I. Introduction

1. 2MASS Overview

It has been 31 years since the last large-area near-infrared survey of the sky was carried out. The Two Micron Sky Survey (TMSS; Neugebauer and Leighton 1969, Two-Micron Sky Survey. A Preliminary Catalogue) scanned 70% of the sky and detected ~5,700 celestial sources of infrared radiation. Since that time there has been a revolution in the development of infrared detector technology. New, large format, sensitive array detectors can now detect astronomical objects over 100 million times fainter than those detected in the TMSS.

The Two Micron All Sky Survey (2MASS) project is designed to close the gap between our current technical capability and our knowledge of the near-infrared sky. In addition to providing a context for the interpretation of results obtained at infrared and other wavelengths, 2MASS will provide direct answers to immediate questions on the large-scale structure of the Milky Way and the Local Universe. The optimal use of the next generation of infrared space missions such as HST/NICMOS, the Space Infrared Telescope Facility (SIRTF), and the Next Generation Space Telescope (NGST), as well as powerful ground-based facilities, such as Keck I, Keck II, and Gemini require a new census with vastly improved sensitivity and astrometric accuracy than that previously available.

To achieve these goals 2MASS is uniformly scanning the entire sky in three near-infrared bands to detect and characterize point sources brighter than about 1 mJy in each band, with signal-to-noise ratio (SNR) greater than 10, using a pixel size of 2.0´´. This will achieve an 80,000-fold improvement in sensitivity relative to earlier surveys.

2MASS uses two highly-automated 1.3-m telescopes, one at Mt. Hopkins, AZ, and one at CTIO, Chile. Each telescope is equipped with a three-channel camera, each channel consisting of a 256×256 array of HgCdTe detectors, capable of observing the sky simultaneously at J (1.25 µm), H (1.65 µm), and K_s (2.17 µm).

The immediate scientific benefits from the 2MASS survey include:

An unprecedented view of the Milky Way nearly free of the obscuring effects of interstellar dust which will reveal the true distribution of luminous mass and thus the largest structures over the entire length of the Galaxy.
The first all-sky photometric census of galaxies brighter than K_s=13.5 mag, including galaxies in the 60°-wide ``Zone of Avoidance,'' where dust within the Milky Way renders optical galaxy surveys incomplete. The final Catalog of >1,000,000 galaxies will provide a rich statistical database, including photometric measurements in three wavelengths and a few structural parameters for large samples of galaxies in differing environments, measured at wavelengths which are sensitive to the stellar populations dominating the luminous mass.
The statistical basis to search for rare but astrophysically important objects, which are either cool, and thus extremely red (e.g., extremely low-luminosity stars and brown dwarfs), or heavily obscured at optical wavelengths (e.g., dust-obscured AGNs and globular clusters located in the Galactic plane).

The northern 2MASS facility began routine operations in 1997 June, and the southern facility in 1998 March. As of 2000 March, the time of the 2MASS Second Incremental Data release, Catalog quality data have been observerd in an area covering 84% of the sky. Analyses of these data show that they meet and often exceed the Level 1 Science Requirements for the Survey.

The first, limited release of 2MASS data took place in 1998 December. This release, referred to as the 2MASS Sampler, included the derived Point and Extended Source Catalogs and Atlas Images from one night of data from the northern 2MASS facility. That release was designed to introduce the astronomical community to the content and formats of the 2MASS data products.

The 2MASS First Incremental Data Release, also known as the 2MASS Spring 1999 Release, was made in May of 1999, and contained data obtained from the northern 2MASS facility on Mt. Hopkins, AZ between the dates of 1997 June 7 and 1998 January 30. This Incremental Release covered approximately 2483 sq.degrees of northern sky and included a Point Source Catalog containing positions and brightness information for over 20.2 million objects, an Extended Source Catalog containing positions, magnitudes and basic shape information for over 73,000 resolved sources, and an Image Atlas containing over 233,000 J, H and K_s images covering the survey area.

a. The 2MASS Second Incremental Data Release

The 2MASS Second Incremental Data Release contains Image and Catalog data covering 19,681 square degrees of sky, derived from Survey observations made at the northern 2MASS facility on Mt. Hopkins, AZ, and the southern facility on Cerro Tololo, Chile. The observations in this release were obtained between 1997 June 7 and 1999 May 21 UT at Mt. Hopkins and 1998 March 19 and 1999 February 20 UT at Cerro Tololo. The Second Incremental Data Release products include a Point Source Catalog containing positions and brightness information for over 162 million objects, an Extended Source Catalog containing positions, magnitudes and basic shape information for nearly 600,000 resolved sources, and an Image Atlas containing nearly 1.9 million J, H and K_s images covering the survey area. The 2MASS Second Incremental Data Release includes an improved processing of data from the area contained in the First Incremental Data Release, and is intended to supersede the earlier release.

The improvements in the data processing between the First and Second Incremental Data Releases include the following:

Correction of a truncation error in the Source Coordinate Designation construction
Improved identification of artifacts due to bright bright stars not on the scan being processed
Correct handling of singly-detected sources that fall in the overlap regions between tiles
Improved identification of observations that are contaminated by residual background structure due to severe atmospheric OH airglow

This large Incremental Release is being made while survey operations continue in order to provide as early as possible access by the community to 2MASS data products. Significant improvements to the data processing may be made before the final, complete release of the 2MASS Catalogs.

b. About this Document

This Explanatory Supplement provides users of 2MASS products with a description of the Survey and its facilities, and the production and contents of data products comprising the Second Incremental Data Release. The document was drawn in part from the 2MASS Spring 1999 Incremental Release Explanatory Supplement, and has additional information specific to this data release. This Explanatory Supplement is designed to be a living document, in that it will be updated regularly to provide users with the most current analyses and advice regarding 2MASS data products. Consequently, the distribution of this Supplement and those accompanying later large Incremental data releases will be only as WWW hypertext document. Please review them regularly for updates.

[Last Updated: 2000 March 2; by R. Cutri, M. Skrutskie and R. Stiening]

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