The WISE 3-Band Cryo Release contains data taken between 6 August and 29 September 2010 UTC, following exhaustion of solid hydrogen in the outer payload cryogen tank. The the telescope and detectors were still cooled by hydrogen ice in the inner tank during this period, but the thermal emission from the warmer telescope fully saturated the W4 detector, so no useful data were acquired in that band.
WISE scanned approximately 30% of the sky during the 3-Band Cryo survey phase in two broad ecliptic longitude intervals: 42.2°<λ<98.0° and 228.5°<λ<281.7°. See VII.5.a for a description of 3-Band Cryo sky coverage.
As the telescope temperature increased during 3-Band Cryo operations, the W3 exposure time was reduced from 8.8 sec, which was used during the full cryogenic survey, to 4.4 sec, 2.2 sec and finally 1.1 sec. 70% of the 3-Band Cryo W3 data were acquired with 1.1 sec exposure time. See VII.1.a.ii.
The W3 detector began to saturate on the telescope thermal emission before the exposure time could be reduced from 4.4 sec to 2.2 sec. This affected W3 frames in 103 survey scans (07402-07505b), that covered the ecliptic longitude ranges 53.09°<λ<56.68° and 239.81°<λ<240.54° (VII.1.a.ii). W3 frames from these scans were omitted from 3-Band Cryo processing, so there will be W1 and W2 data in those intervals that have no corresponding W3 image or source data.
3-Band Cryo Source Working Database (WDB) is subject to many of the same limitations and exhibits many the same features as WISE All-Sky Release Catalog and Reject Table. We recommend that you begin by reviewing the Cautionary Notes for those products (I.4.b and I.4.d.i) before making use of the 3-Band Cryo Source Working Database.
The WISE 3-Band Cryo Source WDB is not a highly reliable, well-vetted list of mid-infrared sources like the All-Sky Release Source Catalog. The WDB contains all detections made on the Atlas Images during Multiframe Pipeline processing, including detections of real point-like and extended mid-infrared sources, as well as spurious detections of image artifacts and faint noise spikes and transient pixel events. Spurious detections were not filtered out into a separate Reject Table, as they were for the All-Sky Data Release. The reliability of entries in the 3-Band Cryo is discussed in VII.5.e.
The best way to verify the reliability of an entry in the 3-Band Cryo Source WDB is to examine its images in the 3-Band Cryo Image Atlas and the All-Sky Release Image Atlas.
In addition to visually examining their images, reliable detections in the 3-Band Cryo Source WDB can be selected using the following criteria:
The WSDS Multiframe Pipeline combined multiple Single-exposure images that fall within the footprint of 1.56°x1.56° Atlas Tiles. Sources that fall in the overlap region between Tiles may be detected the processing for each Tile and thus appear in the Source WDB multiple times. You can select one unique apparition of these redundant detections by selecting sources that have use_src=1. See VII.4.d.
On-board encoding of saturated pixels by the WISE payload worked intermittently (bands W1 and W2) or was disabled (W3) during 3-Band Cryo operations (VII.1.a.iii). Saturated pixel tagging was recovered for bright point sources during the frame calibration processing stage. However, it was not possible to tag saturated pixels in regions of very bright extended emission. These regions triggered spurious detections with very bright fluxes. This was most severe in W3 during the period with exposure times less than 8.8 sec.
This photometric bias between measurements in the 3-Band Cryo WDB and All-Sky Release Catalog increases towards fainter fluxes, and is stronger in regions with higher source density. This bias is similar to the offsets seen between the All-Sky Release Catalog and Spitzer Space Telescope flux measurements described in VI.3.c, and the two biases may be related in their origin. See VII.5.b.ii.
The loss of on-board saturated pixel encoding during the 3-Band Cryo survey phase leads to large uncertainties in the estimated brightness and exact location of very bright sources. Consequently, the artifact flagging algorithms (IV.4.g) were less effective.
