V. Atlas and Catalog Generation

2. Scan, Tile and Frameset Selection

The first step in the Atlas and Catalog Generation for the WISE Preliminary Data Release was to select the input data set for Multiframe processing, and to define the set of Atlas Tiles for which single-exposure images would be coadded and sources extracted. The selection of scans for Multiframe processing is dictated primarily by the scope of the release. The Tiles on which the coadds will be generated are defined by the spatial coverage of the selected scans. The selection of individual framesets and frames to be used in the Multiframe processing is regulated by data quality.

a. Scan Selection

The WISE Preliminary Data Release includes survey data acquired during the first 105 days of survey operations, 14 January 2010 to 29 April 2010 UTC. 3,192 survey scans containing 754,854 framesets were conducted during this period, beginning with scan_id 00936a and ending with 04125a. The area of sky covered by these scans is shown in Figure 1. The coverage region spans two broad ecliptic longitude ranges, approximately 27°<λ<133° and 201°<λ<310°. The frame depth-of-coverage nominally provided by these scans is illustrated by the different colors in Figure 1. The coverage depth ramps down at the outer longitude boundaries because of the progression of scans across the sky dictated by the WISE survey strategy.

The Single-exposure Images and Source Database contain the calibrated image sets and extracted source lists from all 754,854 of the framesets contained in the 3,192 scans included in the scope of the Preliminary Release.

Figure 1 - Ecliptic sky plot showing the raw frameset depth-of-coverage for scans 00936a and 04125a. The key on the left side of the image indicates the depth of coverage encoded by each color, and in parenthesis, the cumulative area (in square degrees) having that coverage or greater. This is not the Preliminary Release area coverage.

b. Tile Selection

The full sky is tessellated into 18,240 Atlas Tiles for the purpose of coadding Single-exposure images and extracting sources in Multiframe processing. The subset of Tiles used for the WISE Preliminary Release was selected such that all Tiles were fully enclosed within the scan coverage area shown in Figure 1. Additional clearance from the outer longitude boundaries was required to ensure a minimum of six frameset coverages, on average, to insure effective pixel outlier rejection in the image coaddition processing. Six or more coverages are achieved by the selected scans within the ecliptic longitude ranges

10,466 Atlas Tiles are fully contained within these longitude ranges. Two of these tiles that were closest to the ecliptic poles, 2691p651 and 0909m682, were excluded from Multiframe processing because their extreme coverage depth exceeded memory limitations in the first-pass processing Multiframe pipeline. These Tiles will be included in the second-pass processing and final data release.

Table 1 contains a listing of the names (coadd_id) and position information for the resulting 10,464 Atlas Tiles that comprise the Preliminary Data Release Atlas and Catalog area.

Table 1 - Preliminary Release Atlas Tile List

c. Frameset and Frame Selection

Not all of the 754,854 framesets contained in scans 00936a through 04125a were used in the Preliminary Release Multiframe processing. Framesets, and in some cases individual band-frames were excluded from the image coaddition step if they did not satisfy minimum quality requirements for the survey. Frameset and frame exclusion was done both as a static filtering of the inputs to the Multiframe pipeline, and as dynamic frame filtering in the coaddition step (IV.5.a).

Framesets and/or frames were filtered out of the inputs to the Multiframe pipeline (i.e. static filtering) in the following cases:

• Zero quality score - 31,522 framesets were excluded from Multiframe processing because they were assigned a net quality score of zero during the Scan/Frame processing Quality Assurance step. Zero quality scores were assigned when a frameset failed to complete scan/frame processing successfully for any reason, if the frameset exhibited degraded image quality caused by spacecraft tracking excursions, or if high noise levels due to scattered light from the moon or other very bright sources or elevated electronic noise levels were diagnosed in the images.


