V. Catalog Generation

4. Tile Edges and Overlap Resolution

a. Tile Edges

Point sources are required to be >10´´ from Tile edges, and extended sources must be >15´´ from Tile edges to be candidates for passing from the Working Point and Extended Source Databases to the respective Catalogs. For this purpose, a Tile Edge is defined to fall along the great circles interpolated between the four Tile corners that are listed for each Tile in the Release in the Scan Information Table. These safety boundaries serve three purposes:

  1. The three arrays in each 2MASS Camera are aligned on the sky to within 1-2 pixels. Although small, the misalignment produces a 2´´-3´´ region of less than three-band coverage around the Tile borders within which a source may not be reliably detected in all survey bands, resulting in a bandmerged source missing detections in one or more bands.

  2. The point and extended source photometric routines (cf. IV.4b and IV.5) may produce incorrect brightness estimates for sources that that fall close enough to a Tile edge that the source or sky apertures intersect the edges. Use of the safety boundaries omits these sources.

  3. The RA tracking stability of the 2MASS telescopes during the fast declination scanning (~1°/minute) is excellent. Figure 1 shows the RA difference (in arcseconds) between the astrometrically-reconstructed frame centers and the starting of-date RA for a typical 2MASS scan. The maximum deviation from the interpolation between the endpoints is < 2´´. A histogram of the maximum RA deviation from a great circle path on the sky for all of the scans of Tiles included in the Second Incremental Data Release is shown in Figure 2; the most common maximum deviation is only ±2´´. The small tails of the distribution indicate that rare excursions of up to 7´´-8´´ are possible, though, so the true Tile edges may lie slightly inside or outside the great circle interpolated edges. The safety boundaries will compensate for these cases.

Figure 1Figure 2

b. Overlap Resolution

Adjacent Survey Tiles overlap by a minimum of 52´´ in right ascension and approximately 8.5´ in declination (cf. III.2.a). Sources that fall in the overlap regions between Tiles may be detected two or more (in the corner regions and near the poles) times during the Survey. Thus, the Working Databases can contain multiple apparitions of such sources. The Point and Extended Source Tables in the 2MASS Sampler data release (1998 December) intentionally included all detections of sources in the overlaps regions to provide users an idea of the duplication of sources between Tiles.

The 2MASS Incremental Data Release Catalogs are intended to contain a unique entry for each source on the sky. Because the Catalogs are also to have a high degree of photometric uniformity, it is necessary to select only one apparition of a multiply-detected source in the Tile overlaps regions rather than averaging the properties from multiple detections. To avoid introducing subtle sensitivity biases, a simple geometric algorithm is used to select which apparition of a source is selected as a candidate for the Catalogs. Special care is made in handling faint sources near the detection limit of the survey that may not be detected, or in the case of extended sources, resolved in all scans that cover their position.

For all sources that fall in the nominal Tile overlap regions, less the safety borders described above, all possible apparitions are found by positionally matching sources in adjacent scans using a search radius of 2´´ for point sources, and 5´´ for extended sources. A particular apparition of a source is a candidate for inclusion in the Catalog using the following criteria:

Note that if a source in the overlap region is not detected in all possible coverages, it will be passed into the final Catalog only if it is on the proper side of a geometrical boundary. This normally affects faint sources near the detection limits of the Catalogs, but may also affect brighter sources that either move between the time adjacent Tiles are observed (e.g., asteroids or high proper motion stars), or variable brightness sources. The handling of sources not detected in all coverages for this Release is different than in the Catalog Generation for the 2MASS First Incremental Data Release. In the First Incremental Data Release, a non-multiply-detected overlap source would always be a candidate for the Catalog, regardless of its position. Thus, the First Incremental Data Release Catalogs had a slight sensitivity enhancement in the Tile overlap areas.

i. Graphical Examples of Overlap Source Resolution

Figures 3-18 below show graphically the result of the resolution of point sources in Tile overlap regions for a set of eight Tiles in the Second Incremental 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. 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 3Figure 4Figure 5Figure 6
970609n Scan 100 south970609n Scan 100 north 970609n Scan 101 south970609n Scan 101 north

Figure 7Figure 8Figure 9Figure 10
970609n Scan 102 south970609n Scan 102 north 970609n Scan 103 south970609n Scan 103 north

Figure 11Figure 12Figure 13Figure 14
970609n Scan 104 south970609n Scan 104 north 970609n Scan 105 south970609n Scan 105 north

Figure 15Figure 16Figure 17Figure 18
970609n Scan 106 south970609n Scan 106 north 970609n Scan 107 south970609n Scan 107 north

c. Unbounded Tile Edges

The overlap source resolution described above requires knowledge of the actual position of adjacent Tiles. If a Tile in the Second IDR has edges unbounded by other Tiles, then all sources out to the safety boundary on the unbounded edges will be candidates for passing into the Catalogs. In future Releases, the adjacent Tiles will be included, and the resolution of sources in the overlaps will be revised.

[Last Update: 11 February 2000, by R. Cutri, S. Wheelock and H. McCallon]

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