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FITS converter

A .fits file is the standard data format of astronomy: one or more header-data units, each a block of human-readable 80-character keyword cards followed by an N-dimensional array or a table. Essentially every professional telescope and space observatory writes it, and the format has been backwards-compatible since 1981.

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Batch files can each use a different output. Nothing uploads for local conversions.

Working inputs include camera RAW, browser-local audio/video, PDF, CBZ/CBR comics, office documents, ebooks, markup, 3D models, structured text, images, and archives.

Where FITS comes from

Don Wells, Eric Greisen and Ron Harten defined FITS in 1979 so radio and optical observatories could exchange data on nine-track tape, and the IAU adopted it in 1982. Its governing principle became known as 'once FITS, always FITS': no change may break an existing valid file. Hubble, Chandra, Spitzer, the Sloan Digital Sky Survey and JWST all use it, and NASA's archives hold decades of data that is still readable with a modern library — which is the strongest practical argument for the rule.

How FITS works

  • Headers are ASCII cards of exactly 80 characters, in the form KEYWORD = value / comment, padded into 2880-byte blocks — a size inherited from magnetic tape.
  • BITPIX gives the data type: 8, 16, 32 and 64 are integers, -32 and -64 are IEEE floats.
  • FITS is big-endian, which sets it apart from nearly every other format still in use.
  • NAXIS and NAXIS1..n give the dimensionality, so an image, a data cube and a spectrum differ only in those keywords.
  • World Coordinate System keywords map array indices to sky coordinates, which is what turns a pixel grid into an astronomical observation.
  • A file may hold several HDUs: an image followed by binary tables of extracted sources is a common layout.

When to use FITS

  • Telescope and space-observatory imagery and spectra
  • Astronomical catalogues stored as binary tables
  • Reading archival observations from NASA, ESA or ground-based surveys
  • Rendering a scientific image to PNG for a presentation or a quick look

Strengths and limitations

Strengths

  • Headers are readable text, so a file partly describes itself even in a hex editor
  • Backwards compatibility has genuinely held for over forty years
  • Carries full scientific metadata, including coordinates, instrument and provenance

Limitations

  • The 2880-byte block padding wastes space and reflects hardware nobody has used in decades
  • Big-endian byte order trips up naive readers on every modern CPU
  • Rendering to an ordinary image format necessarily discards scientific precision

Compatibility

Astropy, CFITSIO, DS9, FITS Liberator, ImageMagick and Aladin read FITS, and most astronomy archives distribute it directly. Novus Convert decodes bounded image and fixed-table HDUs locally and writes JSON, CSV, or — for rank-two images — a grayscale PNG. That PNG is a preview, not data: it is produced with explicit warnings that the scientific dynamic range has been mapped to eight bits and that the WCS coordinate mapping does not survive.

Header cards, including WCS and instrument keywords, are read and exported into the JSON output; PNG carries no channel for them, which is why the loss is stated rather than assumed obvious.

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