IV. 2MASS Data Processing

4. Point Source Detection and Photometry


This section contains descriptions of the source detection, photometry, band-merging and optical source association operations conducted during 2MASS pipeline processing. While the photometry techniques described herein are optimized for point source characterization, in fact all sources, including true point sources, multiple stars, nebulae and galaxies, are detected and measured in this phase of processing. Because resolved objects are not filtered out of the PSC, users should be aware that the point source photometry listed for non-point-like sources may not accurately represent their true brightness (e.g. I.6.b.xii). True extended sources are distinguished from point-like and multiple star detections, and characterized using optimized photometry algorithms in the extended source processing phase (IV.5).

Point sources are detected and characterized in each scan of a Tile in different ways depending on their brightness relative to the saturation levels of the 2MASS 51 ms "Read_1" and 1.3 s "Read_2" exposures. The default magnitudes listed for each PSC source represent what are the appropriate measurements in each band relative to saturation levels, as determined by the automated processing.

Aperture photometry of high SNR, non-saturated sources on the 1.3 s exposures is assumed to define the true photometric scale for 2MASS. In the absence of confusion, such measurements best capture the total flux of points sources and are least influenced by effects such as focal plane distortion. Therefore, profile-fitting magnitudes, and those derived from aperture photometry on the "Read_1" frames are normalized to the curve-of-growth-corrected aperture photometry of bright sources from the "Read_2" frames (IV.4.c.i). For consistency, the nightly photometric calibration transformations applied to all point and extended sources, and to the Atlas and Galaxy Postage Stamp Images are derived using the curve-of-growth-corrected aperture photometry of standard stars observed in the calibration observations (IV.8).

[Last Updated: 2003 March 10, by R. Cutri]

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