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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.
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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
| Keyword | HDU | Notes | |
|---|---|---|---|
APERTURE | P or E | Name of aperture used for exposure | |
BUNIT | P or E | Brightness unit for array values, e.g., erg/cm2/s/Angstrom or Jy for flux density | |
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 (zero-padded) incrementing from 1. Note that this information can alternatively be represented in a PROVENANCE extension. See Provenance Metadata for details. | |
XPOSURE | P or E | Effective 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.
| Keyword | HDU | Notes | |
|---|---|---|---|
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 | Increment 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. | |
CRPIXj | P or E | Location of the reference point in the image for axisj, in array pixel units. | |
CRVALi | P or E | World coordinate value at the reference location for axisi | |
CTYPEi | P or E | World coordinate type for axisi. | |
CUNITi | P or E | Physical units of CRVAL for axis i. Note: units for celestial coordinate systems must be degrees. | |
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. | |
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 |
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.
| Keyword | Notes | |
|---|---|---|
FLAGS | Data quality (binary) flags, with zero indicating no pathologies | |
FLUX | Flux(es) for the associated wavelength(s), in units of BUNIT | |
VARIANCE | Variance in the flux(es) at the associated wavelength(s) | |
WAVELENGTH | Wavelength(s) for the associated flux(es) |