VI. Analysis of the 2MASS Second Incremental Release Catalogs

2. Global Photometric Properties

c. Photometric and Astrometric Accuracy Using Tile Overlaps

S. Wheelock generated information on sources seen in the overlap regions of the 25,545 scan pairs in the Second Incremental Data Release. The files she created included position and magnitude differences for each source identified as having a match in the overlap zone. We generated summary statistics for each file, including means and for all sources seen in each scan pair (clipping magnitude differences greater than 1 mag as unreliable). We evaluated these statistics for all stars at a limit where the signal-to-noise ratio should be very good (J,H,K_s=13,12.5,12 mag). We excluded sources brighter than 8 mag to avoid contamination with very bright objects. The average number of high quality sources in an overlap region is 115, with a low of 7 and a high of over 5000. The total number of sources considered here is 3,415,987. The statistics per scan-pair should give a good measure of the positional and magnitude repeatability of the sources seen in those scans.

Table 1 and the figures below provide an overview of the quality of the released data. Figures 1 and 2 show histograms of the scan-to-scan offsets in RA and Dec, respectively, while Figures 3, 4, and 5 show histograms of the scan-to-scan offsets in J, H, and K_s, respectively. The very low values of the dispersions around zero offset (<~0.1´´ and <0.01 mag) is a testimony to the quality of the individual measurements and to their overall uniformity.

Table 1. Overlap Statistics
Quantity	Value
(RA1-RA2)	0.090"
(DEC1-DEC2)	0.089"
(J1-J2)	0.0096 mag
(H1-H2)	0.0094 mag
(K1-K2)	0.0096 mag

Figure 1	Figure 2

Figure 3	Figure 4	Figure 5

Figure 6 shows the dispersions in RA and Dec plotted against each other. We found only one scan with RA offsets greater than 1´´. Scans with offsets greater than 0.75´´ in either RA or DEC are shown in Table 2 (below). Most are scans with problems in RA and all are scans with relatively few stars.

Figure 6

Table 2. Scans with RA/DEC Offsets > 0.75"
Scan	Number	RA	DEC	DRA	pop RA	mean RA	DDEC	pop Dec	mean Dec
out2_980502n9	11	178.82	16.12	1.081	0.242	0.073	0.214	0.135	0.041
out2_980301n106	>7	174.76	27.13	0.916	0.137	0.052	0.419	0.242	0.091
out3_981201n145	14	160.99	25.35	0.752	0.237	0.063	0.244	0.315	0.084
out3_981227s93	17	152.57	-2.84	0.821	0.640	0.155	0.164	0.237	0.057
out3_990120s115	15	175.79	-3.52	0.827	0.498	0.129	0.496	0.251	0.065
out3_990120s90	18	174.20	-3.53	0.813	0.216	0.051	0.459	0.157	0.037
out3_990124s115	16	193.49	-3.78	0.824	0.228	0.057	0.375	0.199	0.050
out3_990126s87	14	176.39	-3.30	0.918	0.159	0.043	0.230	0.132	0.035
out3_990216s96	18	191.63	-8.32	0.908	0.612	0.144	0.437	0.191	0.045
out3_990219s113	17	225.50	-2.89	0.830	0.482	0.117	0.203	0.278	0.067
out3_990221n115	9	217.25	26.10	-0.193	0.110	0.037	0.910	0.401	0.134
out3_990518n15	8	197.19	27.50	0.845	0.202	0.072	0.108	0.116	0.041

Figures 7 and 8 show dispersions in J vs. H and H vs. K_s. Common mode problems indicative of calibration problems can be seen for a few scans. Scans with photometric offsets greater than 0.04 mag in two or more bands are listed in Table 3. These scan pairs may have problems with their overall calibration and may need to be rescanned. Finally, we list scans in Table 4 with offsets greater 0.04 mag at > 3 significance in any one band. These scans might have PSF or other problems in one scan.

Figure 7	Figure 8

Table 3. Scans with Correlated Photometric Offsets > 0.04
File	Number	RA	Dec	Mean J	Sigma Pop J	Sigma Mean J	Mean H	Sigma Pop H	Sigma Mean H	Mean K	Sigma Pop K	Sigma Mean K
out_970608n51	21	248.00	20.81	-0.055	0.022	0.005	-0.041	0.025	0.006	-0.049	0.035	0.008
Out_970614n19	17	223.64	20.85	0.049	0.031	0.007	0.043	0.032	0.008	0.053	0.039	0.009
Out2_980115n93	12	190.97	15.10	0.043	0.071	0.021	0.047	0.061	0.017	0.056	0.046	0.013
Out2_980302n80	11	166.61	26.14	0.022	0.040	0.012	0.048	0.070	0.021	0.048	0.068	0.020
Out3_980705s45	5125	271.63	-27.00	0.053	0.072	0.001	0.046	0.094	0.001	0.033	0.083	0.001
Out3_990331n35	7	144.37	16.16	0.035	0.025	0.009	0.044	0.036	0.013	0.042	0.023	0.009
Out2_971119n118	131	75.21	27.35	0.071	0.045	0.004	0.053	0.039	0.003	0.060	0.042	0.004
out2_980319s142	5250	271.50	-27.35	-0.053	0.078	0.001	-0.055	0.094	0.001	-0.043	0.082	0.001

Table 4. Scans with single Band Offsets > 0.04 mag
J	out_970614n80	H	out_970614n19	K	out_970614n19
J	out_970614n19	H	out_970608n51	K	out_970608n51
J	out_970609n79	H	out2_980516n28	K	out_970608n28
J	out_970609n43	H	out2_980425s64	K	out2_980530s77
J	out_970608n51	H	out2_980319s142	K	out2_980418s63
J	out_970608n28	H	out2_980319s136	K	out2_980406s28
J	out2_980530s77	H	out2_980301n105	K	out2_980401s103
J	out2_980515s66	H	out2_971217n123	K	out2_980331n79
J	out2_980511n35	H	out2_971119n118	K	out2_980319s142
J	out2_980508n36	H	out3_980705s45	K	out2_980115n93
J	out2_980505n51	H	out3_980719s152	K	out2_980109n156
J	out2_980412n12	H	out3_980727s120	K	out2_980106n39
J	out2_980406s41	H	out3_980814s23	K	out2_971129n138
J	out2_980328s101	H	out3_980814s24	K	out2_971119n118
J	out2_980319s142	H	out3_980814s25	K	out3_980705s36
J	out2_980311n104	H	out3_980814s7	K	out3_980716s144
J	out2_980302n77	H	out3_980916s89	K	out3_980716s79
J	out2_980130n77	H	out3_981002s39	K	out3_980804s67
J	out2_980128n95	H	out3_981030s14	K	out3_980815s108
J	out2_980120n27	H	out3_981101s21	K	out3_980815s42
J	out2_980113n11	H	out3_981212s13	K	out3_980828s119
J	out2_980112n26	H	out3_990207n142	K	out3_980916s96
J	out2_980108n24	H	out3_990310n77	K	out3_990216s94
J	out2_980107n70	H	out3_990311n56	K	out3_990216s95
J	out2_980107n29	H	out3_990323n126	K	out3_990331n35
J	out2_980106n27	H	out3_990324n91	K	out3_990512n32
J	out2_971119n118	H	out3_990331n35	K	out3_990512n49
J	out3_980624s119
J	out3_980705s45
J	out3_980707s48
J	out3_980918s70
J	out3_981014s63
J	out3_981019n14
J	out3_981112s35
J	out3_981113n92
J	out3_981118n41
J	out3_981201s107
J	out3_981213s14
J	out3_981222s112
J	out3_990127s85
J	out3_990130s102
J	out3_990210s98
J	out3_990306n59
J	out3_990310n56
J	out3_990313n138
J	out3_990313n139
J	out3_990321n112
J	out3_990321n113
J	out3_990331n31
J	out3_990331n42
J	out3_990425n20
J	out3_990426n36
J	out3_990506n49
J	out3_990510n30

[Last Updated: 2000 Jan 24 by C.A. Beichman. Modified 2000 Jul 21 by S. Van Dyk.]