s-l-r-f, s-v-f and ch-sid

 So, if I put the correct velocity in place using s-v-f:

 >> s-v-f
 Output in different vel frame? (Y/N) [N] y
 Velocity frame? (TELLuric, LSR, HELIocentric, GEOcentric) [ LSR] 
 Velocity law definition? (OPTical, RADio, RELativistic) [RAD] 
 Velocity in new frame? (km/s) [  7.00] 
 >>

and then changing the x-axis to frequency using set-x we get the result following in Fig. :

 

From these data the two HDO lines at 310.5333 (upper x axis; lower sideband) and 313.7506 GHz (lower x axis; upper sideband) are clearly seen to be present. Of course, the spectrum is reversed now because of the change in axis coordinate, but it makes line identification a lot easier. A subsequent observation at a shifted velocity confirmed this result on this occasion.

Just to show that this all works as expected, we can use a combination of setting the correct line rest frequency (with s-l-r-f) and adopting the other sideband (using ch-sid as appropriate) to display the two lines of interest to good advantage. In this I anticipate the questions SPECX will ask. Then for the line in the upper sideband at 313.7506 GHz:

 >> s-l-r-f 313.7506
 >> s-v-f \y\lsr\rad\7.0\ 
 >> n
 
 Plot opened; sequence no. 024
 
 Warning ** Rest frequency set using values from SET-LINE-REST-FREQ.
 Do S-L-R-F. to use defaults from header
 
 -- sxgdevice --/xwindow         SPECXDIR:SPECX_PGPLOT.PS
 >>

Note the warning SPECX issues, to let you know that you have modified the frequency/velocity scale. If you want to revert to the default settings you need to type s-l-r-f 0.0. Taking a narrower velocity range the plot looks like that in Fig. :

 

Here we see that our line is centered at about 0 km/s, because we have referred the scale to an LSR velocity of 7 km/s, the actual velocity of this source.

Once again, note that although the correct LSR velocity was chosen when the observations were made, as seen in the `header' below the plot, this is largely irrelevant to this discussion. We can choose to concentrate on any line by a correct choice of rest frequency, sideband and velocity.

For an equivalent display of the lower sideband line we need to change the sideband also:

 >> s-l-r-f 310.5333 
 >> s-v-f \y\lsr\rad\7.0\
 >> ch-sid
 
 Warning ** Rest frequency set using values from SET-LINE-REST-FREQ.
 Do S-L-R-F. to use defaults from header
 
 Sector  1: First I.F. = -1.608541 GHz
 
  --- Header entries changed to other sideband ---
      Note that f_rest still refers to frequency  
      used as reference in velocity transformation:
      You should not normally need to change this.
 
 >> s-p-sc
 Do you want automatic scaling of X-axis? (Y/N) [Y] n
 X-axis scale: Beginning and end? [ -50.00   50.00] 
 Do you want automatic scaling of Y-axis? (Y/N) [N] 
 Y-axis scale: Beginning and end? [   2.00   10.00] 
 >>

Note that ch-sid turns on the default x-axis scaling, and we have to reset this. Then we get the plot in Fig. , containing the other HDO line, again centered around 0 km/s:

 

Sometimes, because it's fairly difficult to get everything the way one might want it, it can be useful to collect the commands together in a procedure, which can be edited and re-run until you get the result you want. For example, the following procedure plot three lines which happen to appear in the same spectrum on a common velocity axis. Two of the lines are in the lower sideband, and one in the upper sideband.

!o-fil tests
rea-sp 1 1
s-l-r-f 362.6303
ch-sid
s-v-f\y\lsr\rad\0\
s-p-sc\n\-10 30\n\-2 15\
n
!
rea-sp 1 1
s-l-r-f 362.7359
ch-sid
s-v-f\y\lsr\rad\0\
s-p-sc\n\-10 30\n\-2 15\
off 5
over 1 3
!
rea-sp 1 1
s-l-r-f 365.3634
s-v-f\y\lsr\rad\0\
s-p-sc\n\-10 30\n\-2 15\
off 10
over 1 3
!
cl-pl
!the end

The result of this procedure is shown in Figure .