I. Introduction
6. Cautionary Notes
a. General
i. Preliminary Nature of the Incremental Release Products
The 2MASS Incremental Data Releases are the result of the first processing of data from the Survey performed while Survey observations are ongoing. Rigorous quality assurance procedures used during Survey data processing indicate that the vast bulk of the released 2MASS data products meet or exceed the high scientific standards of the Survey. However, validation of a dataset as large as that being 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. These data do not yet benefit from all experiences that will be gained over the full Survey, in particular the analyses of the full-sky dataset, nor have they undergone all the rigorous analyses that accompany data releases at the end of mission. However, the benefits of releasing data to community now exceed potential risks, and the feedback from the community on the data products and documentation will ultimately contribute to a better final product. The knowledge gained as the survey continues, and from the feedback received from users will be incorporated when the entire 2MASS dataset is reprocessed at the completion of the Survey observations.
NB: Users are strongly recommended to review the caveats listed below.
Magnitudes reported in the Point and Extended Source Catalogs are
in the natural 2MASS photometric systems.
Section III.3.g describes
the calibration procedure. Transformation equations to other photometric
systems are not yet available. However, 2MASS colors for
normal stars lie very close to those reported by
Bessell & Brett (1988, PASP, 100, 1134).
The global photometric uniformity of the 2MASS Catalogs is
enforced by nightly photometric calibration to an extensive set of
standard star fields. However, it will be difficult to assess the
overall photometric uniformity of the survey until most of the sky has
been analyzed. Analysis of the internal consistency of large numbers of
stars in multiply-observed calibration fields indicates that the systemmatic
variations of the 2MASS flux calibration
frame of reference are of order 1% around the sky.
Duplicate Source Rectification
2MASS Survey Tiles are one camera frame
(8.5´) wide in the Right Ascension direction and 6° long in
declination. Successive tiles on the sky overlap
adjacent tiles by approximately one arcminute in the Right Ascension
direction and 8.5´ in Declination leading to
multiple detections for a fraction of the sources.
A single apparation for
multiply detected sources was selected for inclusion in the Point and Extended
Source Catalogs by identifying the occurrence of the
source that fell farthest from the edge of its respective survey tile
(or equivalently closest to a tile center).
The duplicate source procedure operates only on the edges of
tiles that have an overlapping tile available in the area
included in the Spring 1999 data release.
Faint sources falling in the overlap regions between scans that were
not multiply detected, but that met all other catalog selection
criteria, were also passed into the final Catalogs.
This leads to a small systematic overestimation of
the brightness for faint objects in these regions. To avoid
this bias, restrict analyses to sources with SNR>10.
There will also be a small excess of faint sources in the scan overlap
regions since there were two or more opportunites to detect a source.
Consequences for Atlas Images
The 2MASS Atlas Images are derived directly from the
2MASS tile observations. The sky region in
the westernmost 10% of an Atlas Image may
appear on the eastern edge of the adjacent Image.
The 2MASS Survey Visualizer is designed to return the Atlas
Image for which the input coordinate is closest to the
tile center. If the input coordinates for an object
of interest are not accurate (as sometimes occurs for
name-resolved positions), the target may lie on
extreme edge of the coadd. In these cases, it is possible that
a query aimed at returning the adjacent coadd may
yield a more centered image of the source. During
the incremental release period, however, the adjacent
coadds may not be available.
Meteors
Meteor trails frequently appear on 2MASS raw data frames.
Since the camera observes each position on the sky
in six consecutive frames, the coaddition of
frames mitigates the contamination from meteors. However,
meteor trails persist into the Atlas Images, where they often trigger
point and/or extended source detections. These "sources"
are usually characterized by high chi-square values and low
N/M statistics in the case of point sources. Some fraction of these
false detections have been removed from the catalogs,
but the algorithm to identify meteor trails is not 100% efficient.
Users are encouraged
to review the images of interesting sources to rule out
the possibility that they may be meteor trail detections.
Insects, Electronics Glitches, etc.
The rigorous quality assurance procedures applied
to all 2MASS data products have largely identified
rogue artifacts such as airplanes, insects walking
across the camera window, and rare failures of
the camera readout electronics, and identified those
survey tiles for re-observation -- thus excluding
them from the release data. Inspection of a fraction
of the Images in the release suggests a few such artifacts
may remain. Again, users are encouraged to review the images
of sources of interest.
Anomaly Lists
The 2MASS Catalog products are static. No deletions
will be made after a particular release, even for sources that
are known to be artifacts, until the catalog is rereleased
after reprocessing. Separate anomaly lists will be provided
that are updated at regular intervals.
[Last Update: 1999 May 5; R. Cutri, M.Skrutskie, T.Chester]
ii. Photometric System
iii. Global Photometric Accuracy
iv. Scan Overlap Regions
v. Artifacts
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