Data Description

This page describes the MegaPipe data available for download.

Naming Convention

The names of the groups are of two sorts:

One sort looks like this:

D1.2007A
^  ^
|  |_____ version (in this case January 2007)
|________ CFHTLS Deep Field (in this case the D1)

These are images produced earlier but are included in this release as well.

The other kind of group name looks like this:

G002.003.844+33.133
^    ^      ^
|    |      |_____ Dec of centre (3.844 deg)
|    |____________ RA of centre (+33.133 deg)
|_________________ grouping generation

As more data becomes available, the grouping algorithm described here is run again. More groups will be found and existing groups may become deeper or have different bands added to them. The current release represents the second. The first generation (G001) was very restrictive and only used to develop the MegaPipe pipeline. Added to the group name is the filter name and the file type:

G002.003.844+33.133.G.weight.fits
^    ^      ^       ^ ^
|    |      |       | |__ weight image
|    |      |       |____ g' filter
|    |      |____________ Dec of centre (3.844 deg)
|    |___________________ RA of centre (+33.133 deg)
|________________________ grouping generation

Three kinds of files are provided for each filter of each group image (extension .fits) (extension .weight.fits) (extension .cat), described in turn below.

Images

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(counts) + 30.000

If you need Vega magnitudes, consult the following table:

FILTER	Vega-AB MegaCam
u	-0.346
g	+0.092
r	-0.171
i	-0.401
z	-0.554

Weight Maps

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

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.

Catalogues

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

All magnitudes are on the AB system.

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