JWST Science Image Data Product Design

Daryl Swade (Space Telescope Science Institute), Perry Greenfield (Space Telescope Science Institute)
Robert Jedrzejewski (Space Telescope Science Institute)
Jeff Valenti (Space Telescope Science Institute)


Abstract

Science data products for the James Webb Space Telescope (JWST) will be generated within the Data Management Subsystem of the Science and Operations Center at the Space Telescope Science Institute.

Science data from all of the JWST science instruments will be captured in FITS format files. During the initial stages of data processing, each FITS file will contain pixel values from a single exposure taken with a single detector. These exposure-level FITS files consist of a Primary Header Data Unit that contains keywords that apply to all subsequent extensions. Uncalibrated data files have extensions for the on-board science pixel values, reference pixel values, and group information. Calibrated data files have extensions for calibrated pixel values, calibrated reference pixel values, data quality flags, and errors.

The image arrays are formatted as four-dimensional data cubes with axes for image columns and rows, as well as stacked images for each group within an integration and each integration within an exposure. Multiple groups and integrations result from up-the-ramp exposure data collection. The origin of the image data array is chosen to ensure a consistent parity and orientation for pixel data between instruments and detectors.

The FITS header keyword values are populated from multiple sources. Standard FITS keywords describe the data format. Information about the on-board data capture is obtained from the science telemetry. Engineering parameters are obtained from an engineering database. Engineering telemetry event messages provide information on exposure execution times. Proposal information is obtained from the Proposal, Planning and Scheduling database. A spacecraft ephemeris is used along with a telescope pointing model to determine the specific World Coordinate System parameters for each exposure. Calibration reference files are determined through the Calibration Reference Data System.

A number of design issues remain to be determined. These issues include: the data array scheme for spectral data, specifics of the FITS WCS keywords, and the exact definition of exposure time for up-the-ramp data.

Paper ID: P150

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