Because of increasing telescope temperature and reductions in
exposure time, the W3 measurements during the 3-Band Cryo period
exhibit large and changing scatter. This made computation of
the probability of variability encoded in the
var_flg unreliable. Therefore,
no estimate of W3 flux variability is given in the 3-Band Cryo WDB.
3-Band Cryo Atlas Images are subject to many of the same limitations and exhibit many the same features as the images in the WISE All-Sky Release Atlas. We recommend that you begin by reviewing the Cautionary Notes for that product (I.4.c) before making use of the 3-Band Cryo Image Atlas.
Tile selection for the 3-Band Cryo Multiframe Pipeline processing included as much of the 3-Band data as possible, so Tiles at the edges of the coverage area were not required to be completely covered by Single-exposure frame data. Therefore, some of the Tile area may contain NaN'd pixels signifying that there is no frame data available at those locations. The fractional coverage area in these Tiles can be significantly different in the three bands.
Many of the W3 3-Band Cryo Atlas Images were constructed using W3 Single-exposure frames that have very different sensitivity due to increasing temperatures and changing exposure times. Therefore, you cannot use depth-of-coverage reliably to track relative sensitivity for those images. The 3-Band Cryo Uncertainty Maps take into account the variations in the input frames so should be used when computed flux uncertainties from photometry on the Atlas Images.
There are many instances of untagged saturation in the 3-Band Cryo Atlas Images that would otherwise appear as NaN'd pixels (e.g. I.4.c.viii). This occured because of the loss of on-board saturated pixel tagging and occurs most frequently in the W3 images in regions with very bright extended emission. See VII.3.b.iv.1.
This occurs primarily in the regions covered by scans taken when the W3 detectors inadvertently went into saturation during the 4.4s exposure period. This also occurs in a small number of Tiles on the outer boundaries of the 3-Band Cryo survey area when the W3 Single-exposure data were excluded because of elevated scattered light levels when WISE scanned near the moon.
These features are caused by an unmasked row of pixels in the input Single-exposure images that had slightly higher responsivity than was tracked in the responsivity maps. Rotating position angles of scans covering the poles smeared this bad-pixel row over an elliptical footprint.
The 3-Band Cryo Single-exposure Source WDB is similar in character to the WISE All-Sky Release Single-exposure WDB, so they have many of the same limitations and features. We recommend that you begin by reviewing the Cautionary Notes for the All-Sky Release Single-exposure WDB in I.4.d.ii and the 3-Band Cryo Source WDB in above. The features listed below are unique to the 3-Band Cryo Single-exposure WDB.
The W3 photometric zero point during the 3-Band Cryo survey phase varied considerable because of the increasing operating temperatures and reducing exposure times. The calibration of the zero point did not adequately track the most rapid of these changes, as discussed in VII.3.g, resulting in photometric residuals with respect to photometry in the the All-Sky Release Catalog. These residuals were corrected when the Single-exposures were coadded during Multiframe Pipeline processing, so these biases are not present in the 3-Band Cryo Atlas Images or the Source WDB extracted from the Atlas Images.
We recommend that you review the Cautionary Notes for the All-Sky Release Single-exposure Images in I.4.d.iii and the 3-Band Cryo Atlas Images above before making use of the 3-Band Cryo Single-exposure images. The features listed below are those that are unique to the 3-Band Single-exposure images.
The exposure time given in the EXPTIME FITS keyword in the headers of the W3 Single-exposure images is erroneously listed as 8.8 sec after the W3 exposure time was reduced. The correct W3 exposure times are listed in the 3-Band Cryo Single-exposure Frame Metadata Table.
Short timescale variations in the W3 photometric calibrations were not adequately tracked during calibration of the 3-Band Cryo Single-exposure data. This resulted in photometric zero points that were applied to the Single-exposure images that produce photometry that has has residuals with respect to the All-Sky Release Catalog, as discussed in VII.3.g. These residuals were corrected when the Single-exposures were coadded during Multiframe Pipeline processing, so these biases are not present in the 3-Band Cryo Atlas Images.
Last update: 2012 December 28