VI. Analysis of the Release Catalogs

1. Comparison of Achieved Performance of the All-Sky Release Catalogs with Level 1 Science Specification

The 2MASS project has prepared an All-Sky Release, very similar in characteristics to previous Incremental Releases. This Release increases the integrated sky coverage in the public domain from 47% to virtually 100%, with improved processing of all sources. The All-Sky Release data products include a Point Source Catalog containing positions and brightness information (see II.2a) for over 470 million objects, an Extended Source Catalog containing positions, magnitudes and basic shape information (see II.3a) for nearly 1.6 million resolved sources, and an Image Atlas containing nearly 5 million J, H and Ks images covering the survey area.

In constructing this document, the 2MASS team has drawn upon and summarized both internal and Explanatory Supplement documentation. To provide the most complete information, many of the links here point to the original detailed analyses. In all cases, however, the 2MASS team has arranged this page so that all primary information is available in summary in the first level below this front page.

The 2MASS All-Sky Release includes improved processing of data from the area contained in the Incremental Data Releases, and is intended to supercede these earlier releases. This summary outlines the major improvements in the final data processing pipeline.

The 47% of the sky delivered in the Second Incremental Release has been in the public domain since 2000 March and has led to hundreds of publications -- with most now coming from outside the 2MASS collaboration. This rigorous practical testing of the preliminary product has yielded only minor comments on the data's shortcomings. All evidence points to the All-Sky Release being of better quality than its predecessor. For reference, this link documents the reports associated with the development and validation of the final pipeline.

The release preview page supplies the details of sky coverage and source counts. This Explanatory Supplement is available as web pages, as was the case for the previous Releases. Section I.6 is of particular interest. It contains a compilation of caveats that will be of interest to users of 2MASS data products. The relevant subsections are:

Below is a summary of science team analysis of the release characteristics in the context of the Survey's Level 1 specifications.

2MASS photometry has a dynamic range of >20 magnitudes, owing to extraction software that addressed three different exposure regimes. The Survey's nominal frame exposure time was 1.3 sec, obtained by differencing two reads of the NICMOS3 array separated by that time interval. Both reads were independently recorded and saved for future processing. The first read (often referred to as "Read_1" throughout the Supplement) occurred 51 msec after array reset and represents an effective 51 msec exposure image. The second read ("Read_2") occurred 1.351 sec after reset. The point source database includes a rd_flg flag which distinguishes between different techniques for populating the "default magnitude" column for sources, depending on their level of saturation (all XSC photometry derives from the 1.3 sec Read_2-Read_1 difference). Since different techniques can be used in different wavelength bands for a given source, the rd_flg is a three-character flag with an independent value for each band revealing the source of the default magnitude (i.e., j_m, h_m, or k_m) column in the Point Source release database.

This Release includes reliable sources fainter than the Level 1 Specification 10- flux limits (as was also the case for the Second Incremental Release). In order to distinguish between "Catalog" sources and the fainter content of the Release Point Source database, the 2MASS project has created a new column/flag -- photometric quality, ph_qual. This flag contains three characters, one for each band. The ph_qual designation, as the name suggests, represents the photometric uncertainty in the measurement -- with "A" representing measurements with photometric uncertainty consistent with the Level 1 Catalog specifications. "B","C","D", and "E" represent successively poorer measurements. "Catalog" sources include

Category Requirement Performance
Point Source Catalog
Photometric sensitivity 10- at 15.8, 15.1, 14.3 mag at J, H, Ks respectively for |b|>10° met for full unconfused sky;
exceeded for most of sky
Photometric uniformity <4% maximum bias in photometric zeropoint around the sky <2% achieved
Photometric precision <5% 1- for bright stars unsaturated in the 1.3 s exposure <3% achieved
Astrometric accuracy <0.5´´ 1- relative to the reference frame <0.1´´ achieved
Completeness >0.99 at 10- sensitivity limits met
Reliability >0.9995 0.9997 demonstrated in test areas; no known source of unreliability
in excess of specification
Bright star photometry

(Read_1, rd_flg=1)

<2% bias at Read_1 saturation limit (Ks~4.0 mag)
<5% 1- repeatability at Read_1 faint limit (Ks~8.0 mag)
<10% 1- repeatability at Read_1 saturation limit (Ks~4.0 mag)
<4% at worst

2% achieved

2% achieved

Extended Source Catalog
Photometric sensitivity 10- at J<15.0, H<14.2, Ks<13.5 mag met
Photometric precision <10% 1- repeatability for H<13.8 mag 7-10% achieved
Photometric uniformity <10% maximum bias around sky <4% achieved
Completeness >0.90 for |b|>30° met
Reliability >0.80 for 10°<|b|<20°

>0.99 for |b|>20°

achieved 0.93-0.95

achieved 0.992-0.995

General Survey
Sky coverage>95% 99.998% imaged;
99.5% coverage in point sources;
98% coverage in extended source
Other Information
Galactic Plane performance No Requirements

[Last Updated: 2003 March 21; by M. Skrutskie]

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