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 2^h 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 20-22^h 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.

Table 1 - 2MASS Calibration Field Centers

Tile	ra(J2000)	dec(J2000)	glon	glat	elon	elat
90021	6.10619	-1.97294	107.367	-64.025	4.823	-4.237
90294	8.31622	-39.40154	318.916	-77.157	348.904	-38.817
90004	28.66074	+0.71693	154.117	-58.275	26.892	-10.328
90301	51.72678	-39.84268	244.635	-55.506	32.087	-55.852
90247	53.01617	+37.34386	155.043	-15.318	59.866	+17.694
90533	55.26392	+6.93647	179.513	-36.676	54.620	-12.339
90191	66.58918	+3.62342	190.909	-29.778	65.383	-17.838
90013	89.28447	+0.01890	206.632	-12.049	89.220	-23.418
90121	97.37444	-59.65713	268.951	-25.881	119.127	-82.345
90161	105.22232	+48.48935	168.289	21.455	101.129	+25.636
90312	126.40319	-39.09847	257.574	-0.669	145.023	-55.799
92026	128.12790	-1.57084	226.548	21.547	130.978	-19.751
90067	132.81203	+11.84773	215.639	31.905	131.935	-5.579
90091	145.75214	+59.06160	154.888	44.631	125.013	+42.210
92397	170.45775	-13.22047	271.788	44.166	176.597	-15.904
90217	180.44070	-50.05148	294.815	12.036	205.734	-44.539
90266	183.61047	+35.59855	163.599	78.431	167.187	+33.672
90860	185.41757	-0.12034	287.009	61.828	185.021	+2.042
90867	220.24529	-0.45767	351.082	51.867	217.981	+14.457
90273	224.21932	-44.81900	325.155	12.572	235.303	-26.739
90272	224.64303	+37.14173	61.476	61.448	205.765	+50.966
90868	225.11368	-0.65787	356.367	48.363	222.849	+15.740
90565	246.68168	+5.87185	20.521	34.698	243.720	+27.209
90009	246.80780	-24.68901	352.970	16.585	249.005	-2.924
90330	247.89420	+30.14552	50.250	42.071	238.704	+51.210
90279	267.09736	-45.42783	346.065	-8.926	267.803	-22.011
90182	279.89627	+49.09363	78.074	22.009	291.129	+71.808
90547	282.82780	-4.27488	29.111	-1.920	283.507	+18.565
90808	285.48438	-4.48794	30.125	-4.377	286.259	+18.082
90234	307.83812	-49.64775	349.607	-36.216	297.217	-29.722
90813	310.27504	-5.06339	41.333	-26.643	311.324	+12.787
92409	330.11998	+20.84962	77.728	-26.651	340.586	+30.780
92202	331.40247	-11.07477	47.133	-47.917	329.508	+0.608
90893	349.54575	+0.54857	80.124	-54.382	350.607	+4.644
90290	352.64369	+38.31591	105.945	-21.874	11.226	+37.503

iv. Secondary Calibration Stars

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).

Table 2: 2MASS Calibration Fields and Primary and Secondary Calibration Stars

[Last Update: 2005 October 12, by R. Cutri, S. Wheelock, E. Howard]