IV. 2MASS Data Processing

4. Point Source Detection and Photometry

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.

  1. Frame Source Detection and Aperture Photometry
  2. Profile-Fit Photometry
  3. Aperture Photometry and Curve of Growth Correction
  4. Very Bright Stars
  5. The Default Magnitude
  6. Band Merging and Bandfills
  7. Optical Source Associations

[Last Updated: 2000 June 20; by R. Cutri, E. Kopan, K. Marsh, T. Evans, J. Carpenter]

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