I. Introduction

6. Cautionary Notes

b. Point Source Catalog (PSC)

i. Origin and Quality of Photometry

Photometry of point sources is performed several different ways during 2MASS data processing, and the "default magnitudes" listed for each source in the PSC can have different origins depending on source brightness and environment. The source and quality of the "default magnitude" fields in the First Release PSC are summarized by the "rd_flg", "bl_flg" and "cc_flg." It is essential that users refer to these flags when interpreting photometry for any source in the Catalog. Each one of these flags has three characters, each corresponding to one band; the first character is the J-band value, the second is the H value, and the third is the Ks value.

Faint Stars

The majority of sources in the 2MASS PSC have default magnitudes obtained using profile-fit photometry performed simultaneously on the combination of all six individual 1.3-s "Read 2-Read 1" (R2-R1) exposures. These sources are indicated with a rd_flg value of "2" in the appropriate bands. Occasionally, the profile-fitting photometry routines will fail for sources in crowded environments, or that lie in regions with complex backgrounds. If a valid aperture photometry magnitude is available it will be listed in the default magnitude field in the appropriate band. However, such magnitudes are highly uncertain. These objects have a rd_flg value of "4" in the affected bands.

Bright Stars

Sources brighter than 7-8 magnitudes will saturate in the 1.3-s R2-R1 exposures. These objects have default magnitudes from aperture photometry performed on the 51-ms Read1 frames. Such sources have rd_flg=1 in the appropriate band.

Saturated Read1

Stars brighter than approximately 5th magnitude will saturate in even the 51-ms exposures. The default magnitude assigned for these objects is -99.999, and the rd_flg value for the appropriate band is "3".

Bright Star Filler Entries

The very brightest stars in the near-infrared sky saturate the detectors so heavily that they may not be unambiguously detected during processing. Placeholders have been included in the 2MASS First Release PSC to account for these very bright stars. The default magnitudes for these objects is always -99.999, and the rd_flg value is "8" in all three bands.

The following table summarizes the rd_flg values and their significance with respect to the origin of the "default magnitudes" in the PSC records. The "rd_flg" is a 3-character string, where the first character refers to the J-band, the second to H and the third to Ks.

RD_FLG Value (1 per band)
Source of Photometry
Not Detected In That Band - Quoted Magnitude is a 95% Confidence Upper Limit
R1 Aperture Photometry
R2-R1 Profile-Fit Photometry
Saturated in R1 - Default Mag=-99.999
R2-R1 Aperture Photometry - Extremely Uncertain!
Very bright star - Default Mag=-99.999; Position provided from External Catalogs for Informational Purposes Only

Blended Sources

The blend flag (bl_flg) included with each point source record indicates the number of components fit simultaneously in profile fit photometry (rd_flg="2"). This provides a measure of source density and possible confusion

BL_FLG Value (1 per band)
Source not detected in that band
Single profile fit to isolated source
Multiple sources detected in small region and fit simultaneously. Value indicates the number of components fit

Bright Star Artifacts

The CC_FLG also encodes several other conditions that indicate challenges to point source photometry, such as confusion during bandmerging, and during the duplicate source rectification procedure. The following table summarizes the possible values in the CC_FLG.

CC_FLG Value (1 per band)
Nature of Artifact or Confusion
Source is unaffected by artifacts, or source not detected in that band
Persistence Image from Bright Star
Nearby Diffraction Spike
Horizontal "Stripe" due to Bright Star
Confusion with nearby source, or during duplicate source rectification
Confusion in Bandmerging

ii. Out-of-Field Bright Source Artifacts

The brightest stars can cast artifacts into adjacent survey tiles. Diffraction and scattered light masking has been carried out across tile boundaries within the combined database of all tiles in the First Release, using positions of bright 2MASS detected stars and bright stars drawn from a sample of near-infrared bright sources taken from the several sources (cf. IV.7), (some of which may have been too bright for 2MASS to detect as point sources). The external artifact "seed" list likely contains objects which, either due to an errant flux in the external catalogs or variability, was not a bright source in the 2MASS observations. This will yield an occasional mask on the sky which has no apparent associated extremely bright source.

iii. Photometric Uncertainties

Several measures of photometric uncertainty are provided for each 2MASS "default magnitude". The first, "msig" is the pure measurement error returned by the respective photometry algorithm. "Msigcom" is the root-sum-square combination of "msig" with the uncertainty in the nightly photometric zero-point offset, the estimated flat-fielding residual (0.005 mags), and the R1 photometric normalization uncertainty, for bright sources. Finally, the scan signal-to-noise ratio, "snr", is the ratio of the source brightness to a characteristic "point source noise" derived from the mode of the distributions of "msig" for all sources in a tile.

5% of the bright H-band sources have approximately 50% larger than typical uncertainties due to an as yet unidentified problem with one of the production PSF's used to fit the point source profile for a particular image size. The fluxes are these sources appear not to be biased in any significant way relative to the rest of the survey sources. Similarly about 15% of the K-band sources appear to have about 30% smaller than typical uncertainties due to a similar phenomenon.

iv. Cross-scan Photometric Bias

The profile-fit photometric algorithm is sensitive to small variations in the shape of the PSF across the focal plane. As a result, fluxes of sources in the First Release PSC at the extreme east-west edges of the arrays can be biased as much as a few percent relative to the center of the scan.

iv. High source density regions

In crowded regions the source extraction threshold is automatically raised in response to elevated confusion noise so that extracted source flux limits are brighter than those that prevail at high galactic latitude. Analysis has shown that the PSF extraction uncertainties are a reasonable representation of the confusion noise in these dense regions.

v. H-band Detection Thresholds and Atmospheric OH Airglow

The point source extraction threshold algorithm that responds to confusion noise in high source density regions also responded to elevated noise due to low frequency structure in the image backgrounds due to OH airglow. This results in an effective loss in sensitivity in the H-band relative to J and Ks in severe airglow conditions, leading to a number of J and Ks detected sources.

An improved noise estimator that is not sensitive to the large scale structure in the airglow backgrounds is now being used to process data obtained later in the survey.

vi. Asteroids

Sources which are at the positions of known asteroids at the time of the observations are flagged in the "mp_flg" column. These are positional associations, and not necessarily identifications. Therefore, they remain in the PSC.

vii. Extremely faint sources

Sources with SNR>7 fluxes in any one band were entered into the Catalog. Fluxes in the remaining two bands may lie well below this threshold, may be entirely dominated by noise, or may be reported as upper limits. The magnitudes in these bands will typically be unphysically faint, or will have associated uncertainties that imply a non-detection (i.e. 95% confidence upper limits).

viii. Optical Associations

Every 2MASS source was positionally associated with an optical counterpart drawn from the ACT or USNOA catalogs. At high latitudes approximately 90% of the sources have optical counterparts. For convenience, the 2MASS PSC contains ACT or USNO-A B and R band fluxes and positional offset information from the detected 2MASS source. These are positional associations, however, and not necessarily identifications.

ix. Position Reconstruction

2MASS positions are tied to the Hipparcos Tycho reference frame on a tile-by-tile basis. Typically there are an abundant number of Tycho stars in a tile and the astrometric solution yields positions accurate to <0.2" for bright sources. A small fraction of tiles contain few Tycho stars and the positional solution may random walk to an amplitude of as much as 1". The astrometric uncertainties account for this random walk.

There is also known to be a ~0.09" bias in the reconstructed declinations of the bright Read1-only-measured stars relative to fainter stars. No measurable bias exists in right ascension.

[Last Updated: 1999 May 5; by R. Cutri and M. Skrutskie]

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