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Comment: Published by Scroll Versions from space DraftMASTDOCS and version 23.04

Metadata that apply to spectra

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.

Excerpt Include
HLSP Metadata Color Codes
HLSP Metadata Color Codes

One-dimensional spectra are commonly organized as an image or as a table (or a table row); two-dimensional spectral images may contain multiple spectra. Spectra may be represented in a FITS IMAGE or a BINTABLE extension. These cases are discussed below.

Metadata Common to All Spectra


APERTUREP or EName of aperture used for exposure

BUNITP or EBrightness unit for array values, e.g., erg/cm2/s/Angstrom  or Jy for flux density

DETECTORP or EID of detector used for exposure

DISPRSRP or EName of dispersive element used, or 'MULTI' if more than one defined the passband.

DISPRnnP or EName(s) of dispersive element(s) used for exposure if more than one was used, with nn (zero-padded) incrementing from 1. Note that this information can alternatively be represented in a PROVENANCE extension. See Provenance Metadata for details.

XPOSUREP or EEffective exposure duration in seconds, exclusive of dead time

Metadata for Spectra Stored as Images

In this case, spectral pixel values are stored in an image array (primary or extension), and header keywords contain the parameters of the dispersion relation (i.e., mapping of pixel to wavelength). Multiple arrays would be used to contain concomitant quantities, such as variance and data quality.


CDi_jP or ETransformation 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.

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 axisj, in array pixel units.

CRVALiP or EWorld coordinate value at the reference location for axisi

CTYPEiP or EWorld coordinate type for axisi.

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

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 present if CDi_j keywords are present.

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

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

Metadata for Spectra Stored as a BINTABLE

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. Note that spectra may be organized such that the values for each field run sequentially with row number; alternatively each table row may contain a spectrum, where table cells contain 1-D arrays.



Data quality (binary) flags, with zero indicating no pathologies

FLUXFlux(es) for the associated wavelength(s), in units of BUNIT

VARIANCEVariance in the flux(es) at the associated wavelength(s)

WAVELENGTHWavelength(s) for the associated flux(es)