Introduction

The Flexible Image Transport System - FITS - was originally developed in the late 1970s to enable the exchange of astronomical image data between computers of different type, with different word lengths and different means of expressing numerical values. Although the IEEE numerical formats have been widely adopted by the computer industry during the past twenty years, and in 1989 the FITS Standard was revised to utilize them, to this day computer manufacturers have yet to agree upon a single standard for bit order. In addition, independent of the numerical values themselves, a standard is essential for expressing the relationship of the data to the instrument with which they were obtained, to the position on the sky or association with wavelength, or with other general descriptive information that collectively constitutes the metadata for the observation. FITS has evolved over the years, encompassing new and more complex data structures in accord with the increasing sophistication of new astronomical instruments, and providing support for much more than the "images'' implied by the name. Images, spectra, data cubes, text tables, and binary tables are all supported, and with a variety of conventions in nomenclature and structure these basic elements have been combined to accommodate data spanning the range from digital (and digitized) images to output from computational simulations. Moreover, FITS has been immensely successful as a community-wide data format standard. No other scientific community has had anything like the success the astronomy community has had with FITS, and we are envied by many other communities for this cohesiveness. We have a process for amending and adding to the standard that assures broad community participation, and although this sometimes makes the process of change rather slow it helps to assure community support and compliance. All major astronomical software packages read and write FITS format data, and many have adopted FITS not only for exchange with other programs and facilities, but as a native run-time data format. The inherent inefficiencies of FITS (such as sequential header records, which when the allocated space is filled and a new header record is desired to be written, requires all following data to be rewritten) have been offset by the tremendous improvements in CPU and I/O efficiencies of modern desktop computers.