions of the two filters are significantly different, so i.MP9701 images were not combined with i.MP9702 ones.
There are 9 W1 fields observed with the new filter ( CFHTLS_W_020241-104400, CFHTLS_W_020631-104400, CFHTLS_W_021021-104400, CFHTLS_W_021410-104400, CFHTLS_W_021800-104400, CFHTLS_W_022150-104400, CFHTLS_W_022539-104400, CFHTLS_W_022929-104400 and CFHTLS_W_023319-104400) and 2 W3 fields observed with both the old and the new filter (CFHTLS_W_141754+533431 and CFHTLS_W_143553+523831). For the W3 fields the i and y data have been stacked separately, as for the deep data.
For the Deep, we had to split the data into 2 samples with different names. The deep stack with i.MP9702 are named *_i_*.fits and the i.MP9702 will are named *_y_*.fits .
| Input images in Deep and Wide | - | - | - |
| CFHTLS Wide W1 list : W1 lists | CFHTLS Wide W2 list : W2 lists | CFHTLS Wide W3 list : W3 lists | CFHTLS Wide W4 list : W4 lists |
| CFHTLS Deep D1 list : D1 lists | CFHTLS Deep D2 list : D2 lists | CFHTLS Deep D3 list : D3 lists | CFHTLS Deep D4 list: D4 lists |
| Product | Description : more details in the T0005 explanatory document (recommended) |
| Stacked Images | All stacks are produced by a median of a set of input images, weighted accordingly, and combined using SWarp, with a Lanczos3 interpolation kernel. Equatorial Coordinate system, J2000 (ICRS). Projection: TAN.
The SWarp configuration file contains CENTER, IMAGE_SIZE , PIXEL_SCALE parameters which are used for each tile. For T0005, as for previous releases, all images have 19354x19354 pixels, 0.186 arc-second/pixel. The center positions of each stack are fixed and correspond to the center positions of each CFHTLS pointing, as pre-defined in the CFHTLS Deep and Wide center list. The "Cartesian" W(i,j) tile organisation and name definition of Wide target positions are defined in the W1 target list, the W2 target list, the W3 target list and in the W4 target list. Astrometric reference catalog used: 2MASS. Each image is composed of one single FITS file (all 36 MEF extensions are combined). FILENAME: CFHTLS_[D*/W]_[filter]_[RA][DEC]_T0005.fits , where the ''D*'' represents Deep D-25 (25% best seeing images) or D-85 (85% best seeing images) data and ''W'' for all Wide fields. NEW: [filter] is u*,g,r,i,z OR y=new i-band filter. All stacks and links to Quality-FITS out quality assessment pages are available at
Important new feature for T0005: Images (as well other compressed files at CADC) are compressed using gzip (.gz files) instead of the RICE algorithm used in previous releases. |
| 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 processing and output weight maps are produced during the image stacking using Weightwatcher in 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]_T0005_weight.fits . |
| Chi2 Image | Constructed by combining images using "SWarp" (hereafter=filterlist)- Pixel values are actually sqrt(chi2) , not chi2. There is one Chi2 image per field (using combined g, r and i images only), but ONLY when g, r and i stacks are available. Chi2 image is generated using the swarp.conf of the co-added images, but with the 'COMBINE_TYPE' chi2' option.
FILENAME: CFHTLS_[D*/W]_gri_[RA][DEC]_T0005.fits . |
| Catalogs | There are two types of catalogues: the '.ldac' and the '.cat'. For the .cat catalogues, u,g,r,i and z are separate, but all are based on a chi2 detection image. The .cat files are a ugri(y)z catalogue containing a subset of parameters.
FILENAME : CFHTLS_[D*/W]_[FILTER]_[RA]-[DEC]_T0005.ldac or .cat (see below) |
| 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.
Columns of catalogs with '.cat' extentions are defined in the '.param' files. They are listed in the CFHTLS catalogue parameter list. Note that these catalogues have not been corrected for galactic extinction. However, for stacks having at least gri data, each u, g, r,i (y), z catalogue contains the value of the galactic E(B-V) extinction, as calculated using the Schlegel et al dust map at the object's position (last column added). Users who require z- or u- limited catalogues should use these files with caution. Important notice: The .cat catalogues are produced using specific selection criteria during the SExtractor step applied to the chi2 image and during the u/g/r/i/z merging phase. There are certainly not suitable for all scientific goals. Please have a look at the configuration and parameter list files. In case you need different selections, you may need to produce your own .cat catalogue, or possibly your own chi2 image. FILENAME: CFHTLS_[D*/W]_[FILTER]_[RA]-[DEC]_T0005.cat |
| Mask files | Built automatically using the "polygon" option of the DS9 visualisation tool to create a "*.reg" file which describes, in world co-ordinates (WCS), the regions of each image which should not be used for scientific analysis. The files mask all bright stars and the boundaries of each field. The size of the masked areas at star position is scaled according to their USNO-B B magnitude. Each mask can be used with DS9 (or SAOimage, after they have been transformed to pixel co-ordinates). Polygons may be added or removed according to user wishes It is important that any users check the masks and modify them according to its scientific needs.
