Entries in the WISE All-Sky Release Catalog
and Reject Table are subject to many of the same limitations because they
were produced using the same detection and characterization algorithms
during Multiframe Pipeline processing.
Therefore, you are strongly encouraged to read the
Cautionary
Notes for the All-Sky Catalog before 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.
As the name implies, the All-Sky Release Reject Table contains primarily spurious detections of low SNR noise excursions, image artifacts produced by bright sources and transient single frame events such as cosmic rays and satellite trails. However, the Reject Table also contains detections of real, low SNR sources, reliable measurements of brighter sources that were rejected from the Catalog because of overly conservative artifact flagging or because they fall in very low frame depth-of-coverage regions. 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.
Suggestions for how to identify reliable source detections in the Reject Table are given in II.4.a.ii.
WISE Atlas Tiles overlap each other by a minimum of 3 arcminutes on the celestial equator, and by increasing amounts moving towards the equatorial pole. Objects that fall in Tile overlap regions may be detected and extracted from more than one Tile during Multiframe Pipeline processing. These duplicate extractions are redundant because they are derived from essentially the same input coadded and Single-exposure image data in all overlapping Tiles. The Duplicate Resolution step in Catalog Source selection identifies one of the redundant extractions for inclusion in the Source Catalog (assuming other Catalog source selection criteria are met). The remaining duplicate extractions are contained in the Reject Table.
Redundant source detections from Tile overlap regions can be identified by the best_use_cntr column in the Reject Table entry. This column identifies the source extraction from associated groups of extractions that was selected for inclusion in the Catalog during the Duplicate Resolution process. Reject Table entries that have a value of best_use_cntr that is not equal to its cntr identifier were eliminated from the Catalog because they are redundant detections.
The Single-exposure Source Database is not a reliable "Catalog." The Database contains all source extractions from all Single-exposure images produced during Scan/frame processing regardless of their quality. Unreliable extractions have not been filtered out of the Source Database as they are from the All-Sky Release Source Catalog.
The Single-exposure Source Database contains not only detections and measurements of real astrophysical sources, but also spurious detections of noise excursions, transient events such as hot pixels, charged particle strikes, and satellite streaks, and image artifacts caused by bright sources such as latent images, diffraction spikes, and optical ghosts. Spurious detections in the Single-exposures have not been filtered out. Unreliable entries have not been flagged as spurious in most cases. An attempt has been made to flag detections that are associated with the expected position of image artifacts, but even in second-pass processing that flagging is not perfect.
Because the Single-exposure Source Database contains a large number of unreliable extractions, using the Database to conduct statistical studies of source populations is not recommended.
When in doubt about the reliability of an entry in the Single-exposure Source Database, one of the best things to do is to examine its image on the Single-exposure or All-Sky Release Atlas Images.
WISE obtained multiple, independent observation covering every point on the sky. However, some of the individual WISE Single-exposure framesets were corrupted image smearing due to spacecraft motion, electronic noise, scattered light from the moon or other very bright objects near the field-of-view, as discussed in I.4.d.iii.3. Low quality framesets were identified during the Quality Assurance process in the Scan/frame Pipeline processing, and were subsequently excluded from Multiframe Pipeline processing so they would not contaminate the All-Sky Release Image Atlas or Source Catalog. However, all source extractions made on all Single-exposure images are included in the Single-exposure Source Database, regardless of the quality of the Single-exposure image data.
Several metadata quality parameters have been added to the individual records in the Single-exposure Source Database to help you to identify extractions that come from lower quality individual framesets. These parameters and constraints that can be used to identify extractions from good quality framesets are given below.
WISE typically observed each point on the sky twelve to thirteen times near the ecliptic plane, and more at higher ecliptic latitudes. Therefore, when an astronomical source is bright enough to be detected in the WISE single exposures, it may have multiple, independent entries in the Single-exposure Source database. This feature of the database makes it a useful data set for time-domain studies, such as the search for source motion and/or flux variability. However, because the Single-exposure measurements are susceptible to contamination by transients and artifacts, these studies must be done with caution. Conversely, in conjunction with the Single-exposure Images, the Database can be useful to test whether a source with unusual properties in the Catalog may have been contaminated by unflagged cosmic rays or image artifacts in one or more of its measurements.
Examples of the utility of the Database and Single-exposure Images include WMOPS processing to identify moving object tracklets and variability flagging in the Catalog (see the var_flg).
