FITS Format Guide

FITS (Flexible Image Transport System) is the standard digital file format used in astronomy for storing, transmitting, and processing scientific data. Originally defined in 1981 by a joint effort of NASA and the International Astronomical Union (IAU) FITS Working Group, the format was designed to provide a platform-independent way to exchange astronomical data between different computing systems and observatories worldwide. FITS files use the extensions .fits, .fit, and .fts. The format supports multi-bit integer and floating-point data types (8-bit, 16-bit, 32-bit, and 64-bit), making it capable of representing the full dynamic range captured by modern astronomical instruments. FITS files can contain multiple data extensions (images, tables, and binary tables) within a single file, along with extensive metadata stored in plain-text ASCII headers. The format includes World Coordinate System (WCS) metadata for mapping pixel coordinates to celestial coordinates on the sky.

History of FITS

The FITS format emerged from the need to exchange astronomical image data between observatories using incompatible computer systems in the late 1970s. Don Wells (NRAO) and Eric Greisen (NRAO), along with Ron Harten (Netherlands Foundation for Radio Astronomy), proposed the initial format specification in 1979, which was formally published and adopted in 1981. The IAU endorsed FITS as the standard interchange format for astronomical data at its 1982 General Assembly. Over the decades, the format has been extended to support binary tables (1995), image compression (tile compression with Rice, GZIP, and HCOMPRESS algorithms), and World Coordinate System conventions for precise astrometric calibration. Every major space telescope and ground observatory uses FITS as its primary data format, including the Hubble Space Telescope (HST), James Webb Space Telescope (JWST), Chandra X-ray Observatory, the Very Large Array (VLA), European Southern Observatory (ESO), and the Sloan Digital Sky Survey (SDSS). The IAU FITS Working Group continues to maintain and evolve the standard, ensuring backward compatibility while adapting to modern astronomical data requirements.

Technical Details

FITS files are structured as a sequence of Header Data Units (HDUs). Each HDU consists of an ASCII header followed by an optional data block. The header contains keyword-value pairs in 80-character fixed-length records, providing metadata such as image dimensions (NAXIS, NAXIS1, NAXIS2), data type (BITPIX: 8, 16, 32, 64 for integers; -32, -64 for IEEE floating point), coordinate system information (CRPIX, CRVAL, CDELT, CTYPE for WCS), observation parameters (DATE-OBS, TELESCOP, INSTRUME), and scaling factors (BSCALE, BZERO). The primary HDU can contain an N-dimensional array (typically 2D images, but also 3D data cubes for spectral imaging). Extension HDUs can store additional images, ASCII tables, or binary tables with mixed data types. FITS data is stored in big-endian byte order with 2880-byte block alignment. The format supports image compression via the tile compression convention, where the image is divided into rectangular tiles that are independently compressed using algorithms like Rice, GZIP, or HCOMPRESS, achieving significant compression ratios while preserving scientific accuracy.

Common Applications

FITS is the universal data format of professional astronomy and astrophysics. NASA missions including Hubble, JWST, Chandra, Spitzer, and TESS distribute all science data in FITS format through archives such as MAST (Mikulski Archive for Space Telescopes) and IRSA (Infrared Science Archive). Ground-based observatories worldwide, from the European Southern Observatory's Very Large Telescope to the Keck Observatory, store raw and calibrated observations in FITS. Radio astronomy facilities including ALMA, the VLA, and the Square Kilometre Array precursors use FITS for visibility data and sky maps. Amateur astronomers use FITS for CCD imaging, stacking, and processing with tools like IRAF, SAOImageDS9, Astropy, FITS Liberator, and PixInsight. The format is also used in solar physics (SDO, SOHO), planetary science (Mars rovers, orbital imagers), and high-energy physics (gamma-ray and X-ray observations). Converting FITS images to PNG, JPG, TIFF, or WebP is common when preparing astronomical images for publications, presentations, websites, educational materials, and public outreach.

Advantages and Disadvantages

Advantages

Universal Astronomy Standard: The mandatory data format for all major observatories, space telescopes, and astronomical archives worldwide
High Dynamic Range: Supports 8/16/32/64-bit integer and 32/64-bit floating-point data, preserving the full sensitivity of astronomical instruments
Rich Metadata Headers: Plain-text ASCII headers store comprehensive observation metadata including coordinates, instrument settings, and calibration data
Multi-Extension Files: A single FITS file can contain multiple images, tables, and data arrays for complex datasets
World Coordinate System: Built-in WCS metadata maps pixel positions to precise celestial coordinates for astrometric analysis
Platform Independent: Big-endian binary format with standardized structure ensures portability across all operating systems
Lossless Storage: Data is stored without any lossy compression, preserving scientific accuracy for quantitative analysis
Extensive Tool Support: Supported by Astropy, IRAF, DS9, FITS Liberator, PixInsight, and virtually all astronomical software
Backward Compatible: Files created decades ago remain fully readable by modern software, ensuring long-term archival integrity
Tile Compression: Optional lossless and near-lossless compression algorithms reduce file sizes while maintaining data fidelity

Disadvantages

Not Web-Compatible: Cannot be displayed natively in web browsers; requires conversion for online use
Large File Sizes: Uncompressed floating-point data produces very large files, especially for multi-extension datasets
No Consumer Software Support: Standard image editors (Photoshop, GIMP) cannot open FITS files without plugins or conversion
Complex Structure: Multi-extension HDU architecture requires specialized libraries to parse and interpret correctly
No Color Profile Standards: Astronomical data is typically monochrome or multi-channel; no native sRGB/Adobe RGB color management
2880-Byte Block Padding: Fixed block alignment adds padding overhead, especially for small files
Header Limitations: 80-character record length restricts individual keyword values; long strings require continuation conventions
No Embedded Thumbnails: Unlike JPEG or TIFF, FITS files do not include preview thumbnails for quick browsing
Specialized Knowledge Required: Proper interpretation often requires understanding of BSCALE/BZERO scaling, WCS projections, and data units
No Animation/Video: Designed for static images and data cubes, not for temporal animation or video playback