II. User's Guide to the 2MASS All-Sky Data Release

4. Atlas Images

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, there are 3,942,246 512×1024 images and 179,193 512×698 Atlas Images covering the sky. The J, H and Ks 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. Section IV.3 contains a detailed description of the Atlas Image generation.

2MASS Atlas Images viewed and retrieved on-line via the 2MASS Image Services which are administered by the Infrared Science Archive (IRSA). These services allow interactive and batch access to the images, and include an inventory mode that lists all images that cover a specified 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.

a. Atlas Image Header Description

b. Image Anomalies

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

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