Photometric Calibration
The Large Galaxy ATLAS mosaics are derived from 2MASS "coadd" images, described here. The "coadd" images are calibrated on the 2MASS system (see links below). The mosaic image is calibrated/scaled to the central "coadd" that passes through the nucleus of the galaxy; hence, one zero-point calibration applies to the entire mosaic image.
The conversion of a 2MASS unit of flux ("dn" for data number, corresponding to the pixel value) is as follows:
m = m_0 - 2.5*log(f)
where f is the background-subtracted flux (in "dn" units), m_0 is the zero point calibration magnitude, and m is the desired (calibrated) magnitude. The zero points are given in the FITS header, with key words JMAGZP, HMAGZP and KMAGZP, respectively; see example below. The extracted photometry was calibrated in this fashion.
FITS header
BITPIX = -32
NAXIS = 3
NAXIS1 = 909
NAXIS2 = 909
NAXIS3 = 6
ORIGIN = '2MASS ' / 2MASS Survey Camera
CTYPE1 = 'RA---SIN' / Orthographic Projection
CTYPE2 = 'DEC--SIN' / Orthographic Projection
CTYPE3 = 'JHKs cube' / 1=J,2=H,3=Ks; 4=J,5=H,6=Ks
EQUINOX = 2000. / Equinox
RA = 202.469574 / RA of source, J2000 (deg)
DEC = 47.19526291 / Dec of source, J2000 (deg)
CRVAL1 = 202.469574 / RA at Frame Center, J2000 (deg)
CRVAL2 = 47.19610214 / Dec at Frame Center, J2000 (deg)
CRPIX1 = 455.
CRPIX2 = 455.
CROTA2 = 0. / Image Twist E of N, J2000 (deg)
CDELT1 = -0.0002777777845 / Axis 1 Pixel Size (deg)
CDELT2 = 0.0002777777845 / Axis 2 Pixel Size (deg)
JSKYSIG = 0.5242551565 / GALWORKS J Noise Measurement
HSKYSIG = 1.011476755 / GALWORKS H Noise Measurement
KSKYSIG = 1.019428134 / GALWORKS K Noise Measurement
JMAGZP = 20.86650085 / Photometric J zero point
HMAGZP = 20.6576004 / Photometric H zero point
KMAGZP = 20.04360008 / Photometric K zero point
COMMENT > Planes 1, 2 and 3 represent the background
COMMENT > subtracted images of the J, H and Ks bands;
COMMENT > Planes 4,5,6 represent the background **and**
COMMENT > star subtracted images of the J, H and Ks bands
HISTORY > 2MASS Large Galaxy Mosaic
HISTORY > created by Tom Jarrett, IPAC/Caltech/JPL