This attribute, together with the RefDec attribute, specifies the FK5 J2000.0 coordinates of a reference point on the sky. For 1-dimensional spectra, this should normally be the position of the source. For spectral data with spatial coverage (spectral cubes, etc), this should be close to centre of the spatial coverage. It is used to define the correction for Doppler shift to be applied when using the AST_FINDFRAME or AST_CONVERT method to convert between different standards of rest.

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 V$\ast$SIN(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 AST_SETREFPOS and AST_GETREFPOS may be used to access the value of these attributes directly as unformatted values in radians.

The default for RefRA is " 0:0:0" .

SpecFrame