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. :

Figure: Figure  after changing to a frequency x-axis and providing SPECX with the correct lsr velocity.

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. :

Figure: Figure  after changing to a frequency x-axis and providing SPECX with the correct lsr velocity, and frequency in the upper sideband.

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:

Figure: Figure  after changing to a frequency x-axis and providing SPECX with the correct lsr velocity, and the frequency in the lower sideband.

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.⁸

!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 .

Figure: Bottom through top: lines of HNC (4-3; 362.3603 GHz), H$_2$CO (5$_{05}$-4$_{04}$; 362.7359 GHz) and H$_2$CO (5$_{23}$-4$_{22}$; 365.3634 GHz) observed toward NGC2071IR; all three lines appear in the same spectrum by virtue of the choice of receiver tuning frequency and IF. The first two lines are in the lower sideband, and the third line comes from the upper sideband. The latter is a fairly high-excitation line (57 cm$^{-1}$) and thus is quite weak. Using a combination of s-l-r-f and ch-sid as shown in the text it is possible to plot all three lines on a common velocity scale.