NAXIS

NAXIS is a keyword in the FITS (Flexible Image Transport System) header that specifies the number of axes in the primary data array of a FITS file. It indicates how many dimensions the data array has.

For each axis i from 1 to NAXIS, there is a corresponding keyword NAXISi that gives the length of that axis in pixels. For example, a common 2D image would have NAXIS=2, NAXIS1=1024, and NAXIS2=1024, describing a 1024-by-1024 image. Higher-dimensional data, such as spectral cubes or multi-dimensional datasets, are described by NAXIS3, NAXIS4, and so on.

In a FITS file, NAXIS applies to the primary data unit; extension headers may contain their own NAXIS values for their data arrays. The numbering of axes starts at 1; NAXIS1 is typically the x-dimension (columns) and NAXIS2 the y-dimension (rows), with higher indices representing additional axes. If NAXIS equals 0, the primary HDU contains no data.

NAXIS is part of the FITS standard, which has been used since the 1970s in astronomical data storage. It is commonly used in concert with the WCS conventions to define the coordinate system and axes of the data.