Why Telescopes Use FITS Files Instead of JPEG

Astronomical telescopes record observational data as Flexible Image Transport System (FITS) files rather than consumer formats like JPEG to preserve raw scientific integrity. While JPEG relies on lossy compression, reduced bit depth, and non-linear color encoding designed solely for human vision, FITS preserves every captured photon count with high numerical precision. Additionally, FITS embeds critical observational metadata and supports multidimensional scientific data, making it the non-negotiable standard for quantitative astronomical research.

Preservation of Raw Photon Counts

JPEG uses lossy discrete cosine transform (DCT) compression to reduce file sizes by permanently discarding image details that the human eye typically cannot perceive. In observational astronomy, faint signals from distant galaxies, exoplanet transits, or subtle variations in background noise are mathematically vital. FITS files store the uncompressed, raw values read directly from the sensor's charge-coupled devices (CCDs) or CMOS detectors, ensuring no data points are altered or destroyed.

High Dynamic Range and Bit Depth

Standard JPEG images are restricted to an 8-bit depth per channel, allowing for only 256 discrete brightness levels between pure black and pure white. Astronomical scenes, however, possess extreme dynamic ranges where a brilliant foreground star can sit immediately adjacent to an exceedingly faint nebula. FITS files support 16-bit, 32-bit, and 64-bit integer or floating-point formats, providing thousands to billions of gradations. This depth ensures that bright objects do not saturate while faint structures remain detectable and measurable.

Linear Data vs. Non-Linear Gamma Curves

Telescope sensors respond linearly to light: doubling the exposure time or the brightness of a source doubles the recorded electron count. JPEG files automatically apply non-linear tone curves (gamma adjustments) and color matrices to make an image look visually pleasing on consumer displays. These irreversible adjustments distort the mathematical relationship between pixel value and incoming photon flux. FITS maintains linear sensor responses, allowing astronomers to perform accurate photometry (measuring brightness) and astrometry (measuring position).

Standardized Scientific Metadata

A FITS file is not merely an image container; it consists of structured headers coupled with data arrays. The human-readable ASCII header stores essential metadata describing the observation, including:

• Precise telescope pointing coordinates (Right Ascension and Declination)
• World Coordinate System (WCS) calibrations that map pixel locations directly to celestial coordinates
• Exposure duration, sensor temperature, and readout noise
• Filter passbands, airmass, and observatory environmental conditions

Without this context, an astronomical image cannot be calibrated against dark frames, flat fields, or standard reference stars.

Support for Multidimensional and Non-Visual Data

Unlike JPEG, which is limited to two-dimensional RGB or grayscale raster graphics, FITS handles arbitrary data dimensions. Astronomers routinely use FITS to store:

• Three-dimensional spectral data cubes (spatial coordinates combined with wavelength)
• Time-series observations and polarization measurements
• Tabular catalogs containing millions of detected sources

By handling raw, high-precision, multidimensional datasets paired with comprehensive observational records, the FITS format ensures that astronomical data remains reproducible, mathematically valid, and useful for generations of researchers.