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II. NEOWISE Single-exposure Source Database

II.2. Cautionary Notes

This section summarizes the caveats and warnings on using the Single-exposure Source Database only. See section III.2 for cautionary notes associated with the Single-exposure Image products.

The NEOWISE Single-exposure Source Database is subject to many of the same limitations and exhibits many of the same features as the WISE All-Sky Release, 3-Band Cryo and NEOWISE Post-Cryo Single-exposure Source Databases. We recommend that you begin by reviewing the Cautionary Notes for each of those products in the All-Sky Release Explanatory Supplement before you make use of the NEOWISE Single-exposure Source Database. Many of the key features are repeated here, along with features specific to the NEOWISE Single-exposure Source Database.

  1. The NEOWISE Single-exposure Source Database contains multiple, independent measurements of sources
  2. NEOWISE typically observes each point on the sky 12 to 13 times near the ecliptic plane during each observation epoch, with increasing coverage towards the ecliptic poles (I.2.b.i). Therefore, when an astronomical source is bright enough to be detected in the NEOWISE Single-exposures, it may have multiple, independent entries in the Single-exposure Source Database. Stationary 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. Tips for selecting good quality source measurements are given in II.3.b.

  3. The NEOWISE Single-exposure Source Database does not contain new measurements for the W3 (12 μm) and W4 (22 μm) bands
  4. NEOWISE does not collect data in the W3 and W4 bands because those detectors are saturated by thermal emission from the payload and telescope. Consequently, there are no new measurements available in those bands. However, all NEOWISE detections are positionally cross-correlated with the AllWISE Source Catalog. If a NEOWISE detection has an associated AllWISE Catalog source, the corresponding AllWISE Catalog 4-band photometry is included in the Single-exposure Source Database entry. Stationary sources that have AllWISE Catalog counterparts may have W3 and W4 measurements available, although they will be from a different observation epoch. Of course, this is not applicable for moving solar system objects.

  5. The NEOWISE Single-exposure Source Database contains unreliable detections
  6. The NEOWISE Single-exposure Source Database contains all source extractions from all Single-exposure images produced during Scan/frame processing regardless of their reliability. Therefore, the Database contains not only detections and measurements of real solar system, galactic, and extragalactic 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. The general reliability of the NEOWISE Source Database is discussed in II.1.e.ii, and tips for selecting reliable source detections in the Database are given in II.3.b.

    Because the Single-exposure Source Database contains a large number of unreliable extractions, as well as multiple measurements of the same object, 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 NEOWISE Single-exposure Images.

  7. The Single-exposure Source Database contains extractions from low-quality framesets
  8. NEOWISE obtained multiple, independent observations covering every point on the sky. However, some of the individual Single-exposure framesets may be corrupted by image smearing due to spacecraft motion, electronic noise, scattered light from the moon or other very bright objects near the field of view. All detections made on all of the Single-exposure images, regardless of their quality, are included in the Single-exposure Source Database. Detections from lower-quality images may have poorer photometric and astrometric measurements, or may be more likely to be spurious detections of transient events and artifacts.

    Source Database detections made on lower-quality image sets can be identified using metadata provided with the Database records. Tips on how to use those metadata to identify such detections are given in II.3.a.

  9. Single-exposure source measurements are susceptible to contamination from transient events
  10. NEOWISE acquires 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. Detections that may be contaminated by transients are identified by the w1frtr and w2frtr flags in the Source Database. A discussion of using these flags to exclude spurious and contaminated detections is given in II.3.b.

    The best way to rule out spurious detections of artificial transients is to examine their expected positions on other Single-exposure Images. Spurious transient detections will usually appear in only one image. Moving solar system and background sources must be handled differently, of course.

  11. Single-exposure Database entries that are flagged as being associated with the predicted position of known solar system objects are not necessarily real detections of those objects.
  12. If the predicted position of a known asteroid, comet, planet or planetary satellite at the time of a NEOWISE exposure falls close to the position of a detection from that exposure, the corresponding entry in the Single-exposure Source Database may be flagged as a solar system object association (sso_flg=1). This is a positional association only, and not an identification of a solar system object detection. The majority of these associations are chance alignments between the predicted object position and either a detection of a background source or a spurious detection of noise or other artifacts. The best way to verify that a Single-exposure Source Database entry is a valid solar system object detection is to determine if it is part of a Moving Object Tracklet that has been vetted by the Minor Planet Center (see II.3.c).

