What is a FITS file?
FITS (Flexible Image Transport System) is the standard image format of astronomy, used everywhere from the Hubble archive to the astro camera in your backyard. Capture gear like the ZWO ASIAIR and Seestar, and software such as NINA and SharpCap, save raw captures as .fits (or .fit / .fts) because the format stores the sensor's full linear readout — usually 16 or 32 bits per pixel — together with header metadata like exposure time, gain, and camera model.
That fidelity is exactly why ordinary photo apps can't open FITS files: they aren't ordinary photos yet. Converting one to JPEG, PNG, or TIFF takes three steps this page performs for you automatically.
How this converter works
- Decode. The file's FITS header is parsed and the image data read at full precision — 8, 16, and 32-bit integer and 32/64-bit floating-point data are all supported, including BZERO/BSCALE offset encoding.
- Debayer. Raw one-shot-color captures are stored as a Bayer mosaic — opened naively they look like a grey checkerboard. When the header records a pattern (RGGB, BGGR, GRBG, or GBRG), the converter reconstructs full color automatically.
- Stretch. Linear astro data hides its signal in the bottom few percent of the histogram, which is why FITS files so often "look black". Auto Stretch applies the same midtone-transfer autostretch used by Siril and PixInsight so nebulosity and faint stars become visible. Already-stretched data can be exported with the Linear option instead.
- Export. Download the result as a lossless PNG, a compact JPEG, or a 16-bit TIFF that preserves maximum tonal depth for further processing.
Everything happens in JavaScript on your own device. Nothing is uploaded, nothing is stored, and there is no file-size quota beyond what your browser's memory allows.
Frequently asked questions
Why does my FITS file look black or grey when I open it?
Because the data is linear and unstretched — the target sits in the darkest few percent of the brightness range. Use Auto Stretch here to lift it into view. If it looks like a fine grey checkerboard, it's an un-debayered color capture; this converter debayers automatically.
Which output format should I choose?
Pick 16-bit TIFF if you'll keep processing on a desktop — it keeps the most tonal depth. PNG is lossless 8-bit, ideal for editing and sharing. JPEG is smallest for quick sharing, with slight compression loss.
Is there a file size limit?
No fixed limit — conversion runs in your browser's memory, so full-frame captures of 60+ megapixels work fine on a modern machine.
Are my images uploaded anywhere?
No. The page never sends your data to a server; you can even load it once and use it offline. Your captures stay yours.
Can I convert multiple frames or stack them?
This tool converts one frame at a time and doesn't stack. For stacking, use dedicated software such as Siril or DeepSkyStacker, then bring the stacked result here — or into the PixADair app — for finishing.
Then make it beautiful
The PixADair app for iPhone & iPad opens FITS files natively — and goes further: AI denoise, sharpen, and upscale tuned for deep-sky images, powered by Topaz Labs.
