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 Second Incremental Release, using positions of bright 2MASS detected stars and near-infrared bright stars drawn from 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. Meteor Trail Detections and Other Unreliable Sources
The source selection criteria used to generate
the PSC are designed
to minimize the number of spurious detections along meteor trails.
A relatively small number of residual meteor trail detections persist
in the the Catalog, though, because they satisfy the formal
Catalog selection requirements. These residual sources can be efficiently
identified as having no optical counterpart, a relatively high
2 value (>7) from the profile-fit
photometry in one or
more bands, and are detected in only one of the six 2MASS frames that
cover each point on the sky. These criteria also describe the characteristics
of other very faint "sources" which are likely spurious detections
of noise. Sources that are such suspected unreliable detections
are indicated in the PSC by having a cc_flg value of "U" in one
or more bands.
iv. Photometric Uncertainties
Two measures of photometric uncertainty are provided for
each 2MASS "default magnitude". The first, "<band>_msig" is the
pure measurement error returned by the photometry algorithm (where
<band> is the 2MASS band, either J, H, or Ks).
Because of statistical fluctuations, faint sources will occasionally
have unphysically low measurement errors.
The second photometric uncertainty quoted in the PSC is
"<band>_msigcom",
which is the root-sum-square combination of
"<band>_msig" with
the uncertainty in the nightly photometric zero-point offset, the
estimated flat-fielding residual (0.005 mag), and the
R1 photometric normalization uncertainty, for bright sources.
The "<band>_msigcom" provide a better estimate of total
photometric errors.
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 for these sources appear not to be biased
in any significant way relative to the rest of the survey sources.
Similarly about 15% of the Ks-band sources appear to have
about 30% smaller than typical uncertainties due to a
similar phenomenon.
v. 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 Second Incremental 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.
vi. 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.
In areas of high source density, the probability is high that
an image artifact from a bright sources will
fall on top of one or more real sources. The process of
artifact identification will result in many real sources being filtered
out of the Catalogs, or at least flagged as being affected by artifacts.
This is especially apparent in the cores of globular clusters, and
near the galactic center.
Aperture photometry in regions with source densities in excess
of ~65,000 deg-2 can be corrupted due to confusion in
both the source apertures and sky annuli. This can result in
errors in the empirical aperture curve-of-growth
derivation in a Tile that produce photometric biases in one or more
bands and therefore color biases of up to ~0.2 mags.
This bias is spatially correlated
within a Tile because the curve-of-growth corrections are
derived for and applied to all sources having the same seeing shape,
and the seeing is estimated on spatial
frequencies no smaller
than the scale of an Atlas Image (17 arcminutes
in declination).
Users performing statistical analyses of source colors in
within 10°-15° of the Galactic center, or any other
region where the source density can exceed ~65,000 deg-2
over a significant area, should be aware that such biases can
be present. See Section IV.4c for a detailed
discussion of this bias.
vii. 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 was introduced in pipeline
processing in early March of 1999. Northern data acquired after
21 September 1998 UT and southern data after 13 January 1999 UT
have been processed using the new noise estimator and should
be less sensitive to large scale structure. During the
reprocessing phase after the end of survey observations, all data
will be rerun with the new noise estimator.
viii. Solar System Objects
Sources which are at the positions of known asteroids, comets, planets or
planetary satellites
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. Some fraction of the putative
detections are not valid, but are chance superpositions of the predicted
positions with background sources. This is particularly likely at low
Galactic latitudes where the density of background stars is large.
See Section IV.9 for more details.
ix. Extremely faint sources
Sources with SNR>7 fluxes in any one band
were included in the PSC. 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 remaining bands can be unphysically
faint, or will have associated uncertainties that imply
a non-detection (i.e. 95% confidence upper limits).
x. Optical Associations
Every 2MASS source was positionally correlated with
the ACT or
USNO-A
optical catalogs. At high latitudes approximately
90% of the sources have optical counterparts within 5´´.
For convenience,
the 2MASS PSC contains optical B- and R-band magnitudes from USNO-A
or B- and V-band magnitudes from ACT, and
positional offset information from the detected 2MASS source.
These are positional associations, however, and not necessarily
identifications.
xi. Position Reconstruction
2MASS positions are tied to the
International
Celestial Reference System (ICRS) via the
ACT Reference Catalog
on a tile-by-tile basis. Typically there is an
abundance of ACT stars in a Tile and the astrometric
solution yields positions accurate to <0.2´´ for sources with SNR>20.
A small fraction of Tiles contain very few astrometric reference stars,
or the reference stars may cover only a part of the Tile.
In these cases, the positional
solution may random walk as much as 1´´ from the ICRS frame.
The quoted astrometric uncertainties in the PSC should reflect
this random walk.
There is also known to be a ~0.09´´ bias in the reconstructed
declinations of the bright R1-only-measured stars relative to fainter
stars. No measurable bias exists in right ascension.
[Last Updated: 2000 June 20; by R. Cutri and M. Skrutskie]
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