V. Catalog Generation
1. Overview
Pipeline processing described in Section IV is in reality only the first step towards preparing the final products for 2MASS. The second and equally important phase of data processing is Catalog Generation.
Pipeline scan processing generates Working Databases, which contain all "detections" of point and extended sources drawn from each scan of a Survey Tile. The scan processing detection thresholds are intentionally set low to insure the completeness of the Survey. Therefore, in addition to detections and measurements of valid astrophysical sources, the Working Point and Extended Databases contain:
- A large fraction of low reliability and/or low SNR noise detections of noise.
- Multiple apparitions of true sources that fall in the overlap regions between Tiles and in multiple observations of the same Tile.
- A large number of residual artifacts from bright stars that either fall nearby but not on the processed scans, or from very bright stars for which there is not a valid measurement on which to base artifact cleanup.
- Spurious detections on meteor trails and other transient atmospheric phenomena.
- In the case of the XSC, residual multiple stars and other false resolved sources.
The objective of Catalog Generation processing is to select only those sources from the Working Databases that will result in release Catalogs that satisfy the 2MASS Level 1 Science Requirements for uniformity, sensitivity, completeness and reliability.
The process of generating Catalogs from the Working Databases begins with the selection of observations of Survey Tiles that were taken under conditions of seeing, background level and structure and atmospheric transparency that yield data that nominally satisfy the requirements for sensitivity and uniformity for the Survey. Once the Tiles are selected for the release, then sources from those Tiles are selected that meet SNR and other quality thresholds that insure that the reliability of the catalog will be high enough, but not so strict that the completeness is compromised.
For inclusion in the Second Incremental Data Release Point Source Catalog, a candidate source must meet all of the following criteria:
- Source >10´´ from a tile edge
- Photometric signal-to-noise ratio (SNR)
7 in
at least one band - Goodness-of-fit
2 from profile-fit
photometry <10 in at least one band - Sources brighter than J < 14.5 or H < 14.0 or Ks < 13.5 mag must be detected on >2 individual frames, and on >40% of possible frames.
- Not identified with bright star artifact or meteor trail
- Only one apparition of multiply detected source in overlap region. Singly-detected sources in overlap regions included only if on correct side of boundary between scans.
To be included in the Second Incremental Data Release Extended Source Catalog, a candidate source must meet all of the following criteria:
- Source >15´´ from a tile edge
- Photometric SNR 7 in J or Ks, and
J
15.1 or Ks
13.5 mag - e_score or g_score <1.4
- Only one apparition of multiply detected source in overlap region. Singly-detected sources in overlap regions included only if on correct side of boundary between scans.
- No sources in elliptical region centered on galactic center with semi-major axis of 12.8° (along the plane) and an axis ratio of 0.47.
The following sections provide more detailed descriptions of each of these Catalog Generation processing phases.
[Last Update: 1 March 2000, by R. Cutri]
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