Appendix 4. 2MASS Calibration Scan Working Databases and Atlas Images

1. Overview

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. The survey's calibration strategy is described in Section III.2.d of the Explanatory Supplement.

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. Figure 1 contains a map of the sky showing the location of the survey (filled squares) and special (open squares) calibration fields. A listing of the 2MASS calibration fields, their central positions and objects of note contained in them are given in Table 1. Click on the calibration Tile number to view a 3-color preview image of the field constructed from the Atlas Images from a single calibration scan.

Figure 1 - Equatorial sky map showing location of the 2MASS survey (solid squares) and LMC/SMC (open squares) calibration fields

One calibration field was visited each hour during the night (before October 11, 1997 UT, two fields were observed each two hours). During each visit, six consecutive scans of the field were made in alternating declination direction. Each scan in the set of six was offset from the preceding scan by ~5´´ in RA to avoid systematic pixel effects. Scans of the calibration fields were made using the standard survey freeze-frame scanning strategy, accumulating 7.8 sec of exposure on each point on the sky per scan.

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. The second column in Table 1, labeled Nscn, indicates the total number of photometric scans made of each field.

The calibration scan data collected each night were reduced using the standard 2MASS Production Processing System (2MAPPS) used to process survey data. Photometric and astrometric calibration were handled differently for the calibration scan data because of the presence of the photometric standard stars within the fields, and because there were very few if any Tycho 2 astrometric reference stars in the fields. Features of calibration data reduction that differed from the main survey are described in A4.4.

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. Table 2 summarizes the number of images and sizes of the calibration source and metadata tables that are provided as part of the 2MASS Extended Mission. 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. The initial calibration scan data product release includes the observations of the main survey calibration fields. The Magellanic Cloud calibration scan products will become available in 2007.

The general properties of the Cal-WDBs and Image Atlas are in A4.2. Cal-WDB and meta-data table formats are provided in A4.3. Special aspects of the Calibration data processing are given in A4.4.

Users are strongly encouraged to read the Cautionary Notes on the 2MASS calibration scan data products before using the Working Databases or Image Atlas.

