The data acquisition strategy, data processing system, rigorous quality assurance procedures and extensive data product validation for 2MASS were all designed to generate products of the highest reliability and quality to enable far-reaching research using the properties of the near infrared sky on unprecedented scales and precision. However, validation of a dataset as large as that produced by 2MASS presents a great challenge, and it is expected that there will be problems both known and unforeseen that persist into the release data products.
The following sections contain a summary of known
caveats and cautionary notes that pertain to the 2MASS All Sky Release
as a whole, and to each of the release products.
Section I.6.a presents general Cautionary Notes, and
I.6.b, I.6.c and
I.6.d address essential
information users must be aware of when using the PSC, XSC and Atlas Images,
respectively. Sections II.2.b and
II.3.b are designed to provide you
with guidance and suggestions on how best to use the 2MASS All-Sky Catalogs
to maximize their scientific return, and to minimize any potential pitfalls
due to their known limitations.
i. Sky Coverage
The effective fractions of the sky covered by the 2MASS All Sky Release
Data products are:
The majority of the lost coverage area comes from the masking
influence of bright stars. Approximately one square degree of sky
was not covered by scans included in the All Sky Release area.
The 2MASS telescopes scanned the sky in units of Survey
Tiles which are one camera frame
(8.5´) wide in the Right Ascension direction and 6° long in
Declination. As laid out on the sky, each Tile overlaps adjacent Tiles
by ~51´´ in RA and 8.5´ in Dec.
Small telescope pointing errors sometimes resulted in misplacement
of the scans with respect to the predefined Tile positions, leading
to possible gaps in coverage between Tiles. These gaps were monitored
during the Survey, and special scans were conducted to fill them.
At the completion of the Survey, 21 physical gaps between Tiles remained.
These gaps encompass an area of 0.71 deg2.
Their location is described in III.2.c.
The 2MASS PSC and XSC coverages contain larger effective gaps
than the imaging area because sources were required to lie
>10´´ and >15´´ away from Tile
edges, respectively (V.3).
This Tile-edge safety boundary was designed
to minimize edge-effects that include
non-three-band coverage due to slight misalignments of
the focal plane arrays, distortion at the edges of focal planes,
and partial coverage of extended sources close to Tile edges.
For Tile pairs that overlap by less than <20´´ there will
be an effective coverage gap in the PSC. There are 43 such gaps,
and the total effective missing area in the catalog, including the physical
gaps, is 1.26 deg2.
Similarly, for Tile pairs that overlap by <30´´, there
will be an effective gap in the XSC.
There are 89 such gaps in the XSC, for a total combined
missing area of 1.65 deg2.
The location of these gaps is described in
III.2.c.
Bright stars effectively mask detection of nearby fainter sources
because of the influence of their bright image wings, ghosts images,
diffractions spikes, latent images and other image artifacts
(IV.7).
These regions are masked out during source extraction and characterization
in the data processing to minimize spurious detections and confusing
photometric effects triggered by the artifacts, resulting in further
loss of sky coverage.
The area lost due to bright stars is much larger than any of the physical
or effective gaps in the Tile sky coverage.
The
effective areas lost to bright stars in the PSC are 106.16 deg2
in J band, 156.21 deg2 in H band, and 177.87 deg2 in
Ks band. Approximately 825 deg2 of coverage in the XSC
is lost due to the confusing effects of bright stars. Detailed sky
coverage maps for the XSC are available in II.6.f.
The PSC and XSC lose effective coverage area due to the increasing
effects of confusion in high source density regions in and around
the Galactic Plane. This loss of coverage is not as simply quantified
as those due to the Survey gaps or the influence of bright
stars because the loss is a varying function of Catalog source brightness
with field star density.
The point source extraction threshold is regulated by the
point-source-filtered noise levels on Atlas Images
(IV.4.a). The point source
detection brightness limit rises in response to increasing confusion noise.
Thus, the PSC completeness limit increases in brightness
as the source density increases. This is illustrated
in this GIF "movie," which shows the
differential PSC source counts as a function of source brightness
(see VI.1n).
Coverage loss begins at Ks~12.7 mag, and the areal loss
increases with decreasing
brightness.
