V. Catalog Generation

4. Multiple Source Detection Resolution

2MASS Survey Tiles (see III.2b) overlap each other by 10% (51´´) in right ascension on the equator-most ends, and by 8.5´ in declination on both ends. Because the Tiles are oriented along lines of constant RA, they converge towards the poles resulting in increasing RA overlap in that direction. There are 311 overlapping Tiles that cover the area within approximately 8´ of the celestial poles.

Sources that fall in the Tile overlap regions were observed multiple times during the Survey, and may have been detected each time. Therefore, the WDBs contain multiple, independent measurements for many of these sources. To maintain photometric uniformity in the All-Sky Catalogs, only one apparition of each multiply-detected source was selected for release, rather than combining the multiple sightings. For multiply-detected sources, the apparition that lies farthest from the edges of its respective Tile, independent of source brightness or measurement quality, was selected for the Catalogs. Faint sources that toggle above and below the 2MASS detection limit, variable brightness sources, and moving objects, such as asteroids and high proper motion stars, may not be detected in every scan that covers their position. Sources detected only once in the overlap strips are are also passed to the Catalogs, but with cautionary flagging.

The results of the multiple detection resolution process are encoded in the use_src and dup_src flags carried with each PSC and XSC entry. The values of these flags, described below, indicate if a source falls in a Tile overlap region, if it was detected multiple times in the set of scans (see V.1) used for the All-Sky Data Release, and if confusion was encountered during the resolution process. Sources are selected for the All-Sky Data Release PSC and XSC if they have:

use_src = 1    OR    (use_src=0 AND dup_src=0)

a. Multiple Detection Resolution

The process by which the use_src and dup_src values are set for each source is as follows:

  1. Sources that do not fall in the Tile overlap regions have dup_src set to 0 and use_src set to 1.

  2. Sources that fall in the overlap regions of adjoining Tiles, and that satisfy the selection criteria described in V.2, are positionally correlated using a match radius of 2.0´´ for point sources and 5.0´´ for extended sources.

  3. For each group of positionally-matched detections in the overlap regions:

    1. If there is more than one detection in the match-group, all have dup_src set to 1 to indicate that the source has duplicate detections. If the group contains only one source (i.e., a source had no matching detections in adjacent scans), then dup_src is set to 0. If a source finds a positional match to more than one detection in another scan, then dup_src is set to a value >1, to indicate confusion.

    2. For a source that is not detected in all scans that cover its position, a virtual duplicate is created in the scan(s) in which it was not detected at the equatorial position of the real detection. Virtual duplicates are formed only if they are not within the 10´´ (PSC) or 15´´ (XSC) safety boundaries (V.3) used for source selection in their respective scans.

    3. For each real and virtual detection in the matched group, the minimum of the east-west (dist_edge_ew) and north-south (dist_edge_ns) distances from the source to the nearest respective scan edges is then determined. The scan edges are defined by the great-circle interpolations between actual reconstructed scan corners.

    4. The apparation in each group that has the largest value of min(dist_edge_ew, dist_edge_ns) is then identified. If it is a real source, then that detection has use_src set to 1, and all of the other detections in the matched group have use_src set to 0. If a virtual apparition has the largest value of min(dist_edge_ew, dist_edge_ns), then use_src is set to 0 for all real apparitions in the group.

The requirement for inclusion in the 2MASS All-Sky PSC and XSC (use_src=1 OR [use_src=0 AND dup_src=0]) means that the Catalog sources are:

  1. not in a Tile overlaps region, or
  2. in a Tile overlap region and closer to the center of their respective Tiles than matching detections in overlapping Tiles, or
  3. in the Tile overlap region and not detected in all overlapping Tiles.

In the final case, (c), if the detection of the source in an overlap is closer to the center of its Tile than a detection of it in the adjacent Tile would have been, then the source has use_src=1 and is considered a part of the high reliability Catalog subset of the PSC. Detections farther from their Tile centers than their virtual duplicates in overlapping Tiles have use_src=0 and dup_src=0 and are assigned to the lower SNR extension (see II.2) of the PSC. This distinction is made, because if all single-detected apparitions in the overlaps were passed uniformly into the Catalog, then the overlap regions would have a bias to slightly greater sensitivity, because faint sources effectively have two chances to be detected.

Approximately 1.4% and 3.7% of all sources in the PSC and XSC, respectively, have use_src=0 and dup_src=0. These are predominantly faint, having SNR values less than 10, and are cases where the source is toggling above and below the detection or resolution threshold. For example, the J, H and Ks differential source counts for the 28,819 PSC sources with |b|>75°, and use_src=0 and dup_src=0, are shown in Figure 1. 98.4% of these have SNR<10.

