I. Introduction

3. Executive Summary of 2MASS All-Sky Release Data Products

The 2MASS All-Sky Release PSC contains astrometry and photometry in the three survey bandpasses for 470,992,970 sources. Positions, magnitudes, astrometric and photometric uncertainties, flags indicating the quality of the source characterizations, and associations with Tycho 2 or USNOA-2.0 optical catalog sources and known solar system objects are presented for each source.

Most PSC sources are stars in the Milky Way. However, the PSC also contains point-source processed photometry for virtually all of the resolved sources found in the Extended Source Catalogs, as well as a significant number of unresolved, more distant galaxies.

The general properties (see II.2), format (see II.2a), and a practical users guide to the PSC are presented in II.2. A detailed description of the source detection and photometry algorithms used to generate the the PSC can be found in IV.4. Users are strongly encouraged to read the Cautionary Notes for the PSC before using the catalog.

b. Extended Source Catalog (XSC)

The 2MASS All-Sky Data Release XSC contains positions and photometry in the three survey bandpasses for 1,647,599 objects found to be resolved relative to a single point-spread-function during data processing. The majority of XSC sources are galaxies (~97%), but the catalog also contains resolved sources in the Milky Way such as HII regions, planetary nebulae, reflection nebulae, YSOs, etc. Positions, magnitudes measured in a wide variety of ways, photometric uncertainties, associations with several external catalogs and a number of flags and scores indicating the quality of the measurements, and the likelihood of "extendedness" are presented for each source.

The general properties (see II.3), format (see II.3a), and a practical users guide to the XSC are given in II.3. The algorithms used for extended source detection and characterization are described in IV.5. Users are strongly encouraged to read the Cautionary Notes for the XSC before using the catalog.

c. Image Products

The 2MASS All-Sky Data Release includes 4,121,439 FITS images in the three survey bands constructed from 59,731 scans of 8.5´ × 6° Survey Tiles. The image data from each Tile are divided into twenty-three separate Atlas Images in each band. Twenty-two of these are 512×1024 (1"/pix) in size, and one is 512×698 (1"/pixel). Thus, 3,942,246 512×1024 images and 179,193 512×698 Atlas Images cover the sky. The J, H and K_s images for each region of sky are resampled to the same scale and registered to facilitate 3-color investigations. Positional and photometric calibration data are carried in the FITS header for each image, which is described in II.4.a.

2MASS Atlas Images are accessible via the 2MASS Image Services which are administered by the Infrared Science Archive (IRSA). These services allow interactive viewing and retrieval of one or more images and include an inventory mode that lists all images that cover a point or region on the sky.

Lossy-compressed versions of the Atlas Images, known as the "Quicklook" Images, are also served by the 2MASS Image Services. Compression of the images was carried out using the hcompress procedure, using scale = 4.0 and quant = 1.0 parameters. This achieved an average compression ratio of ~20:1. The Quicklook Images delivered by the IRSA service are restored from the lossy-compressed files in FITS format with full WCS information contained in the image headers. These images are suitable for position measurements, finding charts and visual inspection of the near-infrared sky.

N.B.: Only Atlas Images should be used for quantitative photometric measurements because of the information lost during the Quicklook compression process.

Users are strongly encouraged to read the Cautionary Notes for the Atlas/Quicklook Images, and to view the gallery of anomalies (II.4b) known to exist in the Atlas/Quicklook Images.

ii. Extended Source Postage Stamp Images

Small cut-outs of the uncompressed J, H and K_s Atlas Images around each of the XSC sources in the All-Sky Release are available on-line via the IRSA/2MASS Extended Source Full-Resolution Image Server. The image sizes are scaled to the size of the object, ranging from 21´´ × 21´´ for the faintest galaxies (K_s > 13 mag) to a maximum of 301´´ × 301´´.

The 2MASS Extended Source Images are 6-dimensional FITS image cubes. The first three planes contain three image planes. Each plane contains the J, H and K_s 2MASS image data, respectively. Planes 4-6 contain the J, H and K_s image data with foreground stars removed. The near-infrared sky background emission has been subtracted from each image plane. Positional and photometric calibration information are included in the image header.

[Last Updated: 2004 Sept 2; by R. Cutri]