The extended source detection thresholds are also automatically
increased in response to increasing confusion noise, using
the extracted density of point sources brighter than Ks<14.5
mag as a proxy for confusion noise (IV.5).
As the star density increases, the source processor
requires a source to be increasingly brighter and larger to
be operated on as a true extended object.
This is because of the increasing incidence of close
multiple stars masquerading as extended sources, and because
smaller, fainter galaxies are more heavily contaminated by foreground stars.
The growth of the excluded area with decreasing extended
source brightness is clearly illustrated in this movie showing
differential XSC source counts as a function
of brightness. Extended source coverage loss begins
at Ks~11.5 mag, and the areal loss increases with
decreasing brightness.
ii. Reliability of the All Sky Release Products
The 2MASS All Sky Release data products have been demonstrated to meet or
surpass the Level 1 Requirements
for reliability (cf. VI.5a.ii and
VI.5b.ii).
Every effort has been made to remove unreliable
sources from the Catalogs, and to flag such entries that do remain.
However, the sheer volume of the 2MASS image and source data products make
it impossible to guarantee their perfection.
For example, the target reliability for sources in unconfused regions
of the sky in the 2MASS Catalogs is 99.95% (PSC) and 99%
(XSC for |b|>20o). Even at these strict limits, there
may be >170,000 unreliable point sources and >12,000 unreliable
extended sources.
Extensive discussion of the characteristics of the remaining
unreliable sources in the Catalogs are presented in the
PSC and XSC
Cautionary Notes sections of the Explanatory Supplement.
Examples to help users recognize artifacts that can be found in
the over four million Atlas Images in the All Sky Release are discussed in
the Image Atlas Cautionary Notes section.
The 2MASS All Sky Catalog products are static. No deletions
will be made after their official release, even for sources that
are known to be artifacts. Anomaly lists identifying confirmed
unreliable sources are provided in II.6.g.
These lists will be updated periodically.
iii. Photometric Considerations
Magnitudes reported in the Point and Extended Source Catalogs are
in the natural 2MASS photometric system.
The system bandpasses are described in III.1.b,
and the 2MASS calibration procedure is described in
III.2.c and
IV.8. Users should be aware that
comparisons of 2MASS photometry with measurements made in other
common near infrared photometric systems may show systematic
differences.
Preliminary transformations between the 2MASS and selected other photometric
systems were presented by
Carpenter et al. (2001, AJ, 121, 2851)
using data from the 2MASS Second Incremental Data Release. The
transformations have been updated using data from the All-Sky Release PSC,
and are given in VI.4b.
The global uniformity of the 2MASS photometric calibration is demonstrated
to be better than 1-2% over large spatial scales (c.f. VI.2.a.ii).
The global uniformity is enforced by the Survey's photometric
calibration procedure (III.2.d)
that ties all photometry to measurements of a common network
of standard stars observed hourly each night from both observatories.
Several sources of bias that affect the measurements
reported in the PSC are discussed in detail in
I.6.b.viii.
These include brightness-dependent biases that exist between measurements of
sources that fall in the different brightness regimes of the
Survey observations:
sources that are non-saturated
in the 1.3 s "Read_2-Read_1" exposures, those that are non-saturated
in the 51 ms "Read_1" exposures, and those that saturated the 51 ms
exposures. These are known to cause features in global source count
curves for the PSC.
Spatially correlated biases in PSC photometry are known
to be caused by mismatches between the instantaneous seeing
and the PSF used during profile-fitting photometry measurements,
and normalization errors of the profile-fitting measurements.
These can produce coherent "jumps" in mean source color
correlated with Tile and Atlas Image boundaries.
Finally, point source photometry can be biased due to proximity to
sources of equal or greater brightness because of gradients
induced in the backgrounds by the nearby source.
Data taken under non-photometric conditions due to clouds or other
instances of decreased or variable atmospheric transmission were
identified and rejected from the Survey.
However, during final review of 2MASS data quality following
the completion of observations, two scans were found to
have a brief 10-25% decrease in transparency, presumably
because the telescope scanned over a small cloud or contrail.
Colors of sources in the affected regions are largely unchanged,
however, because the extinction due to the clouds is grey.
It was not possible to reobserve these regions following decommissioning
of the observatories.
