The NEOWISE Post-Cryo Preliminary Release data are superseded by the NEOWISE 2013 Post-Cryo Release products. The Post-Cryo Preliminary Release products were generated during first-pass processing that used early versions of the WISE Science Data System (WSDS) pipelines and instrumental calibrations that are known to have been sub-optimal. This combined with the fact that the first-pass processed Post-Cryo data was not closely scrutinized by the project's quality assurance scientists mean that there are a number of limitations and problems, known and unknown, that persist into the Post-Cryo Preliminary Release products. A number of these deficiencies and problems were corrected during second-pass processing. Therefore, you are recommended to always defer to the 2013 Post-Cryo Release data products.
NEOWISE Post-Cryo Release data products are subject to most of the same limitations and exhibit the same features as the April 2012 All-Sky Release Single-exposure image and extracted source database. We recommend that you begin by reviewing the Cautionary Notes for the All-Sky Release Single-exposure data before making use of the Post-Cryo data.
Section VIII.1.d.ii.1 below contains lists of the most important things that you should know about the Post-Cryo Preliminary Release products to make the best use of them in your research. Click on any item in the list to see a more detailed discussion.
Sections VIII.1.d.ii.2, VIII.1.d.ii.3 and VIII.1.d.ii.4 contain detailed information that pertain to the Post-Cryo Preliminary Release products in general, and specifically to the Single-exposure Source Database and Single-exposure Images, respectively. The Known Solar System Object Possible Association List is subject to the same limitations that apply to the All-Sky Release that are described in I.4.d.iv.
You are strongly encouraged to read and familiarize yourself with all of these Cautionary Notes before using the NEOWISE Post-Cryo Preliminary Release data products.
Image and source data products for the Post-Cryo Preliminary Release were generated during first-pass processing that used early versions of the WISE Science Data System (WSDS) pipelines and instrumental calibrations that were developed for the WISE Cryogenic survey data and were known to be sub-optimal for the Post-cryogenic data characteristics. This combined with the fact that the first-pass processed Post-Cryo data was not closely scrutinized by the project's quality assurance scientists mean that there are a number of limitations and problems, known and unknown, that persist into the Post-Cryo Preliminary Release products. A number of these deficiencies and problems were corrected during second-pass processing. Therefore, you should always defer to the 2013 Post-Cryo Release data products.
The WISE Post-Cryo Preliminary Release contains data taken between 29 September 2010 and 1 February 2011 UTC, following exhaustion of solid hydrogen in the both payload cryogen tanks. The W3 and W4 detectors were fully saturated by the increasing thermal emission from the WISE telescope, so no useful data were acquired in those bands.
WISE scanned approximately 70% of the sky during the Post-Cryo survey phase in two broad ecliptic longitude intervals: 97.1°<λ<221.8° and 280.6°<λ<48.2°. See VIII.1.a.iv for a description of Post-Cryo sky coverage.
The Post-Cryo Single-exposure images were not coadded to generate Atlas Images or a deep extraction Catalog/Database during first-pass processing. Consequently, there are no Post-Cryo Atlas Images or a deep Source Working Database as there were for the All-Sky and 3-Band Cryo Releases.
We recommend that you begin by reviewing the Cautionary Notes for that product if you use the Post-Cryo Preliminary Release Source Database. Some of the key features are repeated here, as are Cautionary Notes specific to the Post-Cryo Preliminary Release Single-exposure Database.
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 Post-Cryo Preliminary 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. An attempt has been made to flag detections that are associated with the expected position of image artifacts from bright stars, but that flagging is not perfect, particularly in the first-pass Post-Cryo processing. Tips for selecting reliable source detections in the Single-exposure Database are given in below
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 Post-Cryo Single-exposure images.
The best way to verify the reliability of an entry in the Post-Cryo Preliminary Release Source Database is to examine its position on the Single-exposure images. For inertial sources, it is also valuable to examine the position on the All-Sky Release Atlas Images.
In addition to visually examining their images, reliable detections in the Single-exposure Source Database can be selected using the following criteria:
During the Post-Cryo survey phase, 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. Inertial sources will appear at the same location on different images, but most solar system objects move significantly between different exposures. 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. See the tips for identifying reliable Single-exposure detections.
WISE was designed to acquire multiple observations of each point on the sky to compensate for individual measurements that may be contaminated by transient events, such as charged particle strikes, satellite streaks, or scattered light artifacts. Individual Single-exposure source detections may be contaminated by or be a spurious detection of such transients. Special care should taken if using Single-exposure Source Database detections made on images taken while WISE was passing through the South Atlantic Anomaly because of the greatly elevated number of charged particle strikes. Such images have a value of saa_sep<0 in the Post-Cryo Single-exposure Metadata Table.
