The MegaPipe image stacking pipeline

The MegaPipe image stacking pipeline Data description

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