Quick description of CFHTLS T0003 data products and filenames
Brief summary of data processing steps
Data processing methods and algorithms are largely unchanged since T0002. An detailled explanatory document of the T0003 release is in progress and will be available soon. It will however be very similar to the T0002 document, so users who urgently need detailled information on the releases can read the T0002 document. Note that the SExtractor parameters are slightly diferent from T0002. They are given in the T0003 SExtractor parameter list.
To summarize, the main steps in data processing are as follows:
- Images are selected using the following criteria: TERAPIX quality class A/B, exposure time > 60s, seeing < 1.1'' (except for u* where this is 1.4")
- After image selection, an astrometric solution is computed using the TERAPIX astrometric tool 'SCAMP'
- Finally, images are coadded using SWARP and stack+weightmap images, catalogues and output quality assesments are produced for each. The summary of each stack available for the T0003 Deep+Wide relase is given on the table T0003.
When at least the g,r and i-band stacks are available, a gri chisquared image is constructed and object catalogues are constructed using SExtractor in 'dual-image' mode to provide a matched object catalogue, one for each filter.
CFHTLS Deep and Wide fields
Positions of Deep and Wide fields in the sky
Data products and filenames
As in previous releases, all stacks and weightmaps are provided in the standard TERAPIX image 'quanta' of 19354x19354 pixels of size 0.186"/pixel
All magnitudes are in the AB MEGACAM instrumental system
Zero-points for all stacks are 30.0 (magnitude=-2.5*log(counts) + 30.0)
For each stacked image, THREE catalogues are available:
- (a) a binary FITS table ldac catalogue extracted using SExtractor in single image mode using as input image the stacked image and its corresponding weight map. Filenames write CFHTLS_D/W_filter_RA-DEC_T0003.ldac. Example for a wide field in r-band with the Megacam stack centered at (02:20:21; -08:52:00) : CFHTLS_W_r_021021-085200_T0003.ldac.
- (b) A chisquared catalogue, extracted using three images: a chisquared detection image derived from the chisquared sum of g', r' and i' stacks, with measurements carried out on the input image and it's corresponding stack. These catalogues contain almost all available SExtractor parameters. Filenames are different for the Deep and the Wide:
Deep chi2: CFHTLS-chsq_gri_RA_DEC_filter_T0003.cat
Wide chi2: CFHTLS_W_filter_022150-060400_T0003.cat
Wide chi2-wo : CFHTLS_W_filter_022150-060400_T0003_wo.cat
where "filter" is u*,g,r,i or z. The "wo" stands for "with offset" and means that an offset has been applied to the catalogues in order to rescale the stellar colour-colour plots. We have chosen the D2 field as a reference frame. However, we recommend to use the non-rescaled catalogues since the comparison with the SDSS data show an excellent agreement (for the Deep and Wide).
The chi2 catalogues contain the following : T0003 SExtractor parameters ;
(c) A merged "panchromatic" chisquared catalogue, comprising a subset of the parameters available in (b). There is one of these catalogues for each pointing, and it contains magnitudes for each object detected in (b) for each of the filters available at this pointing. This catalogue also provides TERAPIX quality information about each extracted object. A more detailed description of each column in this catalogue now follows.
Merged panchromatic catalogues: column description
These catalogues have the following names:
W1/2/3 : CFHTLS_W_ugriz_RA-DEC_T0003_chi2.cat
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 in step (b).)
(1); id : the SExtractor object id. This corresponds to the object ID which appears in catalogue (b)
(2,3); x,y (pixels): object pixel co-ordinates. Pixel scale is 0.186"/pixel
(4,5); RA,DEC; right ascension and declination in J2000 co-ordinates
(6): r2 (pixels): radius enclosing half the light
(7): flag : TERAPIX object flag. Objects with flag 0 are 1 are galaxies or (non-saturated) stars, respectively, outside the masked zones. (Note that star-galaxy separation is only carried out on the i* image to a limit of i*=21.0; fainter than this limit, all objects are identified as stars).
(8,9,10,11,12) : u, g, r, i, z : Object MAG_AUTO magnitudes in the MEGACAM instrumental reference frame
(13,14,15,16,17): uerr,gerr, rerr, ierr, zerr: Object magnitude errors.
(18): E(B-V): The value of the galactic E(B-V) extinction calculated using the Schlegel et al dust map at the object's position.
These catalogues have names like:
CFHTLS_W/D_RA_DEC_T0002/3.reg . For example, the mask namefile of the D1 deep field is CFHTLS_D_i_022559-042940_T0003.reg . "W" and "D" stand for "Wide" or "Deep", "T0002" and "T0003" stand for the mask creation date (release).
These are ASCII text files, which can be read using 'ds9', which indicate which zones in each stack were flagged as being 'bad'. Zone are typically flagged in this way because of proximity to bright stars or border areas. It is however important that each user modifies the mask according to their own scientific goals. The flagged objects and the polygons shapes generated by the Terapix masking process may not be suited.
Data quality assessment and comparisons with previous releases
Magnitudes for bright sources in T0002 and T0003 should agree to within 0.01 magnitudes.
IMPORTANT NOTE Object pixel co-ordinates in T0002 and T0003 are not the same. In some cases this shift can be as much as half a pixel. We recommend that, if wish to use use object pixel positions from T0002, that you recenter these positions on objects from T0003.
Absolute magnitude comparisons: We have compared magnitudes from the SDSS with CFHTLS-T0003 using transformations kindly provided by N. Regnault (SNLS). For non-saturated stellar sources, the CFHTLS and SDSS magnitudes in g,r,i and z filters in the D2 and D3 fields agree to within 0.02 magnitudes. For the CFHTLS-Wide, we only have few common fields between W1 and W3 and some SDDS fields. As shown on these SDSS/Wide comparison plots , the photometry agree within 0.03 magnitude in the filters g',r' and i'
What has changed since release T0002 (released by June, 2005)
This release contains observations taken over a longer period of time. This means substantially deeper stacks for images in fields D2,D3 and D4 and much more Wide fields. There is essentially no change in the depth of the D1 images.
All QualityFITS image quality assessment information has been regenerated using the latest version of the TERAPIX tools (note that TERAPIX image quality assessments have been conserved from T0002).
We now use 'windowed' object position measurements which are around 30% more accurate that the old weighted centroid measurements used in releases T0001 and T0002. All astrometric solutions were computed using these windowed position measurements.
Software versions used in this release
This release has been processed using the following versions of the TERAPIX software tools:
SExtractor version 2.4.4 (2005-10-19)
SCAMP version 1.2.1-MP (2005-10-27)
SWarp version 2.15.6-MP (2005-09-15)
PSFEx version 1.15 (2004-12-11)
WeightWatcher version 1.7 (2005-08-27)
STIFF version 1.10 (2005-01-10)
QualityFITS version 1.12.1 (2005-09-23)