As described in the introduction to section III.3, the detection and measurement of point sources is done several times during the processing of a 2MASS scan. Each detection and measurement step serves a different purpose. Sources are first detected and aperture-photometered from the individual dark-subtracted, flattened and sky-offset corrected "Read 1" (R1) and "Read 2-Read 1" (R2-R1) frames. These detections provide positions to tie the frames together for Atlas Image generation (cf. IV.3), and photometry for objects that are saturated in the R2-R1 (1.3 sec) exposures. Source detection is done on coadded Atlas Images for maximum sensitivity. Both profile-fit and aperture photometry is carried out for these fainter detections, although the measurements are actually carried out on the individual R2-R1 data frames to avoid flux biases caused by masked or aberrant pixels. The entries for each object in the 2MASS Point Source Catalog contain a "default magnitude" field for each of the three survey bands (j_m, h_m and k_m). These values represent what are believed to be the best estimate of a source's brightness in each band. The origin of those magnitudes is described in the "rd_flg" parameter, so users are urged to consult that flag for all objects. The subsections below describe each of the detection and photometry algorithms in more detail.
[Last Updated: 2000 June 20; by R. Cutri, E. Kopan, K. Marsh, T. Evans, J. Carpenter]