Modifying the Velocity and Frequency Axes

 There are occasions when it is helpful to change the x-axis on a spectrum. This is particularly true for observations made with offset frequencies designed to accommodate two or more spectral lines from the same or different sidebands. There are four commands which are very useful in this respect:

1.

set-x enables one to change the x-axis scale;

2.

with set-line-rest-frequency it is possible to modify the plot so that the spectrum appears as if this frequency had been chosen as the rest frequency

3.

set-velocity-frame lets one enter a velocity to which the spectrum will be referred in subsequent plots;

4.

change-sideband shows the spectrum as if it were referred to the other sideband. Use of this command frees the x axis plot scale; to return it to the original setting you will have to undo this effect with s-p-sc.

The effects of these commands tend to cause confusion sometimes, so it may be useful to discuss them a little more. The last three can best be discussed together. One important thing to realise is that the header values of the spectrum are not changed by any of these commands, just the relative positioning of the spectrum in velocity/frequency space for the purpose of plotting. Some parameters, but not all, are carried over into the headers of stored reduced spectra and map files.