Data Description

Naming Convention

The names are of two sorts. The first sort starts with "G" (for "group") followed by a generation number (which is incremented every six months), followed by numbers indicating the RA and Dec of the group centre. For example: G002.003.844+33.133 is and older group centred on RA=33.133 and Dec=3.844. The other sort is based on survey from which the came, either the CFHTLS (with names like D3, W4 etc.) or the NGVS (NGVS+1+1 for example).

The group names are followed by a character(s) indicating the filter UGRIZ. The i.MP9702 filter is denoted "I2".


These are simple FITS files (not multi-extension FITS files) measuring about 20000 pixels by 20000 pixels or about 1 degree by 1 degree, depending on the input dither pattern, and are about 1.7 Gb in size. They have a sky level of 0 counts. They are scaled to have a photometric zero-point of 30.000 in AB magnitudes. That is to say, for each source:

AB_magnitude = -2.5 * log10(ADU) + 30.000

The conversions to Vega and SDSS magnitudes can be found on the filter page.

Weight images

These are delivered with in a compressed format. To uncompress them, use CFITSIO's imcopy. For example:

imcopy G002.003.844+33.133.G.weight.fits.fz G002.003.844+33.133.G.weight.fits

The values of a weight map pixel is proportional to the inverse of the variance of that pixel in the main image.


All magnitudes are on the AB system. The catalogues have a header describing the columns:

#   1 NUMBER          Running object number
#   2 X_IMAGE         Object position along x                         [pixel]
#   3 Y_IMAGE         Object position along y                         [pixel]
#   4 ALPHA_J2000     Right ascension of barycenter (J2000)           [deg]
#   5 DELTA_J2000     Declination of barycenter (J2000)               [deg]
#   6 MAG_AUTO        Kron-like elliptical aperture magnitude         [mag]
#   7 MAGERR_AUTO     RMS error for AUTO magnitude                    [mag]
#   8 MAG_BEST        Best of MAG_AUTO and MAG_ISOCOR                 [mag]
#   9 MAGERR_BEST     RMS error for MAG_BEST                          [mag]
#  10 MAG_APER        Fixed aperture magnitude vector                 [mag]
#  11 MAGERR_APER     RMS error vector for fixed aperture mag.        [mag]
#  12 A_WORLD         Profile RMS along major axis (world units)      [deg]
#  13 ERRA_WORLD      World RMS position error along major axis       [pixel]
#  14 B_WORLD         Profile RMS along minor axis (world units)      [deg]
#  15 ERRB_WORLD      World RMS position error along minor axis       [pixel]
#  16 THETA_J2000     Position angle (east of north) (J2000)          [deg]
#  17 ERRTHETA_J2000  J2000 error ellipse pos. angle (east of north)  [deg]
#  18 ISOAREA_IMAGE   Isophotal area above Analysis threshold         [pixel**2]
#  19 MU_MAX          Peak surface brightness above background        [mag * arcsec**(-2)]
#  20 FLUX_RADIUS     Fraction-of-light radii                         [pixel]
#  21 FLAGS           Extraction flags
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