JWST Data Formats

Various JWST data products are produced in one of several standardized data formats, as described below.

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See also: Understanding Data Files

Multi-extension FITS format

Flexible Image Transport System (FITS) is a standard format for exchanging astronomical data, independent of the hardware platform and software environment. FITS format files consist of a series of Header Data Units (HDUs), each containing two components: an ASCII text header and binary data. The header contains a series of keywords that describe the data in a particular HDU; the data component may immediately follow the header.

For JWST FITS data, the first HDU, or primary header, only contains header information in the form of keyword records with an empty data array, which is indicated by the occurrence of NAXIS=0 in the primary header. The primary header may be followed by one or more HDUs called extensions, which may take the form of images, binary tables, ASDF files, or ASCII text tables. The data type for each extension is recorded in the XTENSION header keyword.

ASDF format

The Advanced Scientific Data Format (ASDF) is a next generation, human-readable, hierarchical metadata structure made up of basic dynamic data types such as strings, numbers, lists, and mappings. Data is saved as binary arrays. It is primarily intended as an interchange format for delivering products from instruments to scientists or between scientists, or, for example, between stages of the calibration pipeline. ASDF files are added to certain JWST calibration pipeline products and are part of the  reference data used by the software. As an example, distortion and spectral models needed to transform detector positions to a world coordinate frame are in ASDF format. 

ECSV format

The Enhanced Character Separated Values (ECSV) format, which is standard for the interchange of tabular data in a text-only format, is used to store a catalog of derived data for sources identified in some steps of the calibration pipeline. This file includes a header section with the definition, data type, and description for the columns, and a data section with as many rows as sources identified in an image. Besides a simple comma-separated delimited text file reader,  these files can also be read, modified, or created using the Astropy ECSV code.



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