The best way to assess the reliability of a Post-Cryo Preliminary Release Single-exposure Database entry is to examine its position on the Single-exposure Images.
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 W1 and W2 system throughput changed with time as the focal plane arrays and telescope warmed during the Post-Cryo period. However, a constant instrumental zero point was used to calibrate W1 and W2 photometric measurements during the first-pass Post-Cryo data processing. This resulted in a a time-dependent bias in the Post-Cryo Database photometry caused by the failure to track the changing throughput (see VIII.3.e.iii). Time-dependent instrumental zero points were used to calibrate the Post-Cryo measurements during second-pass processing, so these photometric residuals are not present in the 2013 Post-Cryo Release Single-exposure Source Database.
The positions of objects in the Post-Cryo Preliminary Release Source Database may be systematically offset from their true position by up to 1" in the declination direction.
Approximately 20% of sources fainter than W1~14.5 mag exhibit this bias, while an increasingly smaller fraction of the sources are biased up to at least W1=13.0 mag. The declination bias was caused by an error in the first-pass pipeline source extraction software that failed to apply a 0.5 Image pixel declination offset to some sources when performing simultaneous PSF fitting of multiple detections in close proximity (i.e. passive deblending). The sign of the declination bias depends on the scan direction and celestial hemisphere in which a source is location. The error does not affect right ascension measurements. See VI.5 of the Explanatory Supplement to the WISE Preliminary Data Release for illustrations of the effects of this error and discussion of its impact.
This bias was corrected in the second-pass WSDS processing pipelines. It is not present in the All-Sky, 3-Band Cryo and 2013 Post-Cryo Release products.
The quoted position declination uncertainties in the Post-Cryo Preliminary Single-exposure Source Database, sigdec, have been inflated to reflect the presence of the bias. See the following section for a description.
The declination uncertainties in the Single-exposure Database, sigdec, were inflated to reflect the presence of the declination bias. The uncertainties were adjusted by adding 0.5" in quadrature to the statistical declination measurement uncertainty. 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.
Users who conduct studies based on the Post-Cryo Preliminary Release Source Database positions should note that the distribution of true position residuals with respect to the 2MASS reference frame is not Gaussian, and will not scale as they would for a Gaussian (see VI.5.b in the Explanatory Supplement to the WISE Preliminary Data Release Products).
You are strongly recommended to use source astrometry from the 2013 Post-Cryo Release Single-exposure Source Database in which the declination error is not present.
The loss of on-board saturated pixel encoding during the Post-Cryo survey phase leads to large uncertainties in the estimated brightness and exact location of very bright sources. The pipeline module that flags source extractions that may be contaminated by bright source artifacts (IV.4.g) requires accurate knowledge "parent" source position and brightness. Consequently, the loss of saturated pixel encoding will result in less accurate artifact flagging.
If you do use the Post-Cryo Preliminary Release Images we recommend that you begin by reviewing the Cautionary Notes for that product. Some of the key features are repeated here, as are Cautionary Notes specific to the Post-Cryo Preliminary Release Single-exposure Images.
WISE Single-exposure images contain many artifacts and anomalies associated with bright sources, artifical satellites and transient detector behavior. Please see the 2013 Post-Cryo Release Image Anomaly Gallery for examples of anomalies that are present in the Post-Cryo Single-exposure images.
Many of these Single-exposure image framesets suffer from the following problems:
You can identify Post-Cryo Preliminary Release 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:
There are many instances of untagged saturation in the Post-Cryo Preliminary Release Single-exposure Images that would otherwise appear as NaN'd pixels (e.g. I.4.c.viii). This occurred because of the loss of on-board saturated pixel tagging as discussed in VIII.1.a.iii.
The W1 and W2 system throughput changed with time as the focal plane arrays and telescope warmed during the Post-Cryo period. However, a constant instrumental zero point was used to calibrate W1 and W2 photometric measurements during the first-pass Post-Cryo data processing. The failure to track the changing throughput resulted in a time-dependent bias in the Post-Cryo Single-exposure images (see VIII.3.e.iii). The 2013 Single-exposure images do not exhibit this photometric residual because time-dependent calibrations were utilized in second-pass Post-Cryo processing.
Single-exposure position reconstruction during first-pass processing assumed that the projected offsets and relative rotations between the W1 and W2 bands were constant with time. While this was true for the cryogenic mission phase data when the WISE focal plane and optics temperatures were extremely stable, the band-to-band relationships did change during the Post-Cryo phase as temperatures increased. These unmodeled changes lead to a systematic offset between the W2 frame positions relative to W1 that increased with time, up to approximately ~170 mas in the cross-scan direction (approximately equal to ecliptic longitude), as described in VIII.3.c.i.
Last update: 2013 May 3