• Anneal recovery periods - The W3 and W4 frames in framesets taken within 2000 seconds following annealing of those detectors were excluded from Multiframe processing. Temperature annealing of the Si:As detectors was performed twice per day to eliminate accumulated long-term latent images produced by bright sources. It required approximately 2000 seconds for the responsivity, dark current and noise characteristics of the W3 and W4 detectors to recover to their pre-anneal levels. The W3 and W4 frames in 18,019 framesets with quality >0 were excluded because of the post-anneal recovery. The W1 and W2 HgCdTe detectors were not affected by the anneals, so are included in Multiframe processing. The DTANNEAL keyword in the Single-exposure image headers provides the elapsed time since anneal (in seconds) for all framesets. That quantity is also available as the dt_anneal in the Single-exposure (L1b) Image Inventory tables.


• Special test observations - A non-standard W4 detector bias setting was used for the purpose of in-flight testing during 11 scans, 03752a through 03761b, taken on 4/16/2010 UTC. The W4 frames in these scans were not used for Multiframe processing. This affected 2668 framesets that have a quality score >0. The W1, W2 and W3 frames in these framesets were used.

The total number of frames that contributed to the full set of coadded Atlas Images for the Preliminary Data Release in each band is listed in Table 2. The frame count differs between bands because some of the static and all of the dynamic frame filtering was band-dependent. Approximately 92%, 92%, 89% and 88% of the frames acquired in the 754,854 framesets in the first 105 days of the WISE survey are included in the construction of the Image Atlas and Source Catalog, in W1, W2, W3 and W4, respectively.

In addition to the frameset and frame-level filtering, pixel-level outlier rejection was performed during the Multiframe pipeline coaddition processing. Pixel outlier rejection affects the coverage on smaller scales in the resulting Atlas Images that can be tracked by their corresponding Depth-of-Coverage Maps. The coverage that was available for the measurement of each Catalog source is tabulated in the Catalog w1m, w2m, w3m and w4m columns.

The achieved depth-of-coverage after static and dynamic frameset and frame filters, and pixel-level outlier rejection in the Multiframe processing is shown in Figure 2. A detailed discussion of survey and data release sky coverage is given in VI.2.

Comparison with the original coverage area of the survey's first 105 days (click on the Figure 2 thumbnail to do this interactively), illustrates several differences of note. The ecliptic longitude boundaries of the Atlas and Catalog are pulled in from the original boundaries because of minimum coverage requirements. The Release does not cover the ecliptic poles because those Tiles are not fully contained in the survey boundaries from the first 105 days. The actual boundaries of the Atlas coverage area are not smooth in ecliptic longitude because the Atlas Tiles are laid out in an equatorial grid. There are selected areas with a notable loss of coverage within the general boundaries of the release. Most significant are the horizontal bands at λ,β= 100°,+45° and 290°,-45°. Early in the survey, the spacecrafts' magnetic torque rods were enabled to dump accumulated momentum when scans approached within 45° of the ecliptic poles. Activating the torque rods resulted in a small jump in the telescope pointing and smearing of the resulting images. Because the smearing occurred near the same point on each orbit, and the smeared images were flagged as having degraded image quality in the QA process, low-coverage "holes" developed at those locations. Later in the survey (2010 May 02), torque rod enabling was staggered between 45, 57.5 and 70° latitude on alternating orbits so that any image smearing would not occur at the same point on the sky on each orbit.

Framesets that contribute to each Atlas Image are tracked in the Preliminary Release Atlas Image Frame Cross-Reference Database. This table contains a listing of frameset identifiers that correspond to each Atlas Tile in each band. This table can be used to determine which framesets were used to construct a particular Atlas Image, or to determine to which Atlas Images a particular frameset or frame contributed.

Figure 2 - Net Coverage in W1 for the WISE Preliminary Data Release. Colors encode the average frame depth-of-coverage in 15x15 arcmin spatial bins. Click on this image to view a larger version that allows an interactive comparison between the achieved coverage and raw scan coverage prior to Tile selection and frameset and frame filtering.

Last update: 2011 March 28