A SkyFrame is a specialised form of Frame which describes celestial longitude/latitude coordinate systems. The particular celestial coordinate system to be represented is specified by setting the SkyFrame's System attribute (currently, the default is ICRS) qualified, as necessary, by a mean Equinox value and/or an Epoch.
For each of the supported celestial coordinate systems, a SkyFrame can apply an optional shift of origin to create a coordinate system representing offsets within the celestial coordinate system from some specified point. This offset coordinate system can also be rotated to define new longitude and latitude axes. See attributes SkyRef, SkyRefIs and SkyRefP
All the coordinate values used by a SkyFrame are in radians. These may be formatted in more conventional ways for display by using astFormat.
AST A Library for Handling World Coordinate Systems in Astronomy