Appendix 1. Introduction to the 2MASS Extended Mission and Ancillary Data Products

J/H/Ks color composite of the 2MASS Combined Calibration Scan Image in rho Ophiuchus

1. 2MASS Extended Mission Overview

The objective of the Two Micron All Sky Survey was to carry out a highly uniform digital imaging survey of the entire sky in three near infrared bands. To this end, the twin 2MASS 1.3 m telescopes on Mt. Hopkins, Arizona and Cerro Tololo, Chile systematically scanned the northern and southern skies between June 1997 and March 2001, collecting 24.5 TB of raw imaging data. In March of 2003, three fundamental data products produced from the raw imaging data were released to the astronomical community as the realization of this goal: a digital Image Atlas containing 4.1 million calibrated FITS images covering the sky; 2) a Point Source Catalog (PSC) containing accurate positions and photometry for ~471 million high quality sources; and 3) an Extended Source Catalog (XSC) that contains accurate positions, photometry and basic shape information for ~1.6 million resolved sources, most of which are galaxies. Before release, these products were heavily scrutinized and verified to meet or surpass the survey's Level 1 Science Requirements for reliability, completeness and uniformity.

Considerably more data were acquired during 2MASS observatory operations than were used in the generation of the All-Sky Data Release products. These include multiple, independent observations of objects in the overlapping boundaries between survey Tiles and in Tiles that were scanned repeatedly, thousands of scans of 8.5' x 1° calibration fields used to derive nightly photometric transformations, and a set of observations of selected targets made with an exposure time six times longer (6x) than the 7.8s exposure time used for each sky location in the Survey. In recognition of the unique scientific value of these these additional data, the 2002 NASA Mission Operations and Data Analysis Senior Review funded an extended data processing phase for 2MASS to prepare and release Ancillary Products from these additional observation data. No new data were acquired during this extended phase.

The objective of the 2MASS Extended Mission is to make available all source extractions and calibrated images from the data acquired during the Survey's operational period. These Ancillary Data Products are intended to complement the highly uniform and reliable All-Sky Release Catalogs and Image Atlas by providing multi-epoch observations of objects when available, and measurements of fainter sources than may be found in the Catalogs. The fainter source population includes: 1) detections in survey scans that fall below the signal-to-noise thresholds used to construct the All-Sky Release Catalogs, 2) objects detected in deep images formed by combining hundreds to thousands of scans of each calibration field, and 3) sources detected in the special long exposure (6x) observations.

The fainter flux limits and time sampling are obtained at the expense of reliability and uniform sky coverage. Furthermore, the 2MASS Extended Mission Ancillary Data Products have not received the same level of validation and scrutiny as the All-Sky Release products. Consequently, users should generally defer to the source data contained in the 2MASS All-Sky Catalogs, whenever possible. The Ancillary Data Products should be used as a resource to learn more about individual objects where detailed follow-up is possible, or to conduct studies over small regions of the sky that do not require the high degrees of uniformity and reliability that characterize the All-Sky Catalogs.

a. The Ancillary Data Products

The 2MASS Extended Mission Ancillary Data Products consist of the basic Image and Extracted Source Databases from Survey, Long Exposure (6x), and Calibration scan data acquired during 2MASS operations, as well as two "value-added" products, the Merged Source Information Tables and Combined Calibration Scan Image and Source data, that are designed to assist users in exploiting the multi-epoch 2MASS observations.

  1. Full Survey Image Atlas and Extracted Source "Reject Tables"
  2. The overall reliability of the Survey Point and Extended Source Reject Tables is much lower than that of the All-Sky Release Catalogs, and they are not a uniform representation of the near infrared sky. They are intended to be a resource for carefully probing beyond the constraints of the Catalogs.

  3. Long Exposure (6x) Image Atlas and Extracted Source Working Databases and Catalogs
  4. The 6x observations cover approximately 590 deg2 on selected astronomical targets such as the Pleiades, Galactic star forming regions, galaxy clusters and the Lockman Hole. The largest areas surveyed in the 6x mode cover the Large and Small Magellanic Clouds (383 and 127 deg2, respectively).

