COMPLICATIONS. 1: Displaying the other sideband.

The command CHANGE-SIDEBAND does the following: First, it calculates the telluric centre frequency. Then it adds or subtracts twice the I.F. (depending on whether the observations are in LSB or USB respectively). Finally, it transforms this telluric frequency to the nominal frame of the observation, files it as F_CEN in the scan header, and changes the sense of the frequency step in the spectrometer (F_INC). GSD spectra stored with storage task V6 or earlier do not have the local oscillator frequency stored, so SPECX instead asks you for the current sideband and I.F.; for V7 data or later SPECX can deduce these from the data in the GSD scan header.

Let us take a specific example. We have a RxB3i spectrum which is centred on CS 7-6 (rest freq. 342.883 GHz) in the lower sideband. The nominal I.F. is 1.5 GHz. In order to look at absolute "rest frame" frequencies, we set the display frame to be that of the source itself:

>> set-velocity-frame
Output in different vel frame? (Y/N) [Y] 
Velocity frame? (TELLuric, LSR, HELIocentric, GEOcentric) [TELL] LSR
Velocity law definition? (OPTical, RADio, RELativistic) [RAD] RAD
Velocity in new frame? (km/s) [  20.0] vlsr

(see Fig 1). (There is a predefined variable "vlsr" equated to the actual value for the current spectrum, so we can quote this in response to the last query rather than looking up the actual value.)

For an AOSC spectrum, we would now regrid to a linear scale (see next section for more details on this). Then we go ahead and change the sideband:

>> change-sideband
Local oscillator frequency not defined...
Current sideband? (U/L) [L] L
First i.f.? (GHz) [ 1.500000] 1.5

 --- Header entries changed to other sideband ---
   Don't forget to SET-LINE-REST-FREQ to set the
   frequency of the line you wanted to look at -
   the old line will appear at large velocities!

As suggested, change the reference frequency to CO 3-2:

>> set-line-rest-freq
Receiver # 1  Line rest frequency? (GHz) [  0.000000] 345.795989

If we have done all this correctly we now get the absolute frequency plot shown below (Fig 2). A macro has been provided to do all this - just type IMAGE-FREQ (this calls specx_command:image.spx).