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Image Metadata

The keywords listed below are required, recommended, or suggested for all FITS files that contain images. In simple FITS files, all keywords must appear in the primary (P) header, but for IMAGE extensions these keywords should appear in the extension (E) header unless they are inherited from the primary header. (Note: WCS related keywords should only appear in the extension for which they apply.) Recommended keywords, if absent, will be computed and inserted prior to ingest; suggested keywords would be beneficial to archival users if present. Headers must also include the basic structural FITS keywords and the list of common keywords.

Keyword HDU Notes
Required Keywords
BUNIT P or E Brightness unit for array values
CD i_j P or E Transformation matrix between pixel axis j and intermediate coordinate axis i. The indicies range from 1 to the value of NAXIS. Note that if the alternative PCi_j notation is used (see below), these keywords must not appear in the header.
CDELTi P or E Scale for axis i at the pixel reference location, in units of deg/pixel. Used in conjunction with PC notation for transformation matrix; ignored for CD notation.
CRPIXi P or E Reference location of axis i, in array pixel units.
CRVALi P or E Reference location of axis i, in world coordinates.
CTYPEi P or E World coordinate type for axis i.
FILTER P or E Name of filter used, or 'MULTI' if more than one defined the passband.
FILTERnn P or E Name(s) of filter(s) used to define the passband, if more than one was used, with nn incrementing from 1.
PCi_j P or E Alternative to CD notation for transforming pixel axis j to intermediate coordinate axis i. Must be used with CDELTi and must not be used with CDi_j.
RADESYS P or E Mnemonic for celestial coordinate reference system (typically 'FK5' or the preferred 'ICRS').
Recommended Keywords
DEC_TARG P or E Declination coordinate of the target or field, in degrees
PSFSIZE P or E Typical spatial extent of the point-spread function, in pix
RA_TARG P or E Right Ascension coordinate of the target or field, in degrees
WCSAXES P or E Number of axes in WCS description, which may exceed the number of pixel array axes
Suggested Keywords
APERTURE P or E Name of aperture used for exposure (if any)
DETECTOR P or E ID of detector used for exposure


  • Keyword records are listed in the above table alphabetically by name for ease of reference; contributor teams may choose the order of most keywords.
  • Deprecated FITS keywords (such as EPOCH) must not be used.