Table 1 - Calibration Field Information

Tile	Nscn	ra(J2000)	dec(J2000)	glon	glat	elon	elat	Notes, Objects of Interest
90021	2025	6.10619	-1.97294	107.367	-64.025	4.823	-4.237	HD2023 - K1III, BRI0021-0214 - nearby, high proper motion M9.5V
90294	2621	8.31622	-39.40154	318.916	-77.157	348.904	-38.817	HD98784 - K0/K1III
90299	468	11.25260	-70.58353	303.708	-46.535	315.929	-63.013	SMC field.
90004	2977	28.66074	+0.71693	154.117	-58.275	26.892	-10.328
90301	3430	51.72678	-39.84268	244.635	-55.506	32.087	-55.852
90247	1962	53.01617	+37.34386	155.043	-15.318	59.866	+17.694	HD18552 - B8Vne
90533	1839	55.26392	+6.93647	179.513	-36.676	54.620	-12.339
90191	2086	66.58918	+3.62342	190.909	-29.778	65.383	-17.838	LHS191 - nearby, high proper motion M6.5V
90400	378	74.90247	-65.73341	276.286	-35.956	0.762	-83.854	LMC field. NGC 1783 - globular cluster
90401	156	78.62001	-71.00065	282.110	-33.373	307.642	-83.962	LMC field.
90013	3515	89.28447	+0.01890	206.632	-12.049	89.220	-23.418
90402	432	93.56589	-69.66665	279.959	-28.523	248.477	-86.623	LMC field. Hodge 11 - globular cluster
90121	562	97.37444	-59.65713	268.951	-25.881	119.127	-82.345
90161	2593	105.22232	+48.48935	168.289	21.455	101.129	+25.636
90312	3501	126.40319	-39.09847	257.574	-0.669	145.023	-55.799
92026	2158	128.12790	-1.57084	226.548	21.547	130.978	-19.751	LHS2026 - nearby, high proper motion dwarf
90067	3692	132.81203	+11.84773	215.639	31.905	131.935	-5.579	M67 - open star cluster
90091	789	145.75214	+59.06160	154.888	44.631	125.013	+42.210	NGC 2950 - SB0 galaxy
92397	2585	170.45775	-13.22047	271.788	44.166	176.597	-15.904	LHS2397a - nearby, high proper motion M8V
90217	1687	180.44070	-50.05148	294.815	12.036	205.734	-44.539
90266	2776	183.61047	+35.59855	163.599	78.431	167.187	+33.672	UGC 7252 - Scd galaxy
90860	2452	185.41757	-0.12034	287.009	61.828	185.021	+2.042
90867	949	220.24529	-0.45767	351.082	51.867	217.981	+14.457	NGC 5719
90273	1780	224.21932	-44.81900	325.155	12.572	235.303	-26.739
90272	1973	224.64303	+37.14173	61.476	61.448	205.765	+50.966
90868	2186	225.11368	-0.65787	356.367	48.363	222.849	+15.740	TVLM 868-53850 - nearby, high proper motion M5V
90565	3396	246.68168	+5.87185	20.521	34.698	243.720	+27.209
90009	1582	246.80780	-24.68901	352.970	16.585	249.005	-2.924	rho Ophiuchus star forming region
90330	1192	247.89420	+30.14552	50.250	42.071	238.704	+51.210
90279	977	267.09736	-45.42783	346.065	-8.926	267.803	-22.011
90182	1703	279.89627	+49.09363	78.074	22.009	291.129	+71.808
90547	671	282.82780	-4.27488	29.111	-1.920	283.507	+18.565	Lynds 547 - galactic dark nebula
90808	1876	285.48438	-4.48794	30.125	-4.377	286.259	+18.082
90234	2076	307.83812	-49.64775	349.607	-36.216	297.217	-29.722
90813	1570	310.27504	-5.06339	41.333	-26.643	311.324	+12.787
92409	1439	330.11998	+20.84962	77.728	-26.651	340.586	+30.780	Abell 2409 - galaxy cluster
92202	2802	331.40247	-11.07477	47.133	-47.917	329.508	+0.608	BRI2202-1119 - nearby, high proper motion M5.5V
90893	2619	349.54575	+0.54857	80.124	-54.382	350.607	+4.644	NGC 7589 - SAB galaxy
90290	1141	352.64369	+38.31591	105.945	-21.874	11.226	+37.503
90298	108	356.63061	-74.50079	308.690	-41.894	303.851	-61.386	SMC field.

a. Multi-Epoch Measurements

The distinguishing feature of the 2MASS calibration field observations is the large number of repeated, highly uniform measurements of each field made with a sampling cadence that ranged from ~1 minute (the mean time separating scans within a calibration observation) to ~3.5 years (the duration of the survey). The sampling frequencies and long-term photometric and color trending data available in the resulting source WDBs and Images are a unique resource for conducting time-domain investigations.

The extensive time sampling of the calibration WDBs make possible identification and detailed analysis of the light curves of long period and irregular variable stars. For example, Figures 2 shows the phase-folded light curves of an eclipsing binary star in the 90312 calibration field, one of 246 red variables identified by Plavchan et al. (2008 ApJS, 175, 191) during a survey for eclipsing M-dwarfs using the full calibration PSWDB. The light curve of an RR Lyrae variable identified in the 90004 field by Becker, Bochanski, Claire and Ivezic (private comm.), shown in Figure 3, reveals exquisite detail made possible by nearly 3000 independent measurements. In high galactic latitude fields, the multi-epoch data enable 3-band variability studies of known active galactic nuclei, and offer the prospect of conducting color-independent, variability-based searches for previously unknown objects. Near-infrared light curves of the radio-quiet QSO RX J1631.4+2952 (z=0.25) and radio-loud quasar B3 1456+375 (z=0.33) generated from the calibration PSWDB are shown in Figure 4.

