BSL Deep-Sky Imaging – Image File Formats

When doing astrophotography, choosing the right image file format is critical for preserving raw scientific data and maximizing the quality of your final stacked images.

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Here is a breakdown of the primary image file formats used in astrophotography and when to use them:
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Unlike standard photography where JPEGs are common, astrophotography relies almost exclusively on RAW data formats.
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These formats store the uncompressed, unprocessed numerical data straight from the camera’s sensor.

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FITS
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(FITS = Flexible Image Transport System)
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The universal standard format used by astronomers and dedicated astronomy cameras.
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It supports high bit depths (16-bit or 32-bit floating point) and stores header metadata about the capture session (telescope, temperature, exposure time, filters).
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XISF
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(XISF = Extensible Image Serialization Format)
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A modern, highly flexible file format native to PixInsight.
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It extends the capabilities of FITS by better supporting metadata, thumbnail previews, and complex multi-channel data.
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Camera Manufacturer RAWs
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The proprietary uncompressed or lossless compressed raw files produced by DSLR and mirrorless cameras.
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For example:
–...CR2 for Canon
–...CR3 for Canon
–...NEF for Nikon
–...ARW for Sony)

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What it is
A high-quality, lossless raster image format.
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When it’s used
TIFFs are widely used during intermediate and final stages of image processing (e.g., after calibration and stacking in software like DeepSkyStacker or Siril).
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Because it is lossless, saving a file as a 16-bit TIFF ensures no data or dynamic range is degraded as you move between editing programs.
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What it is
A lossless compressed bitmap image format.
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When it’s used
Excellent for sharing final processed images online or in presentations.
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Unlike JPEG, PNG compression does not introduce compression artifacts, keeping fine star fields and faint nebulosity clean.
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What it is
A lossy, compressed image format.
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When it’s used
Avoid using JPEGs for capturing or processing astrophotography data.
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JPEG files discard crucial data (compressing images down to 8-bit per channel) and apply irreversible compression algorithms that destroy faint details, create blocky gradients around stars, and clip subtle background nebulosity.
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Only use JPEGs if you need a quick snapshot to share directly from a camera’s memory card.
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