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.

ii. Photometric System

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).

iii. Global Photometric Accuracy

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.

iv. Scan Overlap Regions

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 First Incremental 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.

v. Artifacts


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]

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