V. Catalog Generation
1. Overview - Catalog vs. Working Database
The objective of the Catalog Generation phase of 2MASS data processing was to extract and prepare highly uniform and reliable Point and Extended Source Catalogs that satisfy or exceed the Level 1 Science Requirements of the Survey from the Working Databases (WDBs) produced during final pipeline scan processing (IV).
Scan processing generated WDBs that contain a total of 1,314,981,867 and 2,590,500 "detections" of point and extended sources, respectively, from scans of all Survey Tiles, much more than the number of sources in the PSC and XSC. The WDBs are highly complete at the SNR>7 limits of the catalogs because detection thresholds in scan processing were intentionally set well below SNR=7 (IV.4.a.iii). As a result, the WDBs contain many noise-triggered unreliable sources at low SNR. In addition to data for the sources that eventually comprise the Catalogs, the WDBs contain:
- Multiple detections of sources that were scanned more than once during the survey. Most of these fall in the Tile overlap regions, but ~14% of the all Tiles were scanned repeatedly either for validation purposes or to obtain measurements under better atmospheric conditions.
- A large number of low signal to noise ratio detections that are a mixture of faint, real astrophysical sources and noise excursions.
- Residual multiple stars and other false resolved sources, in the case of the Extended Source WDB.
- All flagged and unflagged detections of image artifacts produced by bright stars
- Detections of transients such as cosmic rays, residual meteor trails and hot pixel events
Catalog Generation draws the subset of sources from the highly complete WDBs to form the highly reliable and uniform release Catalogs. The process begins by first selecting the scans of Survey Tiles that a) were taken under atmospheric conditions that yield data that nominally satisfy the requirements for sensitivity and uniformity for the Survey, and b) provide optimal coverage of the sky (V.2). Sources are then selected from those scans using SNR and other quality filters that ensure the reliability of the Catalogs without compromising the completeness levels (V.3). Single apparitions of multiply-detected sources are then selected without effectively increasing the sensitivity in the Survey in the Tile overlap regions (V.4).
The following sections provide more detailed descriptions of each of the Catalog Generation processing steps.
[Last Update: 2003 March 12, by R. Cutri]
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