## Attributes which Qualify Spectral Coordinate Systems

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:

EpochEpoch
  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). It is needed in order to calculate the Doppler shift produced by the velocity of the observer relative to the centre of the earth, and of the earth relative to the sun.

StdOfRestStdOfRest
  This specifies the rest frame in which the coordinates are correct. Transforming between different standards of rest involves taking account of the Doppler shift introduced by the relative motion of the two standards of rest.

RestFreqRestFreq
  Specifies the frequency which correspond to zero velocity. When setting a value for this attribute, the value may be supplied as a wavelength (including an indication of the units being used, “nm” “Angstrom”, etc.), which will be automatically be converted to a frequency.

RefRARefRA
  Specifies the RA (FK5 J2000) of the source. This is used when converting between standards of rest. It specifies the direction along which the component of the relative velocity of the two standards of rest is taken.

RefDecRefDec
  Specifies the Dec (FK5 J2000) of the source. Used in conjunction with REFRA.

SourceVelSourceVel
  This defines the “source” standard of rest. This is a rest frame which is moving towards the position given by RefRA and RefDec, at a velocity given by SourceVel. The velocity is stored internally as a heliocentric velocity, but can be given in any of the other supported standards of rest.

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.