While Mappings () represent the
relationships between coordinate systems in AST, the coordinate
systems themselves are represented by Objects called Frames (see
Figure below).
A Frame is similar in concept to the frame you might draw around a graph. It contains information about the labels which appear on the axes, the axis units, a title, knowledge of how to format the coordinate values on each axis, etc. An AST Frame is not, however, restricted to two dimensions and may have any number of axes.
Figure: (a) A basic Frame is used to represent a Cartesian coordinate system, here 2-dimensional. (b) A SkyFrame represents a (spherical) celestial coordinate system. (c) The axis order of any Frame may be permuted to match the coordinate space it describes.
A basic Frame may be used to represent a Cartesian coordinate system by setting values for its attributes (all AST Objects have values associated with them called attributes, which may be set and enquired). Usually, this would involve setting appropriate axis labels and units, for example. Routines are provided for use with Frames to perform operations such as formatting coordinate values as text, calculating distances between points, interchanging axes, etc.
There are several more specialised forms of Frame, which provide the
additional functionality required when handling coordinates within some
specific physical domain. This ranges from tasks such as formatting axis
values, to complex tasks such as determining the transformation between
any pair of related coordinate systems. For instance, the SkyFrame
(Figure b,c), represents celestial coordinate systems,
and the SpecFrame represents spectral coordinate systems. Both provide a
wide range of different systems for describing positions within their
associated physical domain, and these may be selected by setting appropriate
attributes.
As with compound Mappings (), it is possible
to merge two Frames together to form a compound Frame, or CmpFrame, in
which both sets of axes are combined. One could, for example, have
celestial coordinates on two axes and an unrelated coordinate
(wavelength, perhaps) on a third (see Figure below). Knowledge of the
relationships between the axes is preserved internally by the process
of constructing the CmpFrame which represents them.
Further reading: For a more complete description of Frames see
AST A Library for Handling World Coordinate Systems in Astronomy