Many spectral 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 velocity systems are all parameterised by a rest frequency—the frequency which defines zero velocity, and all coordinate systems are qualified by a `standard of rest” which indicates the rest frame to which the values refer.
In AST, these free parameters are represented by additional SpecFrameSpecFrame attributes, each of which has a default appropriate to (i.e. defined by) the setting of the main SystemSystem 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 FrameFrame. Such value replacement can be prevented by assigning an explicit value to the attribute, rather than simply relying on the default.
The main SpecFrame attributes which qualify the System attribute are:
For further details of these attributes you should consult their descriptions in Appendix C and for details of the System settings for which they are relevant, see the description of the System attribute (also in Appendix C).
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.