The Single-exposure Source Database is not a "Catalog" in the same sense as the reliable Preliminary Release Source Catalog. It 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.
The Single-Exposure Source Database contains not only detections 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 in first-pass processing that flagging was not yet optimal.
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 images.
WISE typically observed each point on the sky twelve times near the ecliptic, or 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 entries in the Single-Exposure Source DB. This feature of the DB 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.
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 ( 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. This cannot be done for the single-exposure measurements, or course. Therefore, users must use extreme caution when using measurements from the Single-exposure DB.
Detections in the Single-Exposure DB may not have counterparts in the WISE Source Catalog for several reasons.
Aperture photometry measurements made on the Single-Exposure images in the Scan/frame 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 DB 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 meaurement quality flags,
The Image Anomaly Gallery
contains examples of the artifacts commonly encountered in the
Many of these framesets suffer from 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:
Areas covered by the Single-Exposure images that may not be covered by the Preliminary Release Image Atlas include the following.
Single-exposure WISE images have intrinsically different pixel sizes,
are different in each of the four bands. Therefore, sources will not fall
at the same pixel locations between 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, Single-Exposure
images were reprojected ("rubbersheeted") onto a common grid corrected for
distortion (IV.5.a.vii), but the
Single-exposure images have not been reprojected.
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 WISE Single exposures.
When the predicted position of the solar system object coincides with an entry in the Single-exposure Source DB, 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.3.e.iv for tips on how to recognize false associations.
Use the Moving Object Tracklets to provide a vetted list of WISE detections of solar system objects. See >II.4.d.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 January 18