line parameter. The velocities are given in km/s
, in the radio
or optical definition,
with respect to the rest frame of the dataset.
The main reason for `velocity channels' is that they are corrected for
the Earth's motion -- a particular source
velocity component will remain in the same velocity channel regardless of
whether the observatory Doppler tracks or not.
The velocity specification is given in the form
line=velocity, nchan, start, width, step
or
line=felocity, nchan, start, width, step
where nchan is the number of velocity channels to select, start
is the centre velocity of the first channel selected, width is
the width of each channel, and step is the velocity step between
channels. With the velocity form, the radio velocity definition is
assumed, whereas the felocity form uses the optical definition.
Note, however, both forms produce channels at equal increments in
radio velocity ( even the felocity linetype). See
Section 5.4 for more information.
For example
line=velocity,10,1.5,1.0,3.0
will specify 10 channels, having velocities (radio definition) centered at 1.5, 4.5, 7.5, etc, km/s , with respect to the rest frame. Each channel has a width of 1 km/s .
If the start, width and step values cause the velocity channels not to map directly to the underlying correlator channels, a weighted sum of correlator channels is used to determine a velocity channel. If the magnitude of the velocity channel width (the width value in km/s ) corresponds to correlator channel increment, i.e. if
(where 
is the correlator channel increment) then the weighted sum
is equivalent to linear interpolation.
For telescopes that do not Doppler track, such as the ATCA, it will rarely be the case that velocity channels map directly to correlator channels because of the continual change in the observatory's radial velocity.
There are some caveats when using velocity channels: