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,Ks=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 Ks, 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 1Figure 2

Figure 3Figure 4Figure 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. Ks. 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 7Figure 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.]

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