FITS (astronomy) file signature
Magic number at the start of the file · SIMPLE
A FITS astronomy file begins with the ASCII header keyword SIMPLE, the first card of a human-readable text header that precedes the image or table data.
What this magic number means
A magic number is a short, fixed run of bytes at a known position that tells a program what a file really is, no matter what the filename says. If a file starts with the bytes 53 49 4D 50 4C 45 (the text SIMPLE), it is a FITS (astronomy) file.
Formats that use the 53 49 4D 50 signature
These file types in our database carry this signature:
How to check a file's signature
You can read the first bytes of any file yourself. The magic bytes are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.fits
- hexdump -C -n 16 example.fits
- file example.fits
Windows (PowerShell)
- Format-Hex -Path example.fits -Count 16
Python
- open("example.fits","rb").read(6).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the FITS (astronomy) file signature?
FITS (astronomy) files start with the hex bytes 53 49 4D 50 4C 45 (SIMPLE in ASCII). This magic number identifies the format regardless of the file's name or extension.
How do I check a file's magic number?
Open the file in a hex editor, or run a command such as xxd -l 16 example.fits on Linux or macOS, or Format-Hex -Path example.fits -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the FITS (astronomy) signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine FITS (astronomy) file has these exact bytes; a file with the wrong bytes is not really FITS (astronomy), whatever its name says.