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SECTION 8. WORLD COORDINATE SYSTEMS

RESTFRQa -- [floating-point; default: none]

Rest frequency of the of the spectral feature of interest. The physical unit must

be Hz.

RESTWAVa -- [floating-point; default: none]

Vacuum rest wavelength of the of the spectral feature of interest. The physical

unit must be m.

One or the other of RESTFRQa or RESTWAVa should be given when it is meaningful to do

so.

8.4.1

Spectral Coordinate Reference Frames

Frequencies, wavelengths, and apparent radial velocities are always referred to some

selected standard of rest (i.e., reference frame). While the spectra are obtained they

are, of necessity, in the observer's rest frame. The velocity correction from topocentric

(the frame in which the measurements are usually made) to standard reference frames

(which must be one of those given in Table 8.7) are dependent on the dot product with

time-variable velocity vectors. That is, the velocity with respect to a standard reference

frame depends upon direction, and the velocity (and frequency and wavelength) with

respect to the local standard of rest is a function of the celestial coordinate within the

image. The keywords SPECSYSa and SSYSOBSa are reserved and, if used, must describe

the reference frame in use for the spectral axis coordinate(s) and the spectral reference

frame that was held constant during the observation, respectively. In order to compute

the velocities it is necessary to have the date and time of the observation; the keywords

DATE-AVG and MJD-AVG are reserved for this purpose.

DATE-AVG -- [character; default: none]

Calendar date of the mid-point of the observation, expressed in the same way as

the DATE-OBSkeyword.

MJD-AVG -- [floating-point; default: none]

Modified Julian Date (JD ­ 2,400,000.5) of the mid-point of the observation.

SPECSYSa -- [character; default: none]

The reference frame in use for the spectral axis coordinate(s). Valid values are

given in Table 8.7.

SSYSOBSa -- [character; default: TOPOCENT]

The spectral reference frame that is constant over the range of the non-spectral

world coordinates. Valid values are given in Table 8.7.

FITS Standard