WISE was designed to acquire multiple observations of each point on the sky to enable filtering out of transient events that can affect any single measurement. Filtering was implemented in the Multiframe Pipeline processing where many individual exposures are combined. However, filtering could not done for the Single-exposure measurements made during Scan/frame Pipeline. Therefore, users must use extreme caution when using measurements from the Single-exposure database. The best way to assess the reliability of a Single-exposure Database entry is to examine its position on the Single-exposure Images, or better yet on the All-Sky Atlas Image.
Detections in the Single-Exposure database may not have counterparts in the WISE Source Catalog for several reasons.
The corollary to this cautionary note is that if a counterpart to a Single-exposure Database entry is found in the All-Sky Catalog, then it is very likely a reliable extraction.
Aperture photometry measurements made on the Single-exposure images in the Scan/frame Pipeline processing suffer from a number of known deficiencies related to the nature of the data and the measurement algorithms.
Defer to the profile-fit photometric measurements in the Single-exposure database (w?mpro) whenever possible because they are robust to many of these deficiencies. If it is necessary to utilize the aperture photometry, always make use of the aperture measurement quality flags, w?flg.
The Single-exposure Images contain a large and diverse quantity of image artifacts. Examples of the artificial features that be seen in the Single-exposure Images are shown in the Image Anomaly Gallery.
Single-exposure framesets may be affected by the following problems:
Most of the low quality framesets and frames were excluded when selecting frames to construct the coadded Atlas Images and Source Catalog (V.2.c), but all are present in the Single-exposure image archive. Use this archive with caution.
You can identify Single-exposure image sets that are of good quality using several of the keywords in the FITS headers and/or values in the metadata tables as follows:
WISE obtained multiple, independent images of every point on the sky. However, not all of these individual exposures were of sufficient quality to be used to generate the All-Sky Release Image Atlas, and were therefore not used during Multiframe Pipeline processing. The minimum quality requirements for the Image Atlas resulted in a few regions that have no effective coverage, as discussed in VI.2.
Because the Single-exposure Image archive contains all WISE single-exposure images, regardless of quality, you will find Single-exposure image coverage even for regions that have not Coverage in the All-Sky Image Atlas. However, the quality of images in those regions will usually be poor, as described in I.4.d.iii.3, above.
The Single-exposure WISE images have intrinsically different pixel sizes, orientations, and distortions which are different in each of the four bands. Therefore, sources do not fall at the same raw pixel locations in the different bands. These effects are accounted for in the WCS parameters recognized (for example) by the DS9 image analysis software which is in wide use by astronomers, but other image analysis software may not utilize this information.
In constructing the coadded Atlas Images during Multiframe processing, the Single-exposure images were reprojected ("rubbersheeted") onto a common grid corrected for distortion (IV.4.f.vii), but the Single-exposure images have not been reprojected.
The Single-exposure image calibration process in the Scan/frame Pipeline removes instrumental image signatures over a very large range of astrophysical scenes and background levels. There are a few known limitations to the Image Calibrations that are listed below.
The WISE payload electronics encoded pixels in the raw images that saturated the amplifier Analog-to-Digital converters on any sample-up-the-ramp read-outs, and that encoding was captured in the Single-exposure Bit Masks (see IV.4.a.i.2). The mask values were read in downstream processing to identify pixels that should be excluded from computations such as source profile-fitting photometry. On rare occasion, the payload encoding failed for some pixels in the cores of very bright sources where the saturation occured on the first sample measurement. In these cases, the photometric measurements will underestimate the true brightness because of the contribution of the saturated pixel(s) that will have an intensity near zero. The only way to recognize this rare occurrence is that there will be a near-zero pixel value near the core of a bright source that is not properly denoted in the Single-exposure Bit Masks.
The Known Solar System Object Possible Association list contains a tabulation of asteroids, comets, planets and planetary satellites, known at the time of the WISE data processing, that may be within the field-of-view of individual Single-exposures.
When the predicted position of the solar system object coincides with an entry in the Single-exposure Source database, the WISE database entry position and brightness is provides. In the majority of cases the associated WISE detections are not valid detections of the solar system object. They are more likely to be chance associations with a background inertial source or a spurious detection of noise, a single-frame transient or an image artifact from a bright source. See section IV.4.e.v for tips on how to recognize false associations.
The Moving Object Tracklets are the vetted list of WISE detections of solar system objects. See II.4.e.ii for instructions on how to retrieve tracklet information from the MPC and then search the WISE Single-exposure Source Database to extract the WISE measurements corresponding to the tracklets.
Last update: 2012 December 28