V. Atlas and Catalog Generation

3. Catalog Source Selection

The WSDS Multiframe pipeline processing generates a Working Database (WDB) that contains extractions of all "sources" detected on the coadded (L3a) images, and characterized on both the coadded and individual Single-exposure (L1) images. In first-pass processing for the Preliminary Data Release, the source detection threshold is set to SNR>5, a relatively conservative level. Despite this, the fractional reliability (e.g. the ratio of the number of reliable source detections to the number of all source extractions) of the WDB is lower than required for the Source Catalog (>99,9% for SNR>20). In addition to extractions of real astrophysical sources the WDB contains:

The Source Selection criteria for WISE Preliminary Release Source Catalog generation, described below, are designed to draw from the WDB the subset of rows that constitute a reliable Catalog with accurate photometry and positions that also satisfies the completeness objectives of the survey (>95% for SNR>20). Entries in the WDB are selected for inclusion in the Source Catalog if they meet all of the criteria listed below.

a. Tile Edge Boundaries

The position of a WDB entry must be >50" from the boundary of the Atlas Image from which it was extracted to be included in the Preliminary Release Source Catalog. The purpose of this safety boundary is to eliminate entries that may have incorrect aperture photometry in the largest measurement aperture, which has 50" radius.

For this purpose, the distance to an Atlas Image boundary is given by the coadd pixel coordinates of a source (wx, wy). Atlas Images are 4095x4095 pixels in size and the coadd pixel scale is 1.375"/pix. Thus, the edge boundary criterion requires a source to have pixel coordinates in the range

36.4≤wx≤4058.6 pix and 36.4≤wy≤4058.6 pix.

Because of the overlap between Atlas Tiles, any source will usually fall more than 50" from the edge of some Tile. However, because the Preliminary Release does not cover the entire sky, Tiles at the outer longitude boundaries of the Release areas have "unbounded" edges. Sources in the 50" outer boundary of these Tiles will be trimmed from the Catalog. Thus, the Preliminary Release Source Catalog covers a slightly smaller area than do the Atlas Images.

b. Source Reliability Criteria

To be included in the Source Catalog, a WDB entry must have what is considered a high-quality, reliable detection in at least one WISE band. A reliable detection in a band must satisfy all of the following criteria in that band. Note that if a WDB satisfies all of these criteria in a band, it will be included in the Catalog even if the detections in other bands are not deemed reliable.

i. Detection and SNR Limits

Sources selected for the Source Catalog must have a valid detection in the band and that detection must have have SNR>7. The detection validity and SNR measurement is required to be in the profile-fit photometry.

The net selection criterion is:

(w?mpro is not null AND w?sigmpro is not null AND w?snr>7)

where "?" is the band, 1, 2, 3 or 4.

ii. Artifact Flagging

To filter out spurious detections of image artifacts caused by bright sources, a Source Catalog entry must not be flagged as a spurious detection of an artifact in the band. Spurious detections of artifacts are flagged by having a capital letter value in the cc_flags parameter. Thus, the requirement is:

(cc_flags not matches '*[DHOP]*')

iii. Frame Coverage

To be included in the Source Catalog, a WDB entry must be extracted from a region with a minimum depth-of-coverage of at least five independent frames. This translates to a requirement of:

(w?m > 4)

This requirement is designed to eliminate spurious detections in low coverage areas within Atlas Tiles. Because they do not persist on the sky, transient events such as cosmic ray strikes, residual satellite trails, hot pixel events and scattered light from very bright moving objects such as the moon and planets are usually suppressed by outlier rejection during the image coaddition process. However, pixel-level outlier rejection becomes ineffective when four or fewer frames are available for coaddition, and single-frame transients can persist into the coadds and induce spurious detections.

iv. Frame Detection Statistics

Source Catalog entries must be detected at SNR>3 on Single-exposure frames more than 40% of the times it was observed and measurements were possible. This requirement is applied only to sources that are brighter than the approximate Single-exposure detection limits of W1<15.3, W2<14.4, W3<10.1 and W4<6.7 mag. Coupled with these brightness limits, the fractional detection requirement is:

(w?nm/w?m > 0.4)

This requirement leverages the redundant sky observations enabled by the WISE survey strategy to further reject bright, spurious detections of transient events.

c. Duplicate Source Resolution

WISE Atlas Tiles overlap adjacent Tiles by 180" in Declination and Right Ascension on the celestial equator. The RA overlap increases towards the equatorial poles. Source that fall within the Tile overlap region may be detected during Multiframe processing for each Tile, and thus may appear multiple times in the L3 WDB. These detections are redundant because the same Single-exposure frame images contribute to each overlapping Tile. Only one entry from these duplicate source apparitions is chosen from the Catalog.

A WDB entry is selected for inclusion in the Source Catalog if it satisfies the above criteria, and it is selected in the duplicate source resolution processing that identifies one, unique detection of multiple-detected sources in the overlap regions. The duplicate source resolution procedure is described in V.4.

Last update: 2011 April 26

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