8.4. SPECTRAL COORDINATE SYSTEM REPRESENTATIONS

83

Table 8.4.

Allowed Values of RADESYSa

Value

Definition

International Celestial Reference System

ICRS

Mean place, new (IAU 1984) system

FK5

FK41

Mean place, old (Bessel-Newcomb) system

FK4-NO-E1

Mean place: but without eccentricity terms

Geocentric apparent place, IAU 1984 system

GAPPT

1New

FITS files should avoid using these older reference

systems.

that the TIMESYS keyword, as defined in Appendix B be used to specify the time

system.

LONPOLEa -- [floating-point; default: φ0 if δ0 ≥ θ0, φ0 + 180◦ otherwise]

Longitude in the native coordinate system of the celestial system's north pole.

Normally, φ0 is zero unless a non-zero value has been set for PVi 1a, which is asso-

ciated with the longitude axis. This default applies for all values of θ0, including

θ0 = 90◦, although the use of non-zero values of θ0 are discouraged in that case.

LATPOLEa -- [floating-point; default: 90◦, or no default if (θ0, δ0, φp -φ0) = (0, 0, ±90◦ )]

Latitude in the native coordinate system of the celestial system's north pole, or

equivalently, the latitude in the celestial coordinate system of the native system's

north pole. May be ignored or omitted in cases where LONPOLEa completely spec-

ifies the rotation to the target celestial system.

8.4

Spectral Coordinate System Representations

This section discusses the conversion of intermediate world coordinates to spectral co-

ordinates with common axes such as frequency, wavelength, and apparent radial ve-

locity (represented here with the coordinate variables ν, λ, or v). The key point for

constructing spectral WCS in FITS is that one of these coordinates must be sampled

linearly in the dispersion axis; the others are derived from prescribed, usually non-linear

transformations. Frequency and wavelength axes may also be sampled linearly in their

logarithm.

FITS Standard