VI. Analysis of the Release Catalogs

1. Comparison of Achieved Performance of the All-Sky Release Catalogs
with Level 1 Science Specification

m. Atlas Images/PSC/XSC/Postage Stamps Sky Coverage

The Level 1 Specifications stipulate a sky coverage >95% with no gaps greater than 200 sq. deg.

The 2MASS scanning strategy was devised around 59,650 "Tiles" with dimensions of roughly 6° × 8.5´. Observations of these Tiles ensure coverage of the entire sky. Tiles were laid out such that adjacent ones would provide overlap of 10% (approximately 50´´) in right ascension and 8.5´ in declination. Three effects are responsible for decreasing the sky coverage from 100%:

• Holes in sky coverage: Although scans of all Tiles were obtained during the course of the Survey, not all scans fell perfectly on their intended Tile positions. As a result, on rare occasions slight gaps would arise between adjacent scans, either along the east-west edges in RA or along the north-south edges in Dec. Although the presence of such gaps was monitored throughout the course of the Survey, certain "gap fill" observations were never successfully obtained at a quality high enough for inclusion in the Catalogs or Atlas Images.
• Effective gaps between scans: The criteria for Catalog source selection stipulate that a point source be further away than 10´´ from a scan edge and an extended source be further away than 15´´. For scan pairs that overlap by less than 20´´, there will be an effective sky coverage hole in the PSC. Similarly, for scan pairs that overlap by less than 30´´, there will be a hole in the XSC.
• Bright stars: It is not possible to probe to the Level-1 sensitivity specifications in the haloes and diffraction spikes around these objects. However, this area is not lost entirely to the PSC; objects bright enough to be distinguished from bright star debris can still be astrometered and photometered, though with reduced accuracy. Completeness in such regions is, nevertheless, reduced, relative to parts of the sky lacking bright stars. Detailed sky coverage maps for the XSC are available in section II.6f.

TABLE 1: Sky Coverage
Release Product	Area Missed	Sky Coverage	Notes
Atlas Images	0.706 sq.deg.	99.998%	Ten small gaps remain on the sky.
PSC	201.074 sq. deg.	99.513%
			In addition to the missing coverage of 0.706 sq. deg. noted above, there are 46 effective gaps in RA and seven in Dec, where the overlap between adjacent scans falls below 20´´. The total area for these additional gaps is 1.258 sq. deg.
			At H-band where the effect is worst, the total area "lost" to bright stars is 199.110 sq.deg.
XSC	<800 sq. deg.	>98%
			In addition to the missing coverage of 0.706 sq. deg. noted above, there are 112 effective gaps in RA and 52 in Dec where the overlap between adjacent scans falls below 30´´. The total area for these additional gaps is 1.648 sq. deg.
			The extended source processing also requires larger masking around bright stars than does the point source processing. In scans which lack very bright stars, the effective coverage for the XSC exceeds 98%.
Postage Stamps	<800 sq. deg.	>98%	The extended source postage stamps, since these are image cut-outs of sources detected by extended source processing, will have the same incompleteness as quoted for the XSC.

The 2MASS All-Sky Release dataset far exceeds the Survey requirements for >95% sky coverage. As this missing area is scattered over the sky, all contiguous coverage gaps are well below the 200 sq. deg. upper limit as well.

[Last Updated: 2009 Nov 23; by J.D. Kirkpatrick]