II. User's Guide to the 2MASS All-Sky Data Release

2. Point Source Catalog (PSC):

a. Point Source Catalog Column Descriptions

b. How to Use the 2MASS PSC - A PSC Practical User's Guide

c. PSC Source Selection Criteria:

The All-Sky Release PSC contains accurate astrometry and photometry for 470,992,970 near-infrared sources extracted from scans of 59,731 Tiles covering 99.998% of the sky.

Sources in the PSC were drawn from a Working Database of all detections in these Tiles, using the selection criteria described in Section V.3 and summarized in Table 1. The point source Working Database contains approximately a factor of two more detections than are in the PSC; a significant fraction of these are detections of noise and image artifacts. The source detection thresholds in 2MASS pipeline data processing are set intentionally low to insure completeness. The demanding reliability requirements for the 2MASS PSC were achieved by applying the selection criteria listed below.

Users are recommended to review the Cautionary Notes for the PSC in Section I.6.b for known features of and problems in the catalog.

Table 1 - Point Source Selection Criteria for the All Sky Release

Signal-to-Noise Ratio (SNR) and Band-detection Source must be detected in at least one band with a SNR>7
Source must be detected in three bands with a SNR>5

The SNR thresholds can be satisfied by either the SNR derived from the photometric measurement uncertainty or the scan SNR ([jhk]_snr).

To filter out low SNR noise detections
Frame-detection In at least one detected band, the source position must not be masked out because of noisy pixels or other transient events on more than two out of the six (and sometimes seven) frames sampling its position,


SNR>20 sources must be detected at SNR>3 on at least 40% of individual the unmasked frames
The source must have non-saturated detections in all three bands AND it must have been measurable on two frames AND detected with SNR>3 on both of those frames.

Frame detection statistics are tabulated for each PSC source in the ndet column.

Hot pixel, cosmic ray, meteor trail and low reliability source rejection
Artifacts Source must not be identified as a spurious detection of an image artifact from a bright star.

Sources believed to be real, but affected by artifacts, are included in the Catalog, and are flagged using the cc_flg.

Spurious source rejection
Tile Boundaries Source lies >10´´ from edge of Survey Tile To avoid <3-band coverage areas at edges of focal plane and other edge effects
Multiply-Detected Sources For multiply-detected sources in Tile overlap regions, only the single apparition farthest from a Tile edge is selected. Sources in the overlap regions not multiply-detected are always included in the PSC, but are properly flagged so that a user can remove the selection bias resulting from multiple opportunities to appear in the catalog. (see V.4) To eliminate duplicate detections and to avoid sensitivity enhancements in the Tile overlaps through flagging.

The PSC is a Composite Catalog

The PSC generated with the selection criteria above is a composite release that has two distinct components contained in a single database. The first component consists of a higher SNR, high reliability Catalog that meets or surpasses all of the 2MASS Level 1 Requirements in unconfused regions of the sky. Very bright stars that saturate the shortest 2MASS exposures are included in this set even though they do not satisfy the photometric accuracy requirements of the Survey. The second component of the PSC is a lower signal to noise extension that primarily contains sources that reach 0.5-1.0 mag fainter J<15.8, H<15.1 and Ks<14.3 mag levels, where the PSC is >99% complete. The completeness, reliability and uniformity of the extension sources are not as good as the high-reliability Catalog subset of the PSC, and may not satisfy all of the specifications described in the Level 1 Requirements. They are included in the PSC because, when used with some care, they can be a valuable resource for certain classes of investigations in which homogeneous, statistical datasets are not necessary.

The criteria for selecting the high-reliability Catalog sources in the full PSC are listed in Table 2. The criteria are meant to be applied band-by-band, so technically PSC sources that satisfy the Catalog requirements in one band may not satisfy them all bands. These criteria are discussed in detail in I.6.b.

Table 2
PSC Catalog Subset Criterion
(applied band-by-band)
ph_qual="A" OR (rd_flg="1" OR rd_flg="3")

d. General Properties of the PSC:

e. Photometric Properties:

Photometry of bright, non-saturated sources in the 2MASS PSC has a precision of 1-2%. The photometric signal-to-noise ratio is >10 for J<15.8, H<15.1 and Ks<14.3 mag in the unconfused sky. Brightness estimates for stars that saturate the shortest 2MASS exposures are accurate to ~20%.

Photometry of point sources in 2MASS was done using different algorithms depending on how bright a source was relative to the saturation regimes of the Survey observations. The origin of the default magnitudes given for each source is specified by the rd_flg value. Sources that were not saturated on the 1.3 s "Read_2-Read_1" exposures, the vast majority of objects detected in the Survey, were measured using profile fit photometry. These have rd_flg="2" in the appropriate band. Sources that saturated the 1.3 s exposures, but not the 51 ms "Read_1" exposures where measured using aperture photometry on the short exposure frames, and are designated with rd_flg="1" in the appropriate bands. Sources that saturated even the 51 ms exposures have brightness estimates derived from 1-d radial profile fits of the non-saturated portions of their images on the 51 ms exposures. These sources have rd_flg="3" in the appropriate bands.

Figure 12Figure 13Figure 14Figure 15

f. Astrometric Properties:

2MASS PSC positions are reconstructed in the International Celestial Reference System (ICRS) via the Tycho 2 Catalog and are accurate to 70-80 mas over the magnitude range of 9 < Ks < 14 mag. The astrometric accuracy of brighter sources is approximately 120 mas. Fainter sources have astrometric accuracies that decrease monotonically with decreasing source brightness.

The astrometric performance of 2MASS is summarized in Figure 16 which shows RMS position residuals of PSC sources plotted versus Ks magnitude measured against several different references, as described below and discussed in detail in VI.5. In this figure, the two upper panels show the cross-scan (approximately RA) position residuals, and the bottom two panels show the in-scan (approximately Dec) residuals. Sources observed from the northern 2MASS observatory are shown in the two left plots, and those observed from the southern observatory are shown in the right pair of plots. In each panel, the mean positional uncertainty on each axis quoted for PSC sources are shown by the dotted black lines.

g. Solar System Objects:

Cross-Scan RMS Souther
n Cross-Scan RMS Norther
n In-Scan RMS South
ern In-Scan RMS Figure 13
Figure 16Figure 17Figure 18

(Each of the four panels in Figure 16 will appear when the reader clicks on the corresponding region of the figure "thumbnail" above.)

[Last Updated: 2009 November 23, by R. Cutri]

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