1. Top 10 (or so) Things You Should Know About the 2013 NEOWISE Post-Cryo Release Data Products
2. General Caveats
3. Single-exposure Source Database
4. Single-exposure Images
NEOWISE Post-Cryo Release data products are subject to most of the same limitations and exhibit the same features as the 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.
The WISE survey data acquisition strategy and data processing were designed to yield data products of high reliability and fidelity. The second-pass processing that was used to generate the 2013 Post-Cryo Release products uses improved calibrations and algorithms that are tuned for the characteristics of the Post-Cryo survey phase data, and corrects a number of deficiencies known to exist in the earlier reductions. Despite these improvements and the increased level of scrutiny that were applied to the second-pass processing results, there remain a number of known limitations that persist into the 2013 Post-Cryo Data Release products.
The 2013 Post-Cryo Release data products supersede those from the 2012 Post-Cryo Preliminary Release, and should be used in all cases rather than the earlier vintage products.
Section VIII.1.d.i.1 below contains a list of the most important things that you should know about the 2013 Post-Cryo 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.i.2, VIII.1.d.i.3 and VIII.1.d.i.4 contain caveats that pertains to the 2013 Post-Cryo Release products in general, and specifically to the Single-exposure Source Database and Single-exposure Images, respectively. The 2013 Post-Cryo Release 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 Release data products.
We encourage you to review the All-Sky Release Cautionary Notes before making use of the Post-Cryo Release products.
The WISE Post-Cryo 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 the All-Sky Release Single Exposure Source Database before you make use of the Post-Cryo Release Single-exposure Source Database. Some of the key features are repeated here, as are Cautionary Notes specific to the 2013 Post-Cryo Release Single-exposure Database.
The Single-exposure Source 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 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. Tips for selecting reliable source detections in the Single-exposure Database are given 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 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 preferentially 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 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 delivered point spread functions (PSF) distorted slightly as WISE telescope and camera temperatures increased during the Post-Cryo survey phase. Because there was not time to implement infrastructure to handle time-dependent PSFs in the WSDS source photometry pipeline module, profile fitting was carried out using the same 9x9 (spatial) grid of W1 and W2 PSFs that were used during the cryogenic survey phases. The slight mismatch between the model and true PSF that developed as the Post-Cryo survey progressed resulted in the development of a small, systematic underestimation of profile-fit source brightness that increases with decreasing source brightness. The maximum W1 flux underestimation is approximately 5% at the Single-exposure detection limit of W1~16 mag. The maximum flux underestimation is smaller in W2, reaching ~2% at the W2 detection limit. These biases are much smaller than the measurement uncertainties at these flux levels.
Profile-fit measurements of sources brighter than the W1 and W2 saturation limits (W1~8.0 and W2~7.5 mags) are systematically brighter than the measurements of the same sources in the All-Sky Release Catalog. The amplitude of this bias increases with increasing source brightness, ranging from little or no difference near the saturation limit, up to as much as one magnitude near W1 and W2 = 1.0 mag. See VIII.3.e.iii. The flux overestimation of saturated sources may be related to changes in the wings of PSF during that Post-Cryo survey phase that are used to estimate source brightnesses. We recommend that you defer to the All-Sky Catalog and 3-Band Cryo Source Working Database for photometry of saturated sources.
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. In addition, the systematic flux overestimation of saturated sources in Post-Cryo source extraction can lead to over-aggressive flagging of artifacts.
We recommend that you begin by reviewing the Cautionary Notes for the All-Sky Release Single Exposure Images before you make use of the Post-Cryo Release Single-exposure Images. Some of the key features are repeated here, as are Cautionary Notes specific to the 2013 Post-Cryo Release Single-exposure Images.
WISE Single-exposure images contain many artifacts and anomalies associated with bright sources, artificial satellites and transient detector behavior. Please see the Post-Cryo Image Anomaly Gallery for examples of these anomalies.
Some Single-exposure image framesets suffer from the following problems:
You can identify Post-Cryo 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 instances of untagged saturation in the Post-Cryo 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. A level of saturated pixel flagging was recovered for bright point sources using prior knowledge of position and flux, as described in VII.3.c.i. However, the recovery can be ineffective when the prior position and flux information was inaccurate.
The problem in the dynamical calibration that produced the spike in the pixel value distribution, or the "false-mode", was corrected for the second-pass processing of the Post-Cryo data (see VIII.3.b.vii.2). Therefore symptoms of this feature, such as faint source flux underestimation, are not present in the 2013 Post-Cryo source extractions.
The channel-bias correction phase of image calibration is more likely to be affected by complex background emission, leading to elevated signals in channels in the Post-Cryo Single-exposure Images (see VIII.3.b.vi.2). This is due to the unreliable behavior of the reference pixels arising from the higher detector temperatures during the Post-Cryo survey phase. This effect is rare, and more prevalent in W2 than W1.
Last update: 2013 May 17