The objective of multiple source resolution processing in 2MASS final product generation is to select one apparition of sources detected more than once because they fall in scan overlap regions. This process was carried out for the full Point and Extended Source Catalog Generation (CatGen) DBs.
The result of multiple source resolution is that every source in the CatGen DBs have values assigned to the use_src and dup_src flags, that are used in determining whether or not a source. will be used in the final release catalog. A source is accepted for the final catalog if:
and other final selection criteria are met for the Catalogs.
The counts for the different combination of use_src and dup_src values for the point source CatGen DB are given in Sherry Wheelock's web page. A summary is:
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A total of 470,084,953 point sources meet the use_src and
dup_src criteria for the release. A number of these are
subsequently rejected from the PSC because of the cc_flg
requirement.
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A total of 1,722,915 extended sources meet the use_src and dup_src criteria for the release. A number of these are subsequently rejected from the XSC because of the cc_flg requirement.
Multiple source resolution should produce a source list that contains pairs of sources from different scans that have separations of < 2". The source proximity calculation for point sources reveals that there are 19 unique pairs of sources from different scans with separation < 2", and the list of pairs is given below (note that each pair is listed twice).
List of < 2" Separation Source Pairs
Eighteen of the pairs are associated with one scan, 990227s scan 053 (tile 307281), and one is associated with 970724n scan 048 (tile 8333). As noted in night processing QA, both of these scans were offset in RA relatively far from the nominal tile positions. 990227s s053 was offset -442.4" and 970724n s048 was offset -159.4". Because of these large offsets, some of the overlaps between these two scans and adjacent scans were omitted from the multiple source resolution processing. This resulted in the 19 pairs being associated and having proper values of use_src and dup_src being set.
The proper values of use_src and dup_src for these sources are given here.
The 469 million row public data release contains 909,985 rows in which the nearest neighbor is within 4" and the nearest neighbor is in a different scan. I have used these rows to study the duplicate resolution algorithm used by IPAC.
Figure 1 shows the distribution in separation of the sources with neighbors in different scans. There are a few sources with separations < 2". These have been identified as missed duplicates because of an error in the processing which is limited to two scans. The other significant feature shown in this plot is the peak at 2" separation. There are approximately 50,000 sources in the public release in the peak. I did not find evidence that this peak is populated by sources with large proper motion. Figure 2 shows the Julian Day difference between the observations of sources which have a nearest neighbor in a different scan with a separation between 1.8 and 2.2 seconds. The peaks separated by 360 days are an artifact created by the observatory scheduling software. The distribution of the sources in the 2" peak by scan shows that they are preferentially located in scans with low galactic latitude. They appear to be a consequence of confusion.
Table 1. The scan distribution of sources with neighbors between 1.8 and
2.2 arc seconds. Only the scans with the largest number of sources are
listed in the table.
scan_key | count | glon | glat
----------+-------+----------+----------
71794 | 25 | 1.7965 | 0.9086
10379 | 24 | 2.2865 | 0.0981
10376 | 23 | 2.104 | 0.402
10384 | 22 | 2.587 | -0.41
35377 | 22 | 29.5378 | 0.2025
27425 | 21 | 330.3194 | 1.7017
10377 | 21 | 2.165 | 0.3001
26097 | 21 | 339.6748 | 0.8628
10380 | 20 | 2.3471 | -0.0034
2220 | 20 | 9.8373 | -1.0266
10383 | 20 | 2.5272 | -0.3085
11315 | 20 | 354.6753 | 0.9304
26096 | 20 | 339.5987 | 0.95
71795 | 19 | 1.8582 | 0.8077
25518 | 19 | 9.0275 | 0.4326
10375 | 19 | 2.0433 | 0.503
34529 | 19 | 9.6649 | -0.7102
26742 | 19 | 340.4313 | -0.0164
25957 | 18 | 29.4861 | 0.3008
26750 | 18 | 341.0194 | -0.728
49649 | 17 | 316.0657 | -3.148
36757 | 17 | 29.5919 | 0.0973
23365 | 17 | 30.1928 | -1.0729
23364 | 17 | 30.137 | -0.9671
11313 | 17 | 354.5445 | 1.1256
33182 | 17 | 43.7972 | -1.2612
10374 | 17 | 1.982 | 0.6051
The maximum number of sources in the 1.8-2.2 second peak in scans where the galactic latitude is greater than 10 degrees is 8.
I conclude from the above that the 2" search radius used for duplicate resolution is a conservative choice and that the number of missed duplicates in the public release is insignificant.
Last Updated: 21 August 2002
R. Cutri (IPAC), S. Wheelock (IPAC), R. Stiening (UMass)