The SpecFrame class assumes this velocity correction is spatially
invariant. If a single SpecFrame is used (for instance, as a
component of a CmpFrame) to describe spectral values at different
points on the sky, then it is assumes that the doppler shift at any
spatial position is the same as at the reference position. The
maximum velocity error introduced by this assumption is of the order
of VSIN(FOV), where FOV is the angular field of view, and V is the
relative velocity of the two standards of rest. As an example, when
correcting from the observers rest frame (i.e. the topocentric rest
frame) to the kinematic local standard of rest the maximum value of V
is about 20 km/s, so for 5 arc-minute field of view the maximum velocity
error introduced by the correction will be about 0.03 km/s. As another
example, the maximum error when correcting from the observers rest frame
to the local group is about 5 km/s over a 1 degree field of view.
The RefRA and RefDec attributes are stored internally in radians, but are converted to and from a string for access. The format "hh:mm:ss.ss" is used for RefRA, and "dd:mm:ss.s" is used for RefDec. The methods astSetRefPos and astGetRefPos may be used to access the values of these attributes directly as unformatted values in radians.
The default for RefRA is "0:0:0".
AST A Library for Handling World Coordinate Systems in Astronomy