While Mappings (2.1) represent the relationships between coordinate systems in AST, the coordinate systems themselves are represented by Objects called Frames (see Figure below).
![]() |
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. Functions 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 5b,c), represents celestial coordinate systems, the SpecFrameSpecFrame represents spectral coordinate systems, and the TimeFrameTimeFrame represents time coordinate systems. All these 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 (2.3), 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.
![]() |