set-x

SPECX's default state on startup is to make all spectrum plots with the x-axis as velocity. This may not always be what you want for clarity, and the command set-x is provided to enable one to plot with a different x-axis. There are three main choices: points, frequency, and velocity. The `points' option is useful for determining which channels to lose from the spectrum if you are planning to make a map.

For instance, the spectrum below (Fig. ) is typical of those obtained toward OMC-1. Within the band are two lines in which I happened to be interested at the time, neither of which will appear at the expected velocity, because the observing frequency was chosen to allow both lines to appear in the band, and a non-standard DAS mode was used for the observation.

Figure: The result of observing with an offset frequency in a line-rich source.

The two lines (of HDO) are at about $-50$ and $-145$ km/s respectively, but it is rather messy to work this out at altitude. It is actually more useful to work in frequency in this case. So, to change from velocity scale to a frequency scale, use set-x and the following exchange occurs:

 >> set-x
 
 Set units for X-scale:
 Key:    1      Points scale
         2      Frequency scale
         3      Velocity scale
         4      User defined scale
 
    Current units are km/s  
 
 Key?  [3] 2
 Apply polynomial correction to frequency scale? (Y/N) [N] 
 Absolute or relative frequencies? (A/R) [R] a
 
 X-scale units set - GHz

Note that there is more to this command than just choosing an x-axis. One must also choose the origin, effectively, in this case via the absolute/relative frequency switch. If I had chosen `relative' I would have been asked ``relative to what?''.

In this example I chose to turn off the polynomial frequency correction; having this on is useful only for non-linear scales such as that produced by the AOSC. The DAS scale is quite linear by definition. The use of absolute frequency scales enables me easily to see what frequencies my lines have. As shown below in Fig. , this provides both upper and lower sideband frequency scales, on the bottom and top x axes respectively. However, these scales will be correct only if one puts in the correct peculiar velocity for your source; otherwise a velocity of 0 km/s is assumed. Thus if the lines have an appreciable peculiar velocity they will appear to have the incorrect frequency. The velocity is specified by using s-v-f. This therefore brings us to the next section.