Since the impact on photometry was relatively small,
the total area affected by the clouds is <0.25 deg2,
and there were only 1,391 point and 3 extended sources
in those areas, these Tiles were included in the Release.
Table 1 gives the description of the
two scans affected by clouds, including the Tile
number, observatory, observation date and
scan number, scan_key,
and the central right ascension and approximate declination range over
which the transparency was diminished.
PSC and XSC sources in the affected areas can be identified
through the use of scan information (scan_key or date, hemis,
scan) and declination range. The photometric uncertainties
for the affected point sources in Tiles 29270 and 205438 were increased
by adding in quadrature 0.11 and 0.25 mags to the original
uncertainties, respectively, equal to the peak photometric offset induced
by the obscuration in each scan. For the affected XSC sources in these Tiles,
the aperture photometry uncertainties had 0.11 and 0.25 mags
added in quadrature, and the isophotal magnitude uncertainties
had 0.22 and 0.50 mags added in quadrature.
Table 1 - Information on Tiles Affected by Clouds
Physical Gaps
Effective Gaps in the Catalogs
Area Lost Due to Bright Stars
Effective Loss of Faint Source Coverage Due to Confusion
in the Galactic Plane
Anomaly Lists
Photometric System
Photometric Biases
Clouds
| Tile | Observatory | Obs. Date (UT) | Scan Number | Scan_key | Approximate RA (deg) | Declination Range Affected (deg) |
|---|---|---|---|---|---|---|
| 29270 | north | 2000-02-18 | 14 | 54806 | 217.65963 | 81.7 to 83.1 |
| 205438 | south | 2000-07-28 | 49 | 58256 | 298.12385 | 6.0 to 6.5 |
iv. Astrometric Considerations
The astrometric precision of 2MASS is outstanding.
Comparison of PSC positions with those reported in the
Tycho 2 and
UCAC astrometric reference
catalogs demonstrates
accuracy of 70-80 mas with respect to the International Celestial Reference System (ICRS)
over much of the
brightness range for non-saturated sources.
The accuracy of position reconstruction will be slightly
poorer near the declination ends of Survey Tiles,
in regions with a low density of astrometric reference stars,
and near the celestial poles where the telescope tracking was
least stable. The degraded accuracy is reflected
in the position uncertainties quoted in the PSC.
Focal plane distortion corrections were incorporated for point source
position reconstruction, but were not used in the construction
of 2MASS Atlas/Quicklook Images. Offsets ranging from
0.08" and 0.2" may exist between Catalog source positions and those measured
near Tile edges (c.f.
I.6.d.iv).
v. Differences Between the All Sky Release and
Incremental Release Data Products
The 2MASS All Sky Release data products supersede those from
the Incremental Data Releases
The All Sky Release products were constructed using the results
of a complete, final reprocessing of all Survey data. The final
processing software incorporated numerous improvements to the
global photometric and astrometric calibrations, Atlas Image
generation, and source
measurement algorithms. Thus, the overall quality of
data in the All Sky Release is believed to be superior to those
in the Incremental Releases.
Differences in point and extended source extraction thresholds,
handling of confused and blended sources,
and the limits used to select sources for the release Catalogs
mean that some sources in the Incremental Release Catalogs will not appear
in the All Sky Release Catalogs, and vice-versa. This will be
most apparent near the faint limits of the release Catalogs.
For example, the All Sky Release PSC includes sources
down to a signal to noise ratio limit >5 if they are detected
in three bands. The Incremental Release PSC required sources
to have SNR>7 in at least one detected band. The All Sky Release
XSC contains fainter sources in the higher source density regions
near the Galactic Plane than are found in the Incremental Releases
because of improved handling of contamination
by foreground stars in the final processing.
A number of scans of Tiles included in the Incremental
Data Releases have been replaced in the All Sky Release
with scans taken later in the Survey under better atmospheric
conditions. The replacement scans will generally be more sensitive
than the ones they replace, so may contain fainter sources.
[Last Update: 2006 May 31; R. Cutri, M.Skrutskie, T.Chester]
b. Point Source Catalog (PSC)
c. Extended Source Catalog (XSC)
d. Atlas Images
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