V. Atlas and Catalog Generation

1. Overview - From Image Archive and Source Working Database to Image Atlas and Source Catalog

Generation of the Image Atlas and Source Catalog for the WISE Preliminary Data Release, characterizing their properties, and producing this Explanatory Supplement is done during the Final Product Generation (FPG) phase of WSDS data processing.

The elements of the FPG process are illustrated in Figure 1. Input for FPG is the archive of calibrated Single-exposure (L1) images, the database of detections from those images, and metadata and quality assessment produced during Scan/frame processing. The ensuing steps of FPG are:

1. Scan, Tile and Frameset Selection - The first step in the FPG process selects the subset of Single-exposure (L1) data that will be combined during Multiframe processing. Scan selection is driven primarily by the scope of the data release, and defines the spatial boundaries of the Atlas Tiles that will be included in the Release. The subset of Single-exposure (L1) framesets and band-frames in those scans that are the input data for the Multiframe processing is driven by the achieved quality of the individual Single-exposures (L1) as assessed during the Scan/frame data QA process. Scan and frameset selection are described in V.2


2. Multiframe Processing - Although the Multiframe pipeline (IV.5) is functionally part of the WSDS PIPELINES subsystem, operationally it is run during the FPG process. Multiframe processing combines the Single-exposure images that were identified during the Scan/frameset selection step, and builds coadded (L3) Images on Atlas Tile footprints. Sources are then detected on the coadded images, then characterized using both the coadded images and the individual Single-exposure images that contribute to the coadds. Multiframe processing produces Image Sets for each Atlas Tile that contain the calibrated Intensity Images, Depth-of-coverage maps, and Uncertainty maps, and a list of extracted sources. These are compiled and stored in the L3 Image Archive and Source Working Database (WDB).


3. Atlas Image Preparation - For the WISE Preliminary Release, the L3 coadded images produced by the Multiframe pipeline processing are identically the served Atlas Images. No further modification of the images, their headers, or image metadata was performed following the Multiframe processing.


4. Source Catalog Preparation - The Catalog Preparation step selects the subset of entries in the L3 Source WDB that constitute a complete and reliable Source Catalog, and performs final formatting of those columns for the Release. In addition to detections of real astrophysical sources, the L3 WDB produced during Multiframe processing contains spurious detections of low SNR noise excursions, image artifacts, and residual transient events such as cosmic rays, hot pixels, satellite streaks and scattered light that were not completely removed by the outlier rejection step in the image coaddition procedure. The WDB also contains duplicate detections of sources that fall within the overlap regions between Atlas Tiles. Catalog source selection filters out low reliability spurious detections from the WDB using criteria based on SNR, artifact flagging, frame-detection statistics (V.3), and selects one apparition of duplicate detections (V.4) for appearance in the Source Catalog.


5. Validation and Characterization - The final step of FPG is the analysis of the Release Atlas and Source Catalog to assess their characteristics and validate product integrity and quality. The results of these analyses are documented in in section VI.

Output from the WSDS FPG steps consists of the Preliminary Release Image Atlas, Source Catalog, ancillary tables with Atlas and Catalog metadata, and this Explanatory Supplement.

Figure 1 - Final Product Generation Functional Flow

Last update: 2011 April 26