II. The WISE All-Sky Data Products

4. Ancillary Products

a. Reject Table

The WISE All-Sky Reject Table contains 283,887,651 detections that were made on the Atlas Images during Multiframe Pipeline processing that were not included in the All-Sky Release Source Catalog because they did not satisfy the Catalog selection quality criteria described in V.3. These "source" extractions were rejected from the Catalog because:

• they have flux signal-to-noise ratios <5 in all bands,
• they are believed to be spurious detections of image artifacts caused by bright sources, including Mars, Jupiter or Saturn,
• they were detected in an area with less than five Single-exposure image depth-of-coverage, or
• they are redundant detections of sources in Tile overlap regions for which an alternate detection was selected for inclusion in the Source Catalog.

WISE Multiframe Pipeline processing generated a Working Database of all detections on the Atlas Images from which both the Source Catalog and Reject Table are drawn. Therefore, the source detection and extraction algorithms are the same for both and are described in IV.4. Directions for accessing the Reject Table are given in I.3.

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

i. Column Descriptions

The format of the Reject Table is the same as that for the All-Sky Source Catalog and is described in II.2.a.

ii. Using the Reject Table

As a rule of thumb, you should assume that every entry in the Reject Table is spurious until you have verified its reliability by visually examining its position on the Atlas Images.

The Reject Table is not a well-vetted list of reliable mid-infrared source detections. As the name implies, the majority of entries in the Reject Table are likely spurious detections of low SNR noise events, image artifacts, or transient pixel events such as cosmic rays. However, the Reject Table does contain some low SNR extractions of real sources, as well as reliable measurement of bright sources that may have been omitted from the Source Catalog because of overly conservative artifact flagging in some regions, or because they fall in areas with low Single-exposure depth-of-coverage. Used with caution, the Reject Table can be a useful database that augments the Source Catalog, particularly when following-up sources that appear on the Atlas Images but are not found in the Catalog.

The following are some examples where the Reject Table entries may be useful.

• Low SNR sources - Pipeline extracted positions and fluxes for faint sources. SNR threshold for Catalog selection required SNR>5 in at least band. As discussed in VI.6, the reliability of extractions down to the Multiframe Pipeline detection limit (SNR>3.5) is estimated to be >99% in unconfused regions. The reliability is lower in higher source density and complex regions, but still better than approximately 80%. Because of the multi-band nature of source extraction used in the WSDS, source reliability is driven strongly by the band with the highest SNR, and this is most often (but not always) W1. Therefore, Reject Table entries that have w1snr<5 and are not otherwise flagged as contaminated by a bright source image artifact (cc_flags='0000') or in a low coverage region (w1m>4) stand a reasonable chance of being reliable detection.
• Overly conservative artifact flagging - As described in IV.4.g, flagging source extractions in Multiframe Pipeline processing was a complex procedure that could not be perfect in all circumstances. In particular, three areas in which the flagging was known to be overly conservative and could result in the incorrect exclusions of sources from the Catalog were in very high source density regions, near the Ecliptic poles, and in the vicinity of extremely bright stars.
  • In very high source density regions in the Galactic plane, the contrast of image artifacts is lower with respect to the the elevated background emission levels. The background-dependence was compensated by background scaling of the predicted diffraction spike length in the artifact identification procedure. The scaling was not perfect in regions with the highest background levels, and could result in overflagging extractions as spurious detections of artifacts. You can check the reliability of Reject Table entries in high source density and high background regions by examining them on the Atlas Images.
  • Near the Ecliptic poles, the rotation of the WISE scan angle and the pixel outlier rejection used in the image coaddition procedure resulted in diffraction spikes from bright stars would appear significantly shorter than from stars of the same brightness at lower ecliptic latitudes. The effects of scan rotation were compensated by ecliptic latitude-dependent scaling in the artifact identification subsystem. However, this scaling was not perfect and resulted in a net overflagging of extractions as spurious detections of diffraction spike artifacts near the Ecliptic pole (e.g. see Figure 21 in IV.4.g.i). Therefore, Reject Table entries that are within a few degrees of the Ecliptic poles and are flagged as spurious detections of diffraction spikes (cc_flags matches '*D*') may be real source detections. Verify this by examining the source on the Atlas Images.
  • The artifact identification and flagging geometric model used in the WSDS pipelines was based on the position and brightness of the "parent" source that produced the artifact. Sources brighter than approximately 2.0, 1.5, -3.0 and -4.0 mag in W1, W2, W3 and W4, respectively, were heavily saturated and were not reliably extracted. The artifact identification procedure filled in missing bright source detections using a prior table of very bright sources with positions and brightnesses estimated from other sources such as IRAS and 2MASS (see IV.4.g). The accuracy of artifact identification depended strongly on the accuracy of the parent source parameter knowledge, so extractions near the augmented bright sources could be either over- or under-flagging depending how well the predicted position and brightness matched the true values for the sources at the time of the WISE observations. Therefore, Reject Table entries in the vicinity of very bright sources may be reliable. Verify this by examining the source on the Atlas Image.

• Low depth-of-coverage regions - The All-Sky Catalog source selection criteria required WDB extractions to have been made in areas for which there were five or more Single-exposure images available for the source measurement because pixel outlier rejection will effectively suppress spurious detections of single frame events such as cosmic rays, satellite trails, and noise excursions. As illustrated in Figures 1 and 2, the Reject Table contains reliable detections of sources in regions with depth-of-coverage too low to be included in the Catalog.

  The Reject table will also contain spurious detection of single frame events, though, so care must be taken to discriminate between real and false detections in these regions. Some artifacts, such as detections of satellite trails, are easy to identify visually on the Atlas Images by their morphology. However, spurious detections from some cosmic ray strikes and single-pixel noise events can mimic real sources on the Atlas Images because of the smoothing effects of the PSF-interpolation in the coaddition procedure. The most reliable entries in the Reject Table in regions where the depth-of-coverage w?m<5 in all bands, and where the source appears real on the Atlas Image, will have been detected on all of the available Single-exposure images, so the ratios w?nm/w?m will be close to unity. The corollary to this is that the smaller the number of Single-exposure images that were available for the Reject Table extraction, w?m, the less confident you should be in the reliability.

Figure 1 - A 1000x1000 arcsec section of the southwest corner of W1 Atlas Image 1088p696p_ab41. Green boxes mark the positions of All-Sky Source Catalog entries and red box mark the positions of entries in the Reject Table. Many of the Reject Table entries in this the upper left half of this image are reliable source detections that fall in low depth-of-coverage areas.	Figure 2 - The 1000x1000 arcsec section of the southwest corner of the W1 depth-of-coverage image for Atlas Tile 1088p696p_ab41. The All-Sky Catalog source selection criteria required a minimum depth-of-coverage of five or more Single-exposure images on which a source could be measured.

Last update: 2012 February 24