FITS Images

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 images in IMAGE extensions these keywords should appear in the relevant extension (E) header. (Note: WCS related keywords must 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.

The following table(s) of HLSP metadata, to be included in science products, are color-coded:




Headers must also include the basic structural FITS keywords and the list of common keywords.


APERTUREP or EName of aperture used for exposure (if any)

BUNITP or EBrightness unit for array values

CDi_jP 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.

CDELTiP or EIncrement of the world coordinate at the reference point for axis i, in units of deg/pixel. Used in conjunction with PC notation for transformation matrix; ignored for CD notation.

CRPIXjP or ELocation of the reference point in the image for axis j, in array pixel units.

CRVALiP or EWorld coordinate value at the reference location for axis i

CTYPEiP or EWorld coordinate type for axis i

CUNITiP or EPhysical units of CRVAL for axis i. Note: units for celestial coordinate systems must be degrees.

DETECTORP or EID of detector used for exposure

FILTERP or EName of filter used, or 'MULTI' if more than one defined the passband

FILTERnnP or EName(s) of filter(s) used to define the passband, if more than one was used, with nn incrementing from 1 (and zero-pad if nn >9)

PCi_jP or EAlternative 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.

DEC_TARGP or EDeclination coordinate of the target or field, in degrees

PSFSIZEP or ETypical spatial extent of the point-spread function, in pix

RADESYSP or EMnemonic for celestial coordinate reference system (typically 'FK5' or the preferred 'ICRS')

RA_TARGP or ERight Ascension coordinate of the target or field, in degrees

WCSAXESP or ENumber of axes in WCS description, which may exceed the number of pixel array axes

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