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 AST_FORMAT.

OPTIONS = CHARACTER $\ast$ ( $\ast$ ) (Given)

STATUS = INTEGER (Given and Returned)

AST_SKYFRAME = INTEGER

FRAME = AST_SKYFRAME( ’’ , STATUS )

FRAME = AST_SKYFRAME( ’ System = FK5, Equinox = J2005, Digits = 10’ , STATUS )

FRAME = AST_SKYFRAME( ’ System = FK4, Equinox = 1955-SEP-2’ , STATUS )

FRAME = AST_SKYFRAME( ’ System = GAPPT, Epoch = ’ // DATE, STATUS )

• Currently, the default celestial coordinate system is ICRS. However, this default may change in future as new astrometric standards evolve. The intention is to track the most modern appropriate standard. For this reason, you should use the default only if this is what you intend (and can tolerate any associated slight change in behaviour with future versions of this function). If you intend to use the ICRS system indefinitely, then you should specify it explicitly using an OPTIONS value of " System=ICRS" .

• Whichever celestial coordinate system is represented, it will have two axes. The first of these will be the longitude axis and the second will be the latitude axis. This order can be changed using AST_PERMAXES if required.

• When conversion between two SkyFrames is requested (as when supplying SkyFrames AST_CONVERT), account will be taken of the nature of the celestial coordinate systems they represent, together with any qualifying mean Equinox or Epoch values, etc. The AlignSystem attribute will also be taken into account. The results will therefore fully reflect the relationship between positions on the sky measured in the two systems.

• A null Object pointer (AST__NULL) will be returned if this function is invoked with STATUS set to an error value, or if it should fail for any reason.