#### Description:

This attribute specifies the central position of interest in a dual sideband spectrum. Its sole use is to determine the local oscillator frequency (the frequency which marks the boundary between the lower and upper sidebands). See the description of the IF (intermediate frequency) attribute for details of how the local oscillator frequency is calculated. The sideband containing this central position is referred to as the `"` observed`"` sideband, and the other sideband as the `"` image`"` sideband.

The value is accessed as a position in the spectral system represented by the SpecFrame attributes inherited by this class, but is stored internally as topocentric frequency. Thus, if the System attribute of the DSBSpecFrame is set to `"` VRAD`"` , the Unit attribute set to `"` m/s`"` and the StdOfRest attribute set to `"` LSRK`"` , then values for the DSBCentre attribute should be supplied as radio velocity in units of `"` m/s`"` relative to the kinematic LSR (alternative units may be used by appending a suitable units string to the end of the value). This value is then converted to topocentric frequency and stored. If (say) the Unit attribute is subsequently changed to `"` km/s`"` before retrieving the current value of the DSBCentre attribute, the stored topocentric frequency will be converted back to LSRK radio velocity, this time in units of `"` km/s`"` , before being returned.

The default value for this attribute is 30 GHz.

Type:

  Floating point.

#### Applicability

##### DSBSpecFrame

All DSBSpecFrames have this attribute.

#### Note

- The attributes which define the transformation to or from topocentric frequency should be assigned their correct values before accessing this attribute. These potentially include System, Unit, StdOfRest, ObsLon, ObsLat, ObsAlt, Epoch, RefRA, RefDec and RestFreq.

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