This attribute, together with the RefDec attribute, specifies the FK5
J2000.0 co-ordinates of a reference point on the sky. For one-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 converting between
different standards of rest.
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.
RefRA has a default value of zero.
