FITS to J2K Converter
Processing entirely on your device — large files may take a moment.
Flexible Image Transport System and JPEG 2000 Codestream exist for different jobs: FITS (Flexible Image Transport System) is the data format of professional astronomy — every image carries a human-readable header with telescope, exposure and world coordinates alongside the raw pixel data. Convert J2K images exactly like JP2 — to JPG, PNG or modern formats.
This converter runs entirely in your browser using WebAssembly — your files never leave your device, nothing is uploaded, and there are no watermarks, sign-ups or queues. Drop in several files at once to convert them as a batch.
How FITS compares to J2K
| FITS | J2K | |
|---|---|---|
| Compression | None (optionally Rice/GZIP) | Wavelet (lossy + lossless) |
| Transparency | Not supported | Supported |
| Animation | Not supported | Not supported (MJ2 is separate) |
| Metadata | Rich text header (WCS) | XML boxes (EXIF/GPS) |
| Best for | Telescope frames, science data | Archives, medical, cinema |
How to convert FITS to J2K
- 1
Select your files
Drag & drop your FITS files onto the area above, or click to browse. You can add multiple files at once for batch conversion.
- 2
Convert instantly
Click "Convert to J2K". The conversion runs entirely inside your browser using WebAssembly — no upload, no queue, no waiting.
- 3
Download results
Download each converted J2K file individually, or grab everything at once as a ZIP archive.
About the FITS format
FITS (Flexible Image Transport System) has carried professional astronomy data since 1981: every file opens with a plain-text header that records telescope, instrument, exposure, filters and world coordinates, followed by raw pixel arrays — often 16-bit or float, sometimes data cubes.
Values can be negative or far above display range, so FITS never looks right in a photo viewer. Converting to PNG applies a simple scaling pass that reveals the nebula or galaxy in the numbers — good for previews, while science pipelines keep the original.
+Strengths
- ✓Self-documenting science headers
- ✓High bit depth (16/32-bit, float)
- ✓The standard of every observatory
!Limitations
- −Not display-ready
- −Files are large and uncompressed
About the J2K format
JPEG 2000 (JP2) replaced JPG's DCT blocks with wavelet transforms: at the same bitrate it shows fewer blocky artifacts, and its lossless mode compresses better than JPG ever could. It was the format of early digital cinema and remains common in archives, medical systems and geospatial pipelines.
Browser support never arrived — Chrome, Firefox and Safari all skip it — so a JP2 file is often invisible on the web. Converting JP2 to JPG or PNG makes archival material viewable anywhere; converting the other way gives long-term storage better compression.
+Strengths
- ✓True lossless mode
- ✓Better quality per byte than JPG
- ✓ROI / progressive decoding built in
!Limitations
- −No browser support
- −Slow to encode compared to JPG
Why convert FITS to J2K?
There is a concrete reason this conversion exists: Not display-ready J2K sidesteps that — True lossless mode If that is your situation, the tool above settles it in seconds, entirely on your device.
Frequently asked questions
Is this FITS to J2K converter really free?
Yes — completely free, with no sign-up, no watermark and no daily conversion limits. The site is supported by advertising.
Are my files uploaded to a server?
No. The conversion runs entirely inside your browser using WebAssembly, so your files never leave your device — that is what makes it instant and private.
Does it work on Windows, Mac, iPhone and Android?
Yes. Anything with a modern browser works: Windows, macOS, Linux, Android and iOS. No software installation is required.
Will I lose quality converting FITS to J2K?
No — J2K is a lossless format, so every pixel of the converted image matches your original FITS file. You can also adjust the quality/size balance before converting when the target format supports compression.
Will transparent areas survive the conversion to J2K?
Transparency is kept whenever J2K actually supports an alpha channel. If the target format has none (like JPG), transparent areas are flattened onto a white background — that is a property of the format, not of this tool.
Can I convert several FITS files at once?
Yes — drop as many files as you like into the tool and they are converted in one batch. When more than one file comes out, everything is bundled into a ZIP for a single download.