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IV. WISE Data Processing

3. Pipelines

c. The Multiframe Pipeline

The WSDS Multiframe pipeline combines multiple images from one or more scans covering the same region on the sky, detects sources on the combined images, characterizes the detected sources on the single-exposure and coadded images, applies photometric and astrometric calibrations, and identifies and tags detections that may be contaminated by or spurious detections of image artifacts produced by bright sources.

The elements of the Multiframe pipeline processing flow are shown in Figure 1. The Multiframe pipeline takes as input the Single-exposure (Level 1) images and metadata produced by the Scan/Frame pipeline. The Multiframe processing steps are:

  1. Image coaddition - Single-exposure frame images that satisfy minimum quality criteria (V.2) are optimally combined on pre-defined Atlas Tile footprints on the sky in the Atlas Image generation step (IV.5.a). Pixel outlier rejection is used to suppress 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 (IV.4.b).

  3. Source Extraction - Positions and photometry for detected sources are measured in the WPHOT subsystem (IV.4.c). Profile-fit photometry is performed simultaneously in four bands and on all single-exposure images. A variety of aperture photometric measurements are performed on the coadded Atlas Images, including special elliptical aperture measurements for sources identified with entries in the 2MASS Extended Source Catalog.

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

  5. Tile Position Refinement - The positions of sources extracted from each Atlas Tile are updated by meausuring any residual offsets using 2MASS reference stars in each Tile, performing a 3-parameter fit for the mean offset and rotation, and applying the correction to the extracted source positions. This process is described in IV.4.d.ix. The Atlas Image WCS information is not modified in this operation.

  6. 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 compared against benchmarks related to the WISE Level 1 Science requirements. The QA system assigns a numerical quality score to the Multiframe pipeline output for each Atlas Tile.

  7. Archive Loading - Output from the Multiframe pipeline, including the Level 3 coadded images, Source Working Database and metadata tables, are loaded into the WISE data archive (IV.7) following the completion of Multiframe pipeline processing and quality assessment.

The Multiframe pipeline does not include formal and Photometric calibration steps because that calibration is propagated from the Scan/Frame pipeline processing via the MAGZP header keyword values in the Single-exposure images. All Atlas Images were generated to have a constant photometric zero point for each band by throughput scaling the Single-exposure images using the MAGZP values set by the Scan/Frame photometric calibrations.

Output from the WSDS Multiframe pipeline consists of the Level 3 coadded images, Source Working Database and associate metadata. These are the inputs to the Final Product Generation step (V.1) in which the WISE All-Sky Release Image Atlas and Source Catalog are produced.

Figure 1 - The WSDS Multiframe Pipeline operational processing flow

Last update: 2012 March 15

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