III. 2MASS Facilities and Operations
2. Data Acquisition
d. Photometric Calibration Strategy
First-order photometric calibration for 2MASS was evaluated nightly using observations of calibration fields made at regular intervals. Photometry of standard stars in these fields was used to derive the photometric zero points in each of the three survey bandpasses as a function of time during each night. Atmospheric extinction coefficients were derived from 2MASS observations made over long periods. Section IV.8 describes the extraction of nightly calibration information from the calibration observations. A discussion of the absolute calibration of 2MASS photometric systems is given in VI.4.a.
i. Calibration Tiles and Observations
2MASS calibration Tiles are 1° long in declination, and were covered by scans containing 48 frames. Each calibration observation consists of six independent scans of a calibration Tile, made in the same freeze-frame scanning mode and scan rate as the normal Survey Tiles. The six scans were made in alternating directions and each scan is displaced 5´´ in RA from the previous one to minimize systematic pixel effects.
Calibration Tiles were assigned numbers having the form 9nnnn. The values of nnnn are numerical and are derived from the name of the primary calibration star in the field. However, the calibration Tile numbers are not correlated with position on the sky. For example, calibration Tile 90294 is centered on the star S294-D taken from Persson et al. (1998 AJ, 116, 2475).
ii. Calibration Strategy
At the beginning of northern Survey operations, two calibration fields were observed every two hours during the night. Beginning on 1997 October 11 UT, the calibration strategy was modified so that one calibration field was observed approximately every hour during a night. All southern calibration was carried out using the newer strategy. Normal 2MASS operations were started with a calibration observation, and the actual calibration interval was adjusted so that the final calibration observation was coincident with morning twilight.
The calibration strategy emphasized the measurement of the photometric zero point of the night, so a few calibration fields were measured multiple times during a night. The selected fields were alternated so the same field was rarely observed on sequential hours. Repeated measurements of calibration fields during each night at a variety of elevation angles were used to develop long-term atmospheric extinction statistics (IV.8.c).
iii. 2MASS Calibration Fields
Table 1 contains a listing of the 2MASS calibration fields, their Tile numbers and their centers in equatorial, galactic and ecliptic coordinates. All positions are given in decimal degrees.
The calibration fields were selected to be centered on one or more primary calibration stars drawn from either the list of faint near infrared standard stars developed by Persson et al. (1998 AJ, 116, 2475) or the UKIRT group of faint, equatorial near infrared standard stars (Casali and Hawarden 1992, JCMT-UKIRT Newsletter, No. 4, 33) A listing of the calibration stars in each field is given in Table 2. The first star listed in each field is the primary calibrator.
Calibration Tiles were selected so that there would be a set of equatorial and ±30° declination fields on approximately 2h RA centers around the sky, if possible. The equatorial fields were observed from both hemispheres to develop tie points between the observatories. The high declination fields transited close to the zenith at Mt. Hopkins or Cerro Tololo, providing low airmass calibration. Because the Persson et al. and UKIRT lists have very few stars at +30° declination in the 20-22h range, a field was defined in that area and the standards in it were calibrated internally to 2MASS. Incidentally, this field was selected to cover the Abell 2409 galaxy cluster so that long-term monitoring of galaxy photometric performance in the Survey could be made.
Although each 2MASS calibration Tile was centered on one primary calibration star, dozens if not hundreds of high SNR stars were measured during every scan of those Tiles. A network of secondary standard stars was developed using the 600 to 3500 measurements made of each calibration field, and the stars therein, during the Survey. Magnitudes for the primary and secondary standards in each field with the highest degree of internal consistency were developed using the global chi-squared minimization procedure described by (Nikolaev et al. 2000, AJ, 120, 3340). In this procedure, photometric solutions were derived for each night's calibration star measurements that minimize the variance in the solutions over the full Survey. Mean magnitudes and residuals are then computed for each star using the set of optimal nightly solutions. The standard network was carefully cleaned of stars that showed large variances over the Survey, signs of variability, confusion or any other condition that would compromise repeated reliable measurements. Table 2 contains a listing of the calibrated magnitudes and positions for all calibration stars. The nightly photometric calibration solutions were derived relative to these magnitudes (see IV.8).
[Last Update: 2005 October 12, by R. Cutri, S. Wheelock, E. Howard]
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