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.
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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
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Bright Star Artifacts
Diffraction spikes from stars can trigger false detections or alter the flux for a real source. For the brightest stars, sources along the diffraction spikes have been deleted from the PSC. Surviving sources which are possibly contaminated by diffraction spikes have a "D" entry in their confusion flag (cc_flg).
Light scattered within the telescope and camera optics can trigger false detections in the vicinity of extremely bright stars. For the First Incremental Release catalog sources lying within a magnitude-dependent circular regions around bright stars have been removed from the catalog if they are below a specified brightness threshold relative to the bright star. Objects which remain in the PSC but which are close to bright stars have a cc_flg value of "C".
NICMOS3 detector material produces a latent image after exposure to a bright source which fades over a timescale of order 10 seconds. This persistence effect produces a trail of spurious sources in the wake of a bright star at exactly the 2MASS frame spacing. Since the position of these artifacts are well known they are flagged in the catalogs. Since a real source could lie under one of these afterimages, persistence is flagged with a "probability of persistence" and the catalog has been selected to include only sources with <50% persistence probability. Sources with persistence probability between 10%-50% are indicated by a "P" value in the cc_flg in the appropriate band.
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.
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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.
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.
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.
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.
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.
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.
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).
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.
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]
ii. Out-of-Field Bright Source Artifacts
iii. Photometric Uncertainties
iv. Cross-scan Photometric Bias
iv. High source density regions
v. H-band Detection Thresholds and Atmospheric OH Airglow
vi. Asteroids
vii. Extremely faint sources
viii. Optical Associations
ix. Position Reconstruction
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