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V. AllWISE Data Processing

1. Overview

Science data processing for AllWISE program was carried out using elements of the WISE Science Data System (WSDS) that was developed and operated at the Infrared Processing and Analysis Center, California Institute of Technology. AllWISE utilized the WSDS Multiframe Pipeline, QA and Archive elements, augmented with new functionality to support the objectives of AllWISE, and improved to address some known deficiencies known to exist in the earlier WISE data products. The many significant updates to the WSDS for AllWISE processing are summarized in V.1.a.i below.

As described in V.1.b, AllWISE makes use of the W1 and W2 Single-exposure images from the WISE 4-Band, 3-Band Cryo and NEOWISE Post-Cryo survey phases, and the W3 and W4 Single-exposure images from the 4-Band Cryo survey phase. Key improvements were made to pixel calibrations and astrometric solutions for the Single-exposures to improve the overall quality of the AllWISE Data products. These improvements are described in V.2.a and V.2.b.

V.1.a. The Multiframe Pipeline

Figure 1 - The WSDS Multiframe Pipeline operational processing flow

Core data processing for AllWISE used the WSDS Multiframe Pipeline. The elements of the Multiframe Pipeline are shown above in Figure 1. The Multiframe Pipeline takes as input the Single-exposure images and metadata that were originally produced by the Scan/Frame pipeline, and improved for AllWISE as described in V.2.a and V.2.b. The AllWISE Multiframe processing steps are:

  1. Image coaddition - Single-exposure frame images that satisfy minimum quality criteria (V.1.b) are match-filtered, registered and combined on pre-defined Atlas Tile footprints on the sky in the Atlas Image generation step (V.3.a). Throughput matching is performed to adjust for data from the different survey phases, and pixel outlier rejection suppresses transient features and fast moving objects.

  2. Multiband Source Detection - Sources are detected simultaneously in the four WISE bands on a combined signal-to-noise image formed from the four-band coadded images in the MDET source detection step (V.3.b.i).

  3. Source Extraction - Positions, apparent motion and photometry for each detected source are measured in the WPHOT subsystem (V.3.b.ii). Profile-fit photometry is performed simultaneously in four bands and on all Single-exposure framesets to produce one set of calibrated magnitudes per object. Profile-fit fluxes for each detection are are also measured on the individual Single-exposure framesets and output to the Multiepoch Photometry Database. A variety of aperture photometry measurements are performed for each object on the coadded Atlas Images, including special elliptical aperture measurements for sources identified with galaxies in the 2MASS Extended Source Catalog.

  4. Artifact Flagging - Source extractions that may be contaminated by or spurious detections of image artifacts produced by bright sources are identified and tagged in the ARTID subsystem (V.3.d).

  5. QA - Coadd image and extracted source quality is assessed by the Multiframe Quality Assurance operation (IV.6.iii). Performance metrics are generated by each Multiframe pipeline subsystem and general pipeline output, and reviewed by quality assurance scientists to identify anomalies. The QA system assigns a numerical quality score to the Multiframe pipeline output for each Atlas Tile.

  6. Archive Loading - Output from the Multiframe Pipeline, including the coadded Atlas Image sets, Source Working Database and metadata information are loaded into the WISE data archive within IPAC's NASA/Infrared Science Archive (IRSA) following completion of Multiframe processing and quality assessment. The elements of the Archive system are described in section IV.7 of the WISE All-Sky Release Explanatory Supplement.

The basic astrometric and photometric calibration information in the individual Single-exposure images was propagated to the coadded images Atlas and deep source extractions during the Multiframe processing. Small adjustments were applied to the Single-exposure WCS information and photometric zero points during the image coaddition step to take advantage of improvements generated for AllWISE. Pixel values of the individual Single-exposure images are scaled so that all AllWISE Atlas Images have the same photometric zero point within a band.

AllWISE Multiframe Pipeline processing generated 18,240 Atlas Image sets covering the sky, a deep-detection Working Database containing 1,176,421,279 source extractions, the 42,759,337,365 entry Multiepoch Photometry Database, and associated metadata products. The Working Database is split into the Source Catalog and Reject Table during the Catalog Generation step.

V.1.a.i. Synopsis of the WSDS Improvements for AllWISE

V.1.a.i.1. Added Functionality

V.1.a.i.2 Single-exposure Updates

V.1.a.i.3. Improved Calibrations

V.1.a.4. Improved Reduction Algorithms

V.1.a.ii. Basic Data Processing Terminology

orbit - An orbit refers to the observations or block of survey data acquired between the times WISE observed the north ecliptic pole (NEP). The WISE orbital period is approximately 95.3 minutes. Orbit number 1 was defined to start with the fictitious NEP passage immediately prior to WISE launch. Thus, WISE was launched into orbit #1. The AllWISE Products are made from data taken from the start of full cryogenic survey observations in orbit 356 to the point at which data acquisition was halted in orbit 6257.

scan - A scan refers to the observations or block of survey imaging data acquired between two flight system maneuvers, such as reorientation of the instrument boresight near the ecliptic poles, or slews to acquire TDRSS for data downlink contacts. Because maneuvers often took place near the ecliptic poles, a scan is associated with the image data from approximately a half-orbit. However, scans can cover less or more than half of the arc between ecliptic poles. Scans are identified in the Single-exposure image headers, Single-exposure source database and metadata tables by the scan_id.

single-exposure - Single-exposure refers to the unit of data acquisition carried out by WISE while the sky is frozen onto the focal plane detectors, and to the set of imaging data collected during this data collection.

frame - A frame is the image data acquired in a single band from a WISE Single-exposure. A Single-exposure was acquired approximately every 11 seconds during WISE scans. The frame exposure times are 7.7 sec in W1 and W2 and 8.8 sec in W3 and W4. Because scans have different lengths, the number of frame exposures acquired per scan varies. A histogram showing the distribution of the number of frames per scan for all WISE survey scans is shown in Figure 2.

frameset - The set of 1-4 images (frames), one in each band (W1, W2, W3, W4), that are acquired simultaneously during each WISE single-exposure. The same region of sky is imaged in all bands. WISE collected images in all four bands during the 4-Band Cryo phase, W1, W2 and W2 during the 3-Band Cryo phase, and W1 and W2 during the Post-Cryo phase.

coadd - Image data in a single band produced by combining multiple Single-exposure frames that cover the same region of the sky. Coadds are produced in the Multiframe pipeline by resampling, reprojecting and coadding single-exposure frames onto predefined spatial footprints.

coaddset - The set of four corresponding coadds, one in each WISE band (W1, W2, W3, W4), that are generated by combining multiple framesets. Coaddsets are generated by the Multiframe pipeline.

Atlas Tiles - A predefined grid of 18,240 regions on the sky onto which coaddsets are generated by the Multiframe pipeline for AllWISE data release products. Atlas Tiles cover 1.564°x1.564° (4095x4095 pix @1.375"/pix) and are built on an equatorial projection.

Atlas Image - An Atlas Image is the particular case of a coadd image built on the footprint of an Atlas Tile. The AllWISE coadd intensity images are Atlas Images.

Figure 2 - Histogram showing the distribution of the number of frames acquired per scan during all phases of the WISE survey.

Last update: 22 November 2013

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