Which stretch should I use?

Asinh (the default) is a good all-rounder — it brings up faint nebulosity without crushing bright stars. Log is harsher; use it for high dynamic range images like Hubble deep fields. Linear is fine for solar/planetary frames where everything sits in a narrow range.

Why is my image black or saturated?

We clip to the 1st and 99th percentile by default, which usually frames the data well, but extreme outliers can throw it off. Try a different stretch — asinh is most forgiving — or open the file in DS9 / SAOImage for full control over zscale and percentile cuts.

Does it handle data cubes (NAXIS3) or BINTABLE extensions?

Only the first 2D slice of the primary HDU is rendered. Header cards from all HDUs in the file are counted but only the primary header is displayed. For binary tables and 3D cubes use DS9, fv, or astropy.

No. The FITS file is read entirely in your browser. Nothing is sent to a server.

Scientific.fits · .fit · .fts

FITS viewer

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FITS · .fits · .fit · .fts

Open FITS files

Astronomical image and table format.

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About FITS

Astronomical image and table format.

FITS — the Flexible Image Transport System — is astronomy's universal file format, used by every major telescope from Hubble to Vera Rubin. The structure is rigid: 2880-byte blocks of either 80-byte header cards or raw image data, with BITPIX telling you the pixel type (8/16/32/64-bit integers, 32/64-bit floats). We parse the header natively, render the primary image HDU to a canvas, and offer four stretch curves — linear, log, sqrt, asinh — which clip to the 1st/99th percentile by default. Floating-point pixel values are converted using BSCALE and BZERO. Tables, data cubes (3D arrays), and Rice-compressed binary tables aren't supported here — try fv from NASA or DS9 for those.

FAQ

Which stretch should I use?: Asinh (the default) is a good all-rounder — it brings up faint nebulosity without crushing bright stars. Log is harsher; use it for high dynamic range images like Hubble deep fields. Linear is fine for solar/planetary frames where everything sits in a narrow range.
Why is my image black or saturated?: We clip to the 1st and 99th percentile by default, which usually frames the data well, but extreme outliers can throw it off. Try a different stretch — asinh is most forgiving — or open the file in DS9 / SAOImage for full control over zscale and percentile cuts.
Does it handle data cubes (NAXIS3) or BINTABLE extensions?: Only the first 2D slice of the primary HDU is rendered. Header cards from all HDUs in the file are counted but only the primary header is displayed. For binary tables and 3D cubes use DS9, fv, or astropy.
Is my file uploaded?: No. The FITS file is read entirely in your browser. Nothing is sent to a server.

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