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 convention layered upon 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 aperture 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).


The FITS-IDI convention has been re-examined and a new document produced. Errors and incomplete areas in the older documents were addressed. The latest document may be found in AIPS Memo number 114 revised July 2022. It is also available as gzipped PostScript and as pdf files. The memo uses color to show corrections to the original document listed below and to show additions which have followed a comment period.

An older document describing an earlier description of 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 Flatter's first draft and in Diamond's VLBA Correlator Memo No. 108.
It was last updated on August 29, 2000. 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 by Phil Diamond. 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.
Some typographical errors were corrected on December 3 1998 and a number of other revisions and corrections have been made subsequently.

Greisen had to re-type the document in 2009-2011 since no editable version of the documeht was available. Numerous additions and corrections to the earlier versions were made at this point. Greisen's document has been accepted as an IAU (FITS Committee) acknowledged convention.