I. Introduction

3. Executive Summary of 2MASS Second Incremental Release Data Products

a. Point Source Catalog (PSC)

The 2MASS Second Incremental Release PSC contains astrometry and photometry in the three survey bandpasses for 162,213,354 sources. Positions, magnitudes, astrometric and photometric uncertainties, associations with ACT or USNOA optical catalog sources and a number of flags indicating the quality of the photometry are presented for each source.

The PSC also contains point-source processed photometry for virtually all of the resolved sources found in the Extended Source Catalogs.

b. Extended Source Catalog (XSC)

The 2MASS Second Incremental Release XSC contains positions and photometry in the three survey bandpasses for 585,056 objects found to be resolved relative to a single point-spread-function during processing. Positions, magnitudes measured in a wide variety of ways, 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.

c. Image Products

i. Atlas Images

The 2MASS Second Incremental Data Release includes 1,897,017 FITS images in the three survey bands constructed from the scans of 8.5´ × 6° long Survey Tiles. 1,814,538 of the images are 512×1024 (1"/pix) in size. The remaining 82,479 images lie at the ends of Survey scans and are 512×698 (1"/pixel). 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.

20:1 lossy-compressed versions of the Atlas Images, known as the "Quicklook" Images, are also available, and are suitable for many applications such as finding charts and visual inspection of fields. Compression of the "Quicklook" images was carried out using the hcompress procedure. Because of the losses during compression, the "Quicklook" images should not be used for quantitative brightness measurements.

ii. Extended Source Postage Stamp Images

Small cut-outs of the uncompressed J, H and Ks Atlas Images around each of the XSC sources in the Second Incremental Release are available on-line. The image sizes are scaled to the size of the object, ranging from 21´´ × 21´´ for the faintest galaxies (Ks > 13 mag) to a maximum of 101´´ × 101´´. For galaxies larger than 101´´, it is best to use the 2MASS Survey Visualizer, which will display larger (but lossy compressed) 2MASS Atlas Images.

The 2MASS Extended Source Images are 3-dimensional FITS image cubes, containing three image planes. Each plane contains the J, H and Ks 2MASS image data, respectively. 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: 2001 January 31; by R. Cutri]

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