IV. 2MASS Data Processing

4. Point Source Detection and Photometry

Overview

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.

• Sources fainter than approximately J=9, H=8.5 and K_s=8 mag are are unsaturated in the 1.3 s exposures and are detected on the combined set of frames (usually six) covering each point on the sky (IV.4.a). These sources are centroided and photometered using profile-fitting (IV.4.b) and multi-aperture photometry (IV.4.c) performed simultaneously on the six individual frames. The default magnitudes for these are almost always taken from the profile-fitting photometry, although the curve-of-growth-corrected aperture photometry is also provided.



• Sources in the approximate brightness range 4.5 < J < 9, 4 < H < 8.5 and 3.5 < K_s < 8, saturate the 1.3 s exposures, but remain non-saturated on the 51 ms "Read_1" exposures. These objects are detected, centroided and measured using aperture photometry on the 51 ms frames (IV.4.a.i).



• Stars brighter than J<4.5, H<4 and K_s<3.5 saturate even the short 51 ms exposures. These objects are detected and centroided on the "Read_1" frames, and have brightness estimates made using a 1-d radial profile fitting procedure applied to the non-saturated wings of their images on the individual "Read_1" frames (IV.4.a.ii).

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]