  5. Calibration Image Atlas and Extracted Source Working Databases
  6. The 2MASS Calibration scan observations covered each of 35 primary calibration fields between 562 and 3692 times during the life of the Survey, and were used to derive the nightly photometric calibration transformations. Five additional calibration fields in and arround the LMC and SMC were scanned between 108 and 468 times to support the long exposure (6x) observation of those regions. The Calibration Image Atlas and WDBs cover only ~7 deg2 of sky, but the highly redundant nature of the observations make them a unique resource for near-infrared time domain studies such as the identification of variable and moving objects.

  7. Merged Source Information Tables for the Survey, 6x and Calibration Working Databases
  8. Exploiting the multi-epoch nature of the WDBs, the Merged Source Information Tables contain statistics on the distribution of observed positions and photometry for each source that can be used to obtain higher precision photometry and astrometry than what is available from the single epoch measurements, and to identify potentially moving and variable sources.

  9. Combined Calibration Scan Images and Extracted Source Lists
  10. The Combined Calibration Scan images nominally probe 3.5 to 4.5 magnitudes deeper than the main 2MASS Survey (not taking into account source confusion which can be significant at the depth of these images).

b. Extended Mission Product Documentation

The summary of the primary 2MASS mission, observatory facilities, survey strategy, data reduction and release products are given in Sections I through VI of this Explanatory Supplement to the All-Sky Data Release. Documentation describing the 2MASS Extended Mission and the Ancillary Data Products is contained in Appendices 1 through 7 of the Supplement. Users are strongly encouraged to familiarize themselves with the main survey descriptions in Section I-VI before using any 2MASS data products including the Ancillary Products. Particular attention should be paid to the Cautionary Notes (I.6) section.

The remaining parts of Appendix 1 (this section) contain the guide to accessing and using the 2MASS Ancillary Data Products. Section A1.2 presents an executive summary of the contents and scope of each of the products. Data access modes and release schedules are described in A1.3. Cautionary Notes specific to the Ancillary Products and their usage are provided in A1.4 (Read this section!). Naming conventions for sources in the Extended Mission products are presented in A1.5, and Auxiliary tables for each of the Extended Mission data sets, such as the asteroid, comet and planet association lists, are given in A1.6.

An overview of the characteristics, generation and formats of the Survey Point and Extended Source Reject Tables and Full Survey Image Atlas is given in Appendix 2. 2MASS Long Exposure (6x) data acquisition, processing, product generation and table formats are described in Appendix 3. Appendix 4 contains a description of the 2MASS Calibration Scan data processing and data products.

Estimation of the reliability of sources in the various 2MASS Working Databases and Reject Tables is discussed in Appendix 5. Users are urged to review this section and the recommended guidelines for using the reliability flag because of the large fraction of unreliable source extractions present in the Working Databases and Reject Tables.

Appendix 6 contains descriptions of the generation and contents of the Merged Source Information and Cross Reference Tables that combine and link multiple sightings of the same objects within each of the Working Databases.

The production of the Combined Calibration Scan images and extracted source lists is described in Appendix 7, along with discussion of the features of those data and recommendations on their usage.

c. Acknowledgments

The 2MASS Extended Mission Ancillary Products are derived entirely from data acquired during the primary 2MASS mission, which was a joint project of the University of Massachusetts and Infrared Processing and Analysis Center (California Institute of Technology). The primary 2MASS mission was funded by the National Aeronautics and Space Administration and the National Science Foundation.

The 2MASS Extended Mission was funded by the National Aeronautics and Space Administration contract number NAS7-03001.

Please refer to I.7.a for the list of the many individuals and institutions who were responsible for the successful planning, construction, operation and oversight of 2MASS.

Acknowledging the Use of 2MASS Data Products

Please include the following attribution in any published material that makes use of any 2MASS image or tabular data:

"This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation."

[Last Updated: 2008 February 12; by R. Cutri]

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