II. User's Guide to the WISE Preliminary Data Release

2. Source Catalog

The WISE Preliminary Release Source Catalog contains positions and four-band photometry for 257,310,278 objects detected on the Atlas Intensity Images. Positions, magnitudes, astrometric and photometric uncertainties, flags indicating the reliability and quality of the source characterizations, and associations with the 2MASS Point and Extended Source Catalog sources are presented for each source.

A summary of the properties of the Source Catalog and a detailed Catalog format description are presented below. More detailed descriptions of photometric and astrometric characteristics of the Catalog are presented in VI. Descriptions of processing algorithms used to generate the the Source Catalog can be found in IV.4.

Users are strongly encouraged to read the Cautionary Notes before using the Catalog.

a. Format and Column Descriptions

b. Source Selection Criteria

The WISE Preliminary Release Source Catalog contains positions and four-band photometry for 257,310,278 objects extracted from 10,464 Tiles covering approximately 57% of the sky.

Sources in the Catalog were drawn from a Working Database (WDB) of all detections made on the coadded Atlas Images that was generated during Multiframe pipeline processing. The WDB contains 352,839,115 entries, significantly more than the Source Catalog. Many WDB entries are spurious detections of noise and image artifacts. The reliability of the WISE Source Catalog was achieved by selecting entries in the WDB that satisfy the criteria that are described in V.3 and summarized in Table 1.

Table 1 - Source Selection Criteria for the Preliminary Release Source Catalog
Detection Reliability:
Extractions must have a "reliable" detection in at least one band. A reliable band-detection must satisfy all these criteria in that band.
Signal-to-Noise Ratio (SNR)Source must be detected with a SNR>7 Rejection of spurious detections of low SNR noise excursions.
Frame coverage The source must be measurable on four or more single-exposure frames.Rejection of single-exposure transient events that can persist into the coadded images because of low coverage.
Frame detection fractionFor sources brighter than the nominal single-exposure SNR=5 detection limit (W1<15.3, W2<14.4, W3<10.1 and W4<6.7 mag), it must have been detected at SNR>3 in >40% of the frames available for measurement. Rejection of spurious detections of bright single-exposure transients.
ArtifactsExtraction must not be identified as a spurious detection of an image artifact from a bright source.

Sources believed to be real, but whose measurements are affected by artifacts, are included in the Catalog, and are flagged using the cc_flags parameter.

Rejection of spurious detection of image artifacts.
Tile Boundaries Extraction lies >50" from the edge of the Atlas TileTo avoid truncation of the largest measurement aperture used for the source aperture photometry.
Duplicate Source Entries For multiply-detected sources in Atlas Tile overlap regions, the apparition farthest from its respective Tile edge is selected. Sources in the overlap regions not multiply-detected are always included in the Source Catalog if they satisfy all other selection criteria.To eliminate duplicate extractions of the same source

c. General Properties

d. Photometric Properties

e. Astrometric Properties

WISE Source Catalog positions are reconstructed with respect to the 2MASS Point Source Catalog reference frame. The accuracy of the astrometric solution for the sources brighter than W1~13.0 mag is better than is 0.2 arcsec (see Figure 1 in VI.5).

Approximately 25% of the source fainter than W1~14.5 mag have declinations coordinates that can be systematically offset from their true positions by 0.2" to 1." This bias can be seen in a decreasing fraction of sources as bright as 13.0 mag, and the sign of the offset alternates with scan direction and equatorial. The bias was caused by an identified source extraction software error that failed to apply a 0.5 Atlas Image pixel declination offset to some sources when performing simultaneous PSF-fitting with other detections in close proximity (i.e. passive deblending). The error does not affect right ascension measurements. See VI.5 for illustrations and further descriptions of the impact of this error.

There was insufficient time to correct this bias prior to the Preliminary Data Release. Therefore, the quoted declination uncertainties in the Source Catalog, sigdec, have been inflated to reflect the presence of the bias. The uncertainties were adjusted by adding 0.5" in quadrature to the statistical declination measurement uncertainty (see VI.5.b). This yields a maximum absolute deviation for the distribution of uncertainties equal to the what would be produced if the declination error distribution was Gaussian.

Last Updated: 2011 December 28

Previous page   Next page
Return to Explanatory Supplement TOC