Gator Documentation:
2MASS Scan Catalog Caveat
Caveat for spatial constraint searches on the 2MASS All-Sky Survey Scan Info Catalog:
Beware of using cone searches on this catalog.
Currently, only the right ascension and declination of the scan centers (columns ra and dec) are used in resolving searches with spatial constraints. The center of a scan is calculated from the equatorial system unit vectors of the four corners (ra_[1,2,3,4] and dec_[1,2,3,4]) listed in the table. The three bands cover slightly different areas, so these corners are actually the corners of the three-band-coverage area of the scan, and the center of the scan is therefore the center of the three-band-coverage area.
Because of the 2MASS tiling pattern on the sky, scan centers are spaced approximately 7.26 true arcmin apart in RA (after accouting for overlaps) and 6° apart in Dec.* Because spatial constraints are resolved using only the scan center positions, it is difficult to search this table for a list of scans covering a certain area of the sky. A small areal search will return a scan only if the area specified in the spatial constraint happens to lie within the search radius of a scan center. Even a spatial constraint search covering a large area will miss scans in the region of interest. In both cases the radius should be increased by at least 3° to include the center of any scan having even a small corner in the search area. However, such a list of returned scans will include scans that do not actually cover the true area of interest.
A better way to search for scans covering a specific small area is to first search the Point Source Catalog for sources contained in that area, using spatial constraints or user-defined constraints in RA and Dec, and include the scan_key column, found in the long form, in the query output columns. (Using the PSC virtually guarantees that any scan covering part of the area will be included in the output; the lower source density in the XSC means that some scans might be missed.) Then compile a list of unique scan_key values that appear in the output table; these are the cntr values of the desired scans in the Scan Info Catalog. Finally, construct a query on the Scan Info Catalog that includes the list of these values in the User Defined Constraints box; for example, if the unique scan_key values were 10, 11, and 52, put the following into the constraint box:
(cntr = 10 or cntr = 11 or cntr = 52)
Gator will 'AND' this constraint with all other constraints on the query (hence the need for parentheses), and return data for only these scans.
*The positions of tiles on the sky mean that scan centers are usually found near declinations ±86.9° ±80.9°, ±74.9°, ±68.9°, ±62.9°, ±56.9°, ±50.9°, ±44.9°, ±38.9°, ±32.9°, ±26.9°, ±20.9°, ±14.9°, -8.9°, -2.9°, 2.9 or 3.1°, and 8.9 or 9.1°. (See the 2MASS All-Sky Data Release Explanatory Supplement page on Data Acquisition: Survey Strategy for further information on survey tiles.)
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