If an integer axis index is included in the attribute name (e.g. "SkyRef(1)") then the attribute value should be supplied as a single floating point axis value, in radians, when setting a value for the attribute, and will be returned in the same form when getting the value of the attribute. In this case the integer axis index should be "1" or "2" (the values to use for longitude and latitude axes are given by the LonAxis and LatAxis attributes).
If no axis index is included in the attribute name (e.g. "SkyRef") then the attribute value should be supplied as a character string containing two formatted axis values (an axis 1 value followed by a comma, followed by an axis 2 value). The same form will be used when getting the value of the attribute.
The default values for SkyRef are zero longitude and zero latitude.
AST A Library for Handling World Coordinate Systems in Astronomy