Sources not detected in all coverages are handled differently in the All-Sky Release than they were in the 2MASS Second Incremental Data Release (IDR2). Non-multiply-detected sources in the overlap regions that were farther from their Tile centers than their virtual duplicates were not included in the IDR2 PSC.

Figure 1

i. Graphical Examples of Overlap Source Resolution

Figures 2-17 below show graphically the result of the selection of point sources in Tile overlap regions for a set of eight Tiles in the All-Sky Data Release. Each figure shows sources centered on one Tile (specified by the observation date, hemisphere and nightly scan number) that are drawn from either that Tile (outlined in black), or the adjacent Tiles (outlined in other colors). Open circles are sources from the central Tile not in overlap regions (use_src=1, dup_src=0). Filled circles are sources in the overlaps. The color of the filled circle indicates from which Tile the apparition marked for inclusion in the Catalog is selected. Thus, in the first example from scan 100, the red circles on the eastern edge come from the Tile to the west, and the green circles come from the Tile to the east. The black circles are drawn from the central tile. Open squares show objects that fall in the outside safety boundary edge of the central Tile.

Most of the examples show the relatively simple two-scan interface. The example for scan 107 south, illustrates the behavior of the resolution algorithm in the complex boundaries between three Tiles. The algorithm is generalizable to large numbers of overlaps, such as will occur near the equatorial poles.

For each Tile, two figures are presented. One illustrates the southern boundary and the other illustrates the northern.

Figure 2Figure 3Figure 4Figure 5
970609n Scan 100 south970609n Scan 100 north 970609n Scan 101 south970609n Scan 101 north

Figure 6Figure 7Figure 8Figure 9
970609n Scan 102 south970609n Scan 102 north 970609n Scan 103 south970609n Scan 103 north

Figure 10Figure 11Figure 12Figure 13
970609n Scan 104 south970609n Scan 104 north 970609n Scan 105 south970609n Scan 105 north

Figure 14Figure 15Figure 16Figure 17
970609n Scan 106 south970609n Scan 106 north 970609n Scan 107 south970609n Scan 107 north

b. The use_src and dup_src Flags

The multiple source detection results are encoded in the dup_src and use_src flags carried with each source record in the PSC and XSC. A summary of their possible values and meanings are as follows:

The matrices in Tables 1 and 2 give the number and of PSC and XSC sources and percentages of the Catalogs that have each allowable combination of use_src and dup_src.

Table 1 - PSC use_src and dup_src Combinations
use_src 0 1>1
1 385,594,875 (81.9%) 78,431,392 (16.7%) 429,888 (0.09%)
0 6,536,185 (1.4%) N/A N/A

Table 2 - XSC use_src and dup_src Combinations
use_src 0 1>1
1 1,402,327 (85.1%) 179,520 (10.9%) 5,657 (0.3%)
0 60,095 (3.7%) N/A N/A

c. Multiple Detection Resolution Confusion

If more than one matching detection is found for a source in an overlapping Tile, the multiple-detection process is in a confusion state. That source has dup_src set to a value >1 to indicate the confusion -- the exact value is a status flag used during the processing and is not numerically significant. Detections must be closer together than the resolution limit of the 2MASS system for this to occur. Therefore, this type of confusion is usually the result of a multiple source being resolved in one Tile and not another, or a single source that is artifically split, due to a failed bandmerge in one Tile and not another. Confusion in the multiple-detection resolution process is a relatively rare occurrence. There are 429,888 sources in the PSC and 5,657 in the XSC that have dup_src > 1, representing <0.1% and 0.3% of each Catalog, respectively.

The confusion in the multiple-detection resolution process is resolved using the same edge-distance rules as for the unconfused case. The detection that is closest to its respective Tile center is the source that had use_src set to 1, while the remaining detections have use_src=0. Because of the confusion, and because the resolution is done independently of source measurement quality, though, the detection passed to the Catalogs may not be the best available measurement for that object. In general, sources with dup_src > 1 should be examined carefully and used with caution.

d. Unbounded Tile Edges

The overlap source resolution described above requires knowledge of the actual position of adjacent Tiles. For the Tiles enclosing the 21 coverage gaps (see III.2c) in the All-Sky Release, there will not be overlapping Tiles on certain edges. In these cases, all sources meeting the selection requirements described in V.2, out to the safety boundary (V.3) on the unbounded Tile edges, will have use_src=1 and dup_src=0 and will be passed to the Catalogs.

[Last Update: 2003 March 12; by R. Cutri, S. Wheelock]

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