The frequent sampling combined with 2MASS's excellent internal astrometric precision also make the calibration point source WDB a resource for proper motion studies. Several of the 2MASS calibration fields contain known, nearby M-dwarfs, and used them as the primary standard stars (III.2.d). Figure 5 shows the RA and Dec motion of LHS 191 (M6.5V), the primary calibrator in the 90191 field. A least-squares fit to the motion ellipse gives a proper motion of 1.022±0.001 arcsec/yr and a parallax of 0.056±0.002 arcsec, which agree well with the values measured by Monet et al. (1992 AJ, 103, 638).

The calibration point source WDB contains associations with 8540 asteroids and comets. Because of the highly redundant nature of the calibration observations, over half of all detected asteroids are seen six or more times, providing the opportunity for light curve studies of brighter bodies, and orbit refinement. The most frequently observed solar system body in any of the 2MASS data sets is the asteroid (1528) Conrada 1940 CA, which was sighted 112 times in the 90067 calibration field between December 7 and 20, 1999 (UT). The large number of detections occurred because the asteroid fortuitously executed its apparent prograde-to-retrograde swing within the field, as shown in Figure 6. Each apparent "point" in Figure 6 is actually four to six independent calibration scan position measurements, illustrating the internal accuracy of the astrometry. Intensity variations due to the asteroids rotation are seen in the raw and phase-folded light curves of (1528) Conrada shown in Figure 7.

Figure 2 - J, H and Ks light curves for an eclipsing variable in the 90312 calibration field folded to a period of 8.09 days (Plavchan et al. 2008 ApJS, 175, 191).	Figure 3 - Phase-folded J and Ks light curves and magnitude-color behavior for an RR Lyr variable in the 90004 calibration field (Becker et al. private comm.).	Figure 4 - J, H and Ks light curves for two known QSOs that lie in calibration fields.
Figure 5 - Proper motion of LHS 191. Cyan points are positions from individual calibration scans, and black points are average positions from six scans in each calibration field observation.	Figure 6 - J2000 position of asteroid (1528) Conrada from 112 independent detections in the 90067 calibration field between December 7 and 20, 1999 UT.	Figure 7 - (left) Raw J, H and Ks light curves of asteroid (1528) Conrada from the Cal-PSWDB. (right) Phase-folded light curves (period = 6.321 hours) of magnitudes corrected for secular variations from the changing heliocentric and topocentric distance and observing phase angle using the predicted V magnitude (Vpred). The residual periodic variation is due to rotation.

b. Additional Calibration Scan Products

The calibration scan data are unique among the 2MASS data sets because of the highly redundant nature of the observations. This redundancy also makes the Cal-WDBs and Image Atlas somewhat challenging to use as a stand-alone resource, since any query based solely on position and/or flux/color may return a large number of measurements of the same objects.

A useful entry point for inquiries into the Cal-PSWDB and Cal-XSWDB are the Calibration Merged Source Information and Cross-Reference tables which are designed to to help exploit the multi-epoch nature of the calibration field scan data. The merged source tables contain the average brightnesses and positions of all measurements of reliable sources detected in the calibration field observations, as well an assortment of statistics designed to help gauge the quality of combined photometry and astrometry and identify candidate variable, transient or moving objects. The cross-reference tables provide simple access to all sightings of the same object in the respective Cal-WDBs for follow-up analyses. The combined photometry and astrometry of hundreds to thousands of independent measurements yields photometric and internal astrometric precision far in excess of what is possible from the main 2MASS survey (e.g.A6.1) or individual calibration scans. The process used to merge the WDBs and the resulting products are described in Appendix 6.

Appendix 7 describes the Combined Calibration Scan Image products, which yield extraordinarily deep observations of the calibration fields. Virtually all Atlas Images of each field have been combined with optimal weighting and scaling to produce extremely sensitive and well-calibrated images that reach 3.5-4.5 magnitudes deeper than the main 2MASS survey observations. Preliminary source extractions are also provided for each field to provide guidance for calibration to users who wish to perform specialized source detection and characterization on these often complex and crowded fields.

c. Cautionary Notes

i. General

ii. Calibration Scan Point Source Working Database

iii. Calibration Scan Extended Source Working Database

iv. Calibration Scan Atlas Images

[Last Updated: 2008 March 14; by R. Cutri]