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Description
Photometric calibration for 2MASS was performed using observations of calibration fields made at regular intervals during each night of survey operations. Measurements of standard stars in the fields were used to derive the photometric zero point offsets as a function of time during each night. Atmospheric extinction coefficients were derived from 2MASS observations made over long periods.
2MASS calibration fields, or tiles, are 1° long in declination and approximately 8.5' wide in right ascension. There are 35 regular survey calibration fields distributed at approximately two hour intervals in right ascension near declinations of approximately -30°, 0° and +30°. An additional five calibration fields were defined in and around the Large and Small Magellanic Clouds to support the deep observation (6x) campaign towards the end of survey operations.
Over the course of the survey, the regular calibration fields were scanned between 562 and 3692 times in nominally photometric conditions. Equatorial fields were observed from both observatories, so were observed more frequently than those near ±30° which were observed with only one telescope. The special Magellanic Cloud calibration fields were observed 108 to 468 times between November 2000 and February 2001.
Calibration scan Atlas Images, and Point and Extended Source Working Databases (Cal-PSWDB and Cal-XSWDB), analogous to those from the main survey, were produced from the calibration scan pipeline data reduction. The calibration Image Atlas and WDBs are the fundamental processing archives of the data from 73,230 calibration scans taken in photometric conditions during 2MASS survey operations. The Cal-WDBs contain positions, magnitudes and characteristics of 191,464,020 and 403,811 point and extended "source" extractions, respectively. The calibration Image Atlas contains 878,769 FITS images in the three survey bandpasses redundantly covering ~5 deg2 of sky. Unlike the main survey and 6x observations, "Catalogs" have not been derived from the Calibration WDBs.
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