IV. WISE Data Processing

3. The Scan/Frame Pipeline

The WSDS Scan/Frame pipeline operates on individual single-exposure framesets within one or more WISE scans. Level 0 images and metadata that are generated by the Ingest procedure are used as input. Scan/frame processing performs basic instrumental calibration, detects and characterizes sources from individual images, and derives and applies astrometric and photometric calibrations, and flags sources that are positionally associated with the expected location of image artifacts. The Scan/Frame Pipeline also determines if any known solar system objects such as asteroids, comets, planets and planetary satellites were within the field-of-view of the single-exposure framesets.

The elements of the Scan/Frame pipeline processing flow are shown in Figure 1. For each scan processed, repeated calls to the Frame pipeline are made that perform frame image calibration, source detection and extraction, astrometric solutions and the frame level of artifact identification. Scan/frame processing includes a final step in which time-ordered artifact identification and photometric calibration information is derived from a sequence of scans, and then applied to the individual single-exposure images and source lists.

The Scan/Frame processing steps are:

  1. Instrumental Image Calibration - Instrumental signatures are removed from the individual Level 0 images data in the ICAL subsystem (IV.3.a). ICAL performs "droop" corrections, dark-subtraction, non-linearity correction, flat-fielding, and dynamic sky-offset subtraction and transient pixel flagging.

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

  3. Source Extraction - Positions and photometry for detected sources are measured on the calibrated, single-exposure image in the WPHOT subsystem (IV.5.c). Profile-fit photometry is performed simultaneously in four bands, along with a variety of aperture photometry measurements.

  4. Astrometric Calibration - An astrometric transformation is derived for each individual single-exposure frameset by the SFPREX subsystem (IV.3.d). WISE source extractions are matched to astrometric reference stars derived from the 2MASS Point Source Catalog to solve for the conversion between frameset pixel and equatorial coordinates. The astrometric solution parameters are used to set the WCS keywords in the Single-exposure image headers, and to report equatorial positions for entries in the Single-exposure source database.

  5. Known Solar System Object Association - The SSOID subsystem (IV.3.e) determines if any asteroid, comet, planet, or planetary satellite, with an orbit known at the time of data processing, may have been within the field of view of each individual WISE single-exposure. SSOID outputs the list of objects predicted to be in each single-exposure field-of-view. If a single-exposure source detection is near the predicted position of a solar system object, SSOID also outputs the information for the associated WISE source.

  6. Artifact Identification - The ARTID subsystem (IV.3.f.) identifies single-exposure source extractions that are positionally associated with the predicted positions of image artifacts produced by bright sources, including latent images, diffraction spikes and optical ghosts. Sources associated with image artifacts are tagged as either being contaminated by or spurious detections of the artifacts.

  7. Photometric Calibration - Calibration of the photometry of single-exposure source detections is performed by the PCAL subsystem (IV.3.g) using measurements of a network of photometric standard stars. Instrumental zero point magnitudes were derived by computing the mean difference between the true standard star and the measured instrumental magnitudes over hundreds to thousands of standard observations. Photometric zero points are used to set the MAGZP keywords in the Single-exposure image headers, and to report calibrated magnitudes for entries in the Single-exposure source database.

  8. Quality Assurance - Single-exposure image and extracted source quality is assessed by the Scan/Frame component of the QA subsystem (IV.6). Performance metrics generated by each Scan/Frame pipeline subsystem are gathered by the QA system and compared against benchmarks related to the WISE Level 1 Science requirements. The QA system assigns a numerical quality score to the Scan/frame pipeline output for each frameset. These scores were used to monitor WISE performance during survey operations, and are used as input to the FPG system to determine which Level 1 image data to include in the Multiframe processing.

The Scan/Frame pipeline produces the WISE Single-exposure (Level 1) images, extracted source database and metadata that are used as the inputs for the Moving Object and Multiframe pipeline processing. An abbreviated version of the Scan/Frame pipeline was also run on a small percentage of data Level 0 data following each WISE downlink pass for Quicklook quality assessment of science data.

Figure 1 - The WSDS Scan/Frame Pipeline operational flow diagram

Last update: 2011 April 26

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