Information About the FITS Interferometry Data Interchange Format

This web page provides basic information about the FITS Interferometry Data Interchange (or FITS-IDI) format. The FITS-IDI format is a variant of FITS that may be used to transport radio interferometry data.



FITS-IDI files store radio interferometry data and associated calibration data as a sequence of FITS binary tables. Different types of tables are used to carry visibility measurements, information about the aperature synthesis arrays, calibration data and so on.
This format was designed to be used to export raw data from aperture synthesis arrays and to be written in near real-time as the data are correlated. The following VLBI correlators export data in FITS-IDI format. The FITS-IDI format can be read using the AIPS task FITLD. FITS-IDI files containing calibration information in the form of FLAG, SYSTEM_TEMPERATURE, GAIN_CURVE, or PHASE-CAL tables can only be read by the version of FITLD in the 15OCT98 release of AIPS (or later releases).


A document describing the FITS-IDI format is available as both a PDF document and a PostScript document.  This document describes the FITS-IDI format as it is interpreted by the version of FITLD in the 15OCT98 release of AIPS and supercedes the description of the FITS-IDI format in VLBA Correlator Memo No. 108.
This document is maintained by Chris Flatters at NRAO. It is currently in draft form and you should feel free to mail any comments or corrections to
The document was last updated on September 16 1998. A list of errata is available.


The FITS-IDI format was originally proposed as the export format for the VLBA in VLBA Correlator Memo No. 108. This was intended to be a general format that could be used by other instruments and was also adopted by the Canadian S2 correlator and by Mitaka.
Over time, a number of practical difficulties were encountered which led to the FITS-IDI format that was actually implemented at the VLBA being slightly different from that described in the original memo. Many of the changes were negotiated privately between the VLBA correlator group and the AIPS group and were not published. This meant that there was no single document that described the FITS-IDI format.
This was clearly an undesirable situation and a decision was made to produce a document that defined the FITS-IDI document as it was currently implemented. This document would then become the new specification for the FITS-IDI format and would be the basis for future enhancements.
The first draft of the new specification was made public on September 16 1998.

Maintained by Chris Flatters
Last updated on September 16 1998