Masks are constructed based on the positions of bright stars in the USNO-B catalogue. FILENAME: CFHTLS_[D*/W]_[RA][DEC]_T0005.reg (note: there is no more filter in the mask name). |
| Merged chi2 catalogues | The merged catalogues use the catalogue-chi2 to produce a simpler ugriz catalogue by merging their most useful columns into one single file. Each merged catalogue contains the following information (With the exception of the TERAPIX quality flag, all this information is available in each of the separate catalogues produced from the chi2 image. )
Relevant input files:
FILENAME : CFHTLS_[D*/W]_ugriz_[RA]_[DEC]_T0005.cat |
| Galaxy counts | Galaxy number counts per 0.5 mag., per deg2, computed. This is a rough count estimate based on the single filter detection (no chi2 image used at this stage): no mask, no galactic extinction applied (note that based on Terapix past analyses of T0002 and T0003, using the chi2 image and the mask should push the magnitude limit 1.0 to 1.5 magnitude deeper). Magnitudes are SExtractor MAG_AUTO. No extinction correction has been applied. The magnitude system is instrumental AB ; there is one plot per filter. To each plot a comparison to the galaxy count expectation is overplot, based on previous Megacam CFHTLS Deep galaxy counts.
FILENAME: CFHTLS_[D*/W]_[FILTER]_RA-DEC_T0005_gal_histo.ps. [Col. 13 of Synoptic T0005 Table] |
| Stellar colour-colour | Stars were selected by automatically locating the vertical stellar branch in the compactness(rh)-magnitude diagram in a series of 10 arcminutes 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 overploted for comparison. The Pickles input data have not been corrected from atmospheric transmission, nor from galactic extinction. The selected stars of Megacam fields have not been corrected from galactic extinction. Terapix provides one plot per field per filter-triplet and a maximum of three plots when all five filters are available. A compilation of all colour-colour Deep or all colour-colour Wide plots of T0005 is available via the Synoptic T0005 Table (two merged ps file , last row of the table) FILENAME: CFHTLS_[D*/W]_RA-DEC_T0005.[FILTER-1]m[FILTER-2]_[FILTER-2]m[FILTER-3].ps, where ''m'' stands for ''minus'' for the colour term (e.g.: rmi = (r-i) = [FILTER-1=r]m(inus)[FILTER-2=i]). [Col. 14 of Synoptic T0005 Table]. |
| Three colour images | Produced by combining the stacked FITS images in 3 filters (the 3-filterlist, like irg, igu or zir) using STIFF . These 2 x 2 binned colour images are not optimised for public outreach posters. The R,G,B color balance and cuts are primarily optimised for visual inspection as a quality assessment tool. It is primarily used to check filter-to-filter astrometric calibration and whether colour gradients are visible in objects, resulting from poor overlaps between each filter, in particular far from the centre of the field.
FILENAME: CFHTLS_[D*/W]_[3-filterlist]_[RA][DEC]_T0005.jpg . |
| Mag. ZP | Photometric calibration is carried out in two steps.
The magnitudes of objects in the final stacks are computed as follows: m=30-2.5log(counts). In some fields, offsets (probably relating to photometric calibration problems at CFHT) may need to be applied. See the table the CFHTLS(ZP) / SDSS offset table values for more information. In this case the effective zeropoint is (30+Corr). Note, however, all of the catalogues delivered to CADC were computed with a photometric zero point of 30. The magnitude system is instrumental AB |
| Dust : galactic extinction | The E(B-V) is provided directly as a parameter in the final catalogues. It is derived for each object position from the E(B-V) images from Schlegel et al .
