#### Description:

This attribute specifies the frequency corresponding to zero velocity. It is used when converting between between velocity-based coordinate systems and and other coordinate systems (such as frequency, wavelength, energy, etc). The default value is 1.0E5 GHz.

When setting a new value for this attribute, the new value can be supplied either directly as a frequency, or indirectly as a wavelength or energy, in which case the supplied value is converted to a frequency before being stored. The nature of the supplied value is indicated by appending text to the end of the numerical value indicating the units in which the value is supplied. If the units are not specified, then the supplied value is assumed to be a frequency in units of GHz. If the supplied unit is a unit of frequency, the supplied value is assumed to be a frequency in the given units. If the supplied unit is a unit of length, the supplied value is assumed to be a (vacuum) wavelength. If the supplied unit is a unit of energy, the supplied value is assumed to be an energy. For instance, the following strings all result in a rest frequency of around 1.4E14 Hz being used: `"` 1.4E5`"` , `"` 1.4E14 Hz`"` , `"` 1.4E14 s∗∗-1`"` , `"` 1.4E5 GHz`"` , `"` 2.14E-6 m`"` , `"` 21400 Angstrom`"` , `"` 9.28E-20 J`"` , `"` 9.28E-13 erg`"` , `"` 0.58 eV`"` , etc.

When getting the value of this attribute, the returned value is always a frequency in units of GHz.

Type:

  Floating point.

#### Applicability

##### SpecFrame

All SpecFrames have this attribute.
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