V. Catalog Generation

2. Tile Selection

The 2MASS Second Incremental Release contains source and image data drawn from observations of 27,493 Tiles, covering an area of 19,681 sq.deg. (~47%) of the sky. The selection of Tiles for the Release uses the the overall scan "quality" assigned during the Quality Assurance process (cf. IV.10), and several other criteria described in the following sections.

a. Date Range

This period included 329 northern and 239 southern nights that have at least one photometric period.

b. Photometric Quality

Tiles in the date ranges described above that were assigned a photometric quality of 0.5 were candidates for the Release. As described in IV.10, the photometric quality is a combination of the photometric calibration assessment for a night and the photometric sensitivity based upon the measured average seeing and background conditions during the observation.

While the overall photometric quality factor assigned a Tile is a complex combination of parameters, a quality 0.5 usually indicates that the following conditions were met:

• The Tile was observed during a period of at least three continuous hours during a night during which time the RMS residual to the measurement of the photometric zero point (cf. IV.8) was < 0.04 mag
• The combination of average seeing and background indicated that the data from the Tile have a >50% probability of meeting or exceeding the minimum photometric sensitivity requirements for the Survey
• Extended Sources only: The atmospheric seeing did not vary faster than could be tracked within a certain tolerance during pipeline processing, as required for reliable discrimination between point and extended sources
• The scan of the Tile had no other fatal flaws such as focus problems, corruption by an aircraft fly-by, insects on the camera lens, earthquakes shaking the telescope, etc.

Candidate Tile observations for the Second Incremental Release were required to have a measured average seeing FWHM 3.5´´. Seeing conditions worse than this value generally lead to poor photometric sensitivity and begin to compromise the completeness achieved when trying to discriminate between small, faint extended and stellar sources.

d. Residual Background Structure

A significant challenge to accurate H-band photometry of extended sources in 2MASS processing is residual structure in the measured sky background due to atmospheric OH airglow emission. When the airglow emission varies on spatial scales smaller than the scale over which the extended source processing attempts to fit the Atlas Image backgrounds (cf. IV.5), the resulting background estimate and photometry will be compromised. The expected noise in a 2MASS Atlas Image can be predicted from the detector characteristics, mean background illumination and source density (confusion noise). A single 2MASS Atlas Image can have post-fit residual noise levels larger than the predicted values due to the presence of bright stars or galaxies. However, empirical analysis has shown that when more than three Images in a Tile have H-band residual noise greater than the predicted level, the origin is invariably strong small-scale structure in the airglow emission.

e. Multiple Tile Observations

A number of Tiles have been observed more than once during the course of Survey operations for engineering or validation purposes. For the Release Catalogs, a single observation of each Tile is selected for inclusion. The observation of a Tile that has the best assessed quality, considering photometric accuracy, background and seeing conditions, was selected for the Release.

f. Tile Contiguity

The achieved accuracy of 2MASS position reconstruction (cf. IV.6) depends on the number and distribution of ACT astrometric reference stars in each Tile. In Tiles that contain very few or poorly distributed reference stars, the astrometric solution can wander off the reference frame. The global consistency of the astrometric solution for a particular Tile relative to surrounding Tiles, that contain different reference stars, is gauged by examining the repeatability of positions of sources in Tile overlap regions. The positional uncertainties quoted in the PSC are adjusted to reflect the global consistency of the astrometric solutions; sources in "ACT-challenged" Tiles have larger uncertainties than those in Tiles with many and/or well-distributed reference stars. The three-contiguous Tile requirement is driven by the need for accurate assessment of the astrometric uncertainties.

g. Other area considerations