The E(B-V) value derived at an object position is computed using dust.c provided with the Schlegel et al. dust data, with the interp flag set to "y" to return a linearly interpolated value from the 4 nearest pixels. |
| Astrometric and field-to-field photometric calibrations | T0005 astrometry and field to field photometric rescaling were calibrated with the new Terapix tool SCAMP (Bertin et al 2007). 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-positioned 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 (2MASS J-band for T0005) 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 2MASS astrometric reference catalogue. 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 17 mas RMS. Specific T0005 astrometric and photometric calibration details are given here . Note that for T0005, 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 via the Synoptic T0005 Table as a tar file through the 'scamp' link. 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 labeled 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. The photometric rescaling values are the .ahead file located in the same tar file as the astrometric solution (see previous paragraph). For T0005 astrometric/photometric calibration and rescaling were done with SCAMP 1.3.7-MP. Quality assessments are summarized in the series of plots and tables. Both are available via the Synoptic T0005 Table (restricted access to CFHTLS registered users) and then the scamp.xml file [Col. 16-20 of Synoptic T0005 Table]. FILENAME: for input CFHT/Qfits-in images/catalogues #cfhtnumber-p.fits/ldac , the following output files are produced : #cfhtnumber-p.head and #cfhtnumber-p.ahead |
| T0005 SCAMP plots and quality assessment data | The series of SCAMP plots produced for the astrometric calibration and the field-to-field photometric rescaling are all provided in the Synotpic T0005 Table, in the columns ''Nb of files'', using the ''scamp'' link. One can then get the following:
. The 2 chi2/dof values of the best astrometric solutions found for all images calibrated together, for both rows and columns of Megacam images. . 5 plots that display the internal error (x-internal along the x-direction, x-internal along the y-direction, y-internal along the x-direction, y-internal along the y-direction and a x-y internal error in polar coordinates). Internal errors are derived from the cross-identifications and the position differences of objects in the overlapping regions of each Megacam field. . 5 plots similar to the previous ones but for the external errors. They are derived from the cross-identifications and position differences of common stars between the relevant Megacam fields and the 2MASS J-band catalogue. . A series of focal plane distortion maps: one per QrunID and one per filter. . A series of global views of all Megacam fields projected on the sky. There is usually one per group, but for T0005 only one per Wide field (W1/W2/W3/W4), one for Deep-25 and one for D-85 has been produced. . A series of plots showing the internal photometric errors along the x and the y axis. There are 2 plots per photometric instrument. As for the internal astrometric solution, photometric internal errors are derived from the cross-identifications and the position differences of objects in the overlapping regions of each Megacam field.. |
| .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 FLXSCALE).
All T0005 .head and .ahead calibration data can be obtained from the Synoptic T0005 Table, via the 'scamp' link to a tar .tgz file. The .head files of each image contain a multi-FITS header with keywords providing updated WCS astrometric data and photometric rescaling coefficients attached to each CCD. The photometric rescaling values are the .ahead file located in the same tar file as the astrometric solution (see previous paragraph). The .head files can be used to re-SWarp and re-stack FITS images, if needed, without need to run the astrometry software again. |
| Qfits-in | Provides a weight map image, the primary '.ldac' catalogue of objects (used later by SCAMP for the astrometric calibration) and a series of quality assessments produced by the QualityFITS tool for ALL images transferred on Terapix disk (STEP1 products of each release). Input images are then installed on the Terapix processing storage system when the release STEP1 process starts. Output weight map images and Qfits-in output are sent to CADC. The Qfits(in/out) tools use SExtractor, Weightwatcher, SCAMP, SWARP and STIFF and a series of more specific software.
The list of images selected for the T0005 release uses the Qfits-in evaluation as well as the following criteria to produce the astrometric .head calibration files : acquisition between May 26 2003, and Feb. 16, 2008. ; seeing <1.3 (except u band <1.4); airmass < 1.7; exposure time >180 sec. and Terapix quality class A or B (all CFHT grade 4 and 5 images have been graded C by Terapix and are therefore no longer considered during STEP2). 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 theirattached 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 givenabove that havebeendone carefully.), with the selectedobjects 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. All qualityFITS-in data products that are used in T0005, including individual weight map images attached to each Megacamimage, are available at CADC. FILENAME: for input CFHT images #cfhtnumber-p.fits (CFHT)--> #cfhtnumber-p.ldac (catalogue) , and #cfhtnumber-p_weight.fits (weight map image) |
| 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).
The number of images stacked, the total exposure time and the median seeing are automatically included in theQfits-out page. Atablelistsalso the individual images usedineachstack and links each of themto its Qfits-inpage. All Qfits-out were produced with QualityFITS v.1.13.2. At the bottom of eachpageare shownfive full resolution 512x512 sub-images that show the image quality and details at the four corners and the center of each stack. A summary FITS table provides information on stacked images. All QFITS-out data are available from this page (and the associated pages for the wide stacks). |
| 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. |
| Completeness | A completeness estimate is computed in the Qfits-out stage for all released stacks. These plots are avaiable here. The 50% and 80% completeness limits are derived from an automated curve fitting using an ad hoc Atanh function having its inflexion point forced to y=50%. Since the completeness curve should drop quickly when approaching the limit, the 50% and 80% value range in steepest part of the curve, so the mean limiting 50% and 80% magnitude errors are large, about 0.2 mag. |
| Effective FOV | Effective area is derived by subtracting from the MEGACAM fields all the masked POLYGON areas defined using the CFHTLS_[D*/W]_[RA][DEC]_T0005.reg mask files delivered to CFHTLS users. Each '.reg' file is then an input mask to Weightwatcher (see the latest news on the news about Weightwatcher). Overlaps between polygons are taken into account so that no area is subtracted several times. The effective area is same for all filters of a given field, this is the reason why the masks file names are the same for all filters. The effective areas of all T0005 stacks are given in the Synoptic T0005 Table. If necessary, more precise (4 digits) effective area values can be found in column (7) of the T0005 stack summary table.
Note that any users can modify the mask according to his own wishes, but the effective area should then be recomputed |
| Data processing | A detailed description of Terapix T0005 release T0005 explanatory document and extra-information on the processing can be found in the T0002 explanatory document . |
D1, D2, D3 and D4 
