Isophotal Photometry

The standard 2MASS aperture is the ellipse fit to the Ks-band 20 mag arcsec-2 isophote, corresponding to roughly 1-sigma of the typical background noise in the 2.2 µm images. This aperture is determined using the axis ratio (b/a) and position angle (P.A.) of the fit to the 3-sigma isophote, allowing the semi-major axis (r20) to vary so that the mean surface brightness along the ellipse is 20 mag arcsec-2. The integrated fluxes within this ellipse in the background-subtracted J, H and Ks mosaic images are then calculated.