  13. Profile-fit brightnesses of saturated sources in the NEOWISE Single-exposure Source Database are systematically overestimated.
  14. Profile-fit measurements of sources brighter than the W1 and W2 saturation limits (W1~8.0 and W2~7.5 mags) are systematically overestimated, with increasing bias for increasing source brightness. The overestimation ranges from little or no difference near the saturation limit, to more than one magnitude at W1=2 mag, and more than two magnitudes at W2=2 mag. (II.1.c.iv). Approximate empirical corrections for this bias have been derived and are presented in II.1.c.iv.a. However, we recommend that you refer to the All-Sky Catalog and 3-Band Cryo Source Working Database for photometry of saturated sources whenever practical.

  15. Profile-fit and standard aperture photometry in the NEOWISE Single-exposure Source Database underestimate the brightness of resolved sources
  16. The profile-fit (w?mpro) and standard aperture (w?mag) photometry in the Source Database are optimized for point-source characterization. They both systematically underestimate the brightness of sources that are extended relative to the NEOWISE PSF. Catalog sources suspected of being resolved may have values of the profile-fit measurement reduced χ2 goodness-of-fit parameter, w?rchi2>2, or be flagged as being associated with a source in the 2MASS Extended Source Catalog (xscprox is not null).

    Large aperture photometry that has not been curve-of-growth corrected is provided for Database entries and may provide better brightness estimates for extended objects. However, as described below, care should be used with any aperture photometry measurements made on the Single-exposure images because of their susceptibility to contamination from hot and/or masked pixels and transient events.

  17. Aperture photometry measurements in the NEOWISE Single-exposure Database are not as robust as the profile-fitting photometry measurements

    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.

  18. The NEOWISE W2 photometric calibration exhibits small uncorrected seasonal variations of up to ~0.03 mags

    The stability of the photometric calibration throughout the NEOWISE survey is discussed in IV.2.d.ii. The W2 calibration exhibits systemic and seasonal variations of up to ~0.03 mag that are correlated with variations in focal plane temperatures over time. These small calibration variations have not been corrected in the photometry reported in the NEOWISE Single-exposure Source Database.

    The W1 photometric calibration is stable to better than 0.01 mag in W1 over the full survey period.

  19. Flagging of sources in the NEOWISE Single-exposure Source Database that are affected by reflected light (glint) image artifacts is not as effective as in earlier WISE data releases.

    The characteristics of scattered light "glints" produced by bright stars just off the focal planes is much less predictable during the NEOWISE survey than it was during the original WISE mission (section IV.4.g.v in the WISE All-Sky Release Explanatory Supplement). Consequently, the flagging of individual Source extractions in the Database that may be contaminated by reflected light glints is less effective than it was previously. Because the presence and severity of the glints depend strongly on the location of the off-frame bright star, it is usually the case that a source affected by a glint artifact in one exposure may not be affected in other observations.

  20. NEOWISE Source Database astrometry is reconstructed with respect to the positions of sources in the 2MASS Point Source Catalog that have been corrected for proper motion in the >14 years separating the surveys.

    The positions of detections in the NEOWISE Single-exposure Source Database are reconstructed with respect to the positions of a bright, reliable subset of the 2MASS PSC that were corrected for motion between the 2MASS and NEOWISE observation epochs using proper motions from the UCAC4 catalog (Zacharias, N. et al. 2013). Therefore, NEOWISE positions do not need to be corrected for the motion of the reference frame.

    The 2MASS reference source positions were not corrected for proper motion during position reconstruction of detections in the WISE All-Sky, 3-Band Cryo, and NEOWISE Post-Cryo Single-exposure Source Databases. Therefore, you may see small systematic offsets between NEOWISE positions and those from the earlier data sets. Correction for the systematic motion of the reference frame was made for AllWISE position reconstruction, so NEOWISE-AllWISE position comparisons should not show significant biases.

  21. The true observation times for each source detection in the Single-exposure Source Database are slightly earlier than the value given by the Modified Julian Date parameter (mjd).

    NEOWISE source detections are tagged with an observation time that corresponds to the mid-point of the observation times of the single-exposure framesets on which the detection was made. The observing times assigned to the images may differ slightly from true mid-point of the exposure of the framesets because of several systematic and random effects that are described in I.2.c.iii.1. For source detections made on images acquired before June 23, 2015 11:29:30.47 UTC (mjd=57196.47882488), the mid-point of the observation times are approximately 1.57±0.25 sec earlier than the times given in the mjd column. For source detections after that date, the observation times are approximately 0.57±0.25 sec earlier than the mjd value.

Last update: 6 February 2024

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