Celestial coordinate system description
’
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 reference point. This offset coordinate system can also be rotated to define new longitude and latitude axes. See attributes SkyRef, SkyRefIs, SkyRefP and AlignOffset.
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. For a SkyFrame, the Unit attribute describes
the formatted value of a SkyFrame axis, and may for instance be "
h:m:s"
, indicating that a formatted
axis value contains colon-separated fields for hours, minutes and seconds. On the other hand, the
InternalUnit attribute for a SkyFrame is always set to "
rad"
(i.e. radians), indicating that the
unformatted (i.e. floating point) axis values used by application code are always in units of radians
AlignOffset: Align SkyFrames using the offset coordinate system?
AsTime(axis): Format celestial coordinates as times?
Equinox: Epoch of the mean equinox
IsLatAxis: Is the specified axis the latitude axis?
IsLonAxis: Is the specified axis the longitude axis?
LatAxis: Index of the latitude axis
LonAxis: Index of the longitude axis
NegLon: Display longitude values in the range [-pi,pi]?
Projection: Sky projection description.
SkyRef: Position defining location of the offset coordinate system
SkyRefIs: Selects the nature of the offset coordinate system
SkyRefP: Position defining orientation of the offset coordinate system
SkyTol: Smallest significant shift in sky coordinates
AST_SKYOFFSETMAP: Obtain a Mapping from absolute to offset coordinates