... specify[*]
While Papers I and II use the generic intermediate axis number i, we use here the axis number k as the spectral intermediate axis number. The single-character version code a was introduced in Paper I. It has values blank and A through Z.
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... formula[*]
The quoted formulæ apply only at normal optical wavelengths. In the UV and IR spectral domains, atmospheric absorption lines cause refractivity to be a strong, weather dependent function of frequency. See, for example, Mathar (2004).
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... refraction[*]
Also known as Descartes' Law of Sines; see for example, http://wikipedia.org/wiki/Snells_law
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... element[*]
The precise interpretation of CTYPE k keywords that describe TIME is deferred to a possible future paper on temporal coordinates in FITS.
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... frame[*]
Proper relativistic formulæ should be used; see Gibbs & Tao (1997) or Rindler (1982). Better yet, observers should consider using standard software described by Lindegren and Dravins (2003) to perform the conversion from topocentric coordinate to the standardized Barycentric Celestial Reference System.
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... listed[*]
The frame and velocity keywords in Table 13 apply to terrestrial observers only. Observations from spacecraft require additional description of their motions - either a position and velocity or an ephemeris. The required keywords and/or tables for this are beyond the scope of this paper.
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.. elements[*]
Suggested by contributions from William Pence, Arnold Rots, and Lorella Angelini of the NASA Goddard Space Flight Center, Greenbelt, MD 20771.
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Copyright ESO 2006