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Spectral Metadata

The keywords listed below are required, recommended or optional for all FITS files headers: either the primary (P) or extension (E) HDUs, for all files that contain spectra. 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. One-dimensional spectra are commonly organized as an image or as a table, while two-dimensional spectral images may contain multiple spectra, and may be represented in a FITS IMAGE or a BINTABLE extension. These cases are discussed below.

Metadata Common to All Spectra

Keyword HDU Notes
Required Keywords
BUNIT P or E Brightness unit for array values
XPOSURE P or E Effective exposure duration in seconds, corrected for dead time
Suggested Keywords
APERTURE P or E Name of aperture used for exposure
DETECTOR P or E ID of detector used for exposure
DISPRSR P or E Name of dispersive element used, or 'MULTI' if more than one defined the passband.
DISPRnn P or E Name(s) of dispersive element(s) used for exposure if more than one was used, with nn incrementing from 1.

Metadata for Spectra Stored as Images

Keyword HDU Notes
Required Keywords
CDi_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: if the alternative PCi_j notation is used, 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.
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 present if CDi_j keywords are present.
Recommended Keywords
RADESYS P or E Mnemonic for celestial coordinate reference system (typically 'FK5' or the preferred 'ICRS'), if applicable.
WCSAXES P or E Number of axes in WCS description, which may exceed the number of pixel array axes


  • Keyword records are listed in the above tables 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.

Metadata for Spectra as BINTABLEs

Only the keywords common to all spectra are required. However, certain content is required or suggested for fields in the table; the field names are suggested.

Keyword Notes
Required Fields
FLUX Flux for the associated wavelength, in units of BUNIT.
WAVELENGTH Wavelength for the associated flux.
Recommended Fields
VARIANCE Variance in the flux at the associated wavelength
FLAGS Data quality flags, with zero indicating no pathologies