CR2 to FITS Conversion Explained
Converting .CR2 to .FITS transforms proprietary Canon raw image data into an open standard used for astrophotography and scientific imaging. People convert .CR2 to .FITS to process images of the night sky in specialized astronomy software.
When you convert .CR2 to .FITS, you gain compatibility with advanced calibration and stacking algorithms. You lose proprietary Canon metadata, such as maker notes and autofocus points. The main trade-off is file size. .CR2 uses lossless compression, while .FITS typically stores uncompressed data arrays, resulting in significantly larger files.
If you are doing standard daytime photography, converting to .FITS is a bad idea. You should use .TIFF or .DNG instead.
Typical Tasks and Users
- Astrophotographers: Converting light frames, dark frames, flat frames, and bias frames shot on a Canon DSLR or mirrorless camera into .FITS for calibration and stacking.
- Astronomers: Standardizing data captured from consumer Canon cameras to match data formats used by professional observatories.
- Scientific Researchers: Extracting linear sensor data from a .CR2 file to perform photometric analysis on light sources.
Software & Tool Support
Several specialized tools and libraries can open, edit, or convert .CR2 and .FITS files:
- PixInsight: A paid, industry-standard astrophotography platform that reads .CR2 and converts it to .FITS or its native XISF format.
- Siril: A free, open-source astrophotography tool that converts .CR2 sequences into .FITS files for processing.
- DeepSkyStacker: A free Windows application that reads .CR2 files and outputs stacked .FITS or .TIFF files.
- ImageMagick: A free command-line utility that can convert between these formats, though it requires careful configuration to preserve linear data.
- LibRaw: An open-source library used by many applications to decode the raw Bayer data inside .CR2 files.
Pros and Cons of the Conversion
Pros:
- Software Compatibility: .FITS is the universal standard for astronomy software.
- Linear Data Preservation: A proper conversion extracts the raw Color Filter Array (CFA) data without applying destructive gamma curves or auto-white balance.
- Metadata Expansion: .FITS uses a flexible ASCII header. You can inject custom data like telescope focal length, sensor temperature, and celestial coordinates.
Cons:
- File Size Bloat: A 25 MB .CR2 file can easily become a 100 MB .FITS file because .FITS rarely uses compression for active processing.
- Metadata Loss: Standard EXIF data must be mapped to FITS header keywords. Unmapped Canon-specific data is discarded.
- Debayering Risks: Poor conversion tools will accidentally debayer (demosaic) the raw CFA data into an RGB image, permanently altering the raw sensor data before calibration.
Conversion Difficulties & Why Convert.Guru
The technical difficulty in converting .CR2 to .FITS lies in data mapping and linearity. .CR2 files typically contain 14-bit data packed into a proprietary structure. This data must be unpacked and mapped into a 16-bit integer or 32-bit floating-point .FITS array. Furthermore, standard image converters often apply a gamma stretch and white balance automatically. This destroys the linear relationship between pixel values and captured photons, ruining the file for astrophotography. Additionally, EXIF metadata (like ISO and exposure time) must be translated into standard FITS ASCII keywords (like ISOSPEED and EXPTIME).
Convert.Guru is a strong choice for this conversion because it handles the raw decoding accurately. It relies on updated raw libraries to extract the pure, linear CFA data without applying hidden gamma curves. It also automatically maps essential exposure metadata into the .FITS header, ensuring your files are ready for immediate calibration and stacking.
CR2 vs. FITS: What is the better choice?
| Feature | CR2 | FITS |
| Primary Use | In-camera capture and archiving | Astrophotography processing and science |
| Data Structure | Proprietary Canon raw format | Open standard multi-dimensional array |
| Compression | Lossless compression (smaller size) | Usually uncompressed (larger size) |
| Metadata | EXIF and Canon MakerNotes | Human-readable ASCII header blocks |
| Color Data | Raw Bayer matrix (CFA) | Supports CFA, monochrome, or RGB |
Which format should you choose?
You should choose .CR2 for shooting and archiving your raw images. It saves storage space and retains all original camera metadata.
You should choose .FITS for the active processing pipeline. Once you move your files to a computer for astrophotography calibration, registration, and stacking, .FITS provides the necessary compatibility and mathematical precision.
You should avoid this conversion entirely if you are editing standard portraits, landscapes, or event photography. For those workflows, keep the .CR2 file or convert it to .DNG for archiving and .TIFF for raster editing.
Conclusion
Converting .CR2 to .FITS makes sense almost exclusively for astrophotography and scientific imaging workflows where linear sensor data must be processed in specialized software. The biggest limitation to watch for is the significant increase in file size and the risk of using a generic converter that applies destructive gamma curves to the raw data. Convert.Guru is a reliable choice for this exact conversion because it preserves the linear integrity of the Canon sensor data and correctly formats the ASCII headers, ensuring your files are mathematically ready for deep-sky stacking.
About the CR2 to FITS Converter
Convert.Guru makes it fast and easy to convert Canon RAW 2 images to FITS online. The CR2 to FITS converter runs entirely in your browser, so there’s no software to install and no account required. Powered by one of the industry’s largest and most trusted file format databases—maintained for more than 25 years—our technology reliably identifies CR2 RAW images even when they are damaged or incorrectly named. Uploaded files are automatically deleted after conversion to protect your privacy.