Many celestial coordinate systems have some additional free parameters which serve to identify a particular coordinate system from amongst a broader class of related coordinate systems. For example, the FK5 (J2010.0) system is distinguished from the FK5 (J2000.0) system by a different equinox--and the coordinates of a fixed astronomical source would have different values when expressed in these two systems.
In AST, these free parameters are represented by additional SkyFrame attributes, each of which has a default appropriate to (i.e. defined by) the setting of the main System attribute. Each of these qualifying attributes may, however, be assigned an explicit value so as to select a particular coordinate system. Note, it is usually best to assign explicit values whenever possible rather than relying on defaults. Attribute should only be left at their default value if you ``don't care'' what value is used. In certain circumstances (particularly, when aligning two Frames), a default value for an attribute may be replaced by the value from another similar Frame. Such value replacement can be prevented by assigning an explicit value to the attribute, rather than simply relying on the default.
The main SkyFrame attributes which qualify the System attribute are:
This attribute is inherited from the Frame class. It gives the moment in time when the coordinates are correct for the astronomical source under study (usually the date of observation).
This value is used to qualify celestial coordinate systems that are notionally based on the Earth's equator and/or the ecliptic (the plane of the Earth's orbit around the Sun). The position of either of these planes is difficult to specify precisely, so in practice a model mean equator and/or ecliptic are used instead. These, together with the point on the sky that defines the coordinate origin (termed the mean equinox) move with time according to some model which smoothes out the more rapid fluctuations. The SkyFrame class supports both the old FK4 model and the newer FK5 one.
Coordinates expressed in any of these systems vary with time due to movement (by definition) of the coordinate system itself, and must therefore be qualified by a moment in time (the epoch of the mean equinox, or ``equinox'' for short) which specifies the position of the model coordinate system on the sky. This is the role of the Equinox attribute.
Note that it is quite valid and common to relate the position of a source to an equinox other than the date of observation. Usually a standard equinox such as J2000.0 is used, meaning that the coordinates are referred to axes defined by where the model mean equator and ecliptic would lie on the sky at the Julian epoch J2000.0.
For further details of these attributes you should consult their descriptions in and for details of the System settings for which they are relevant, see the description of the System attribute (also in ). For the interested reader, an excellent overview of celestial coordinate systems can also be found in the documentation for the SLALIB library (SUN/67).
The value of these qualifying attributes is most conveniently set at the same time as the System value, e.g. when a SkyFrame is created. For instance:
SKYFRAME = AST_SKYFRAME( 'System=Ecliptic, Equinox=J2005.5', STATUS )
would create a SkyFrame representing an ecliptic coordinate system referred to the mean equinox and ecliptic of Julian epoch J2005.5.
Note that it does no harm to assign values to qualifying attributes
which are not relevant to the main System value. Any such values are
stored, but are not used unless the System value is later set so that
they become relevant.
AST A Library for Handling World Coordinate Systems in Astronomy