5.1.8 Plotting the spectrum of raw data

Figure 5.2: The raw amplitudes for a point-cal at 5 GHz as a function of channel, for 10 integrations averaged together. Note channels 1, 2 and 31 at the edges of the bandpass have poor sensitivity. For bandpass calibration all channels should be retained but for imaging channels 3-30 would be selected.

The visibility amplitudes or phases of the raw data can be plotted as a function of channel using the program splot. A sample plot is shown in Fig. 5.1.8. This is useful to:

• help decide which channels to omit at the band edges in continuum data;
• help decide the bandwidth needed when planning spectral line observations;
• identify time-dependent rfi or other problems, especially affecting the point-cal or flux-cal where this must be avoided at the earliest stages;
• identify the signal in spectral-line data if the target is strong enough, especially important if the pipeline is to be run or to check data quality if no phase-cal has been observed.

 % splot
   splot>dev /xw        ! Open window for plot
   splot>df 99apr09.d:1 ! Open data file
   splot>data a         ! Plot amplitudes
   splot>type bas       ! Plot baseline
   splot>tel ca         !      between CA
   splot>bas mk         !              and MK
   splot>nav 10         ! Average every 10 integrations
   splot>chan 5 250 1   ! Plot channels 5 to 250
   splot>pol ll         ! Select polarization
   splot>source TARGET  ! Select source
   splot>go             ! Start plot       
    #w                  ! Plot data for first 10 integrations on target
    #a                  ! Add averaged data for next 10 integrations
    #r                  ! (or) redraw plot for next 10 integrations
    #q                  ! Exit from plot
   splot>quit           ! Exit from program

In some cases autocorrelation data is also recorded which can be plotted using type auto. If bas or pol are left blank then all baselines to tel or both polarizations are plotted respectively. The maxium number of integrations which can be plotted at any one time is equivalent to the scan length, e.g nav 15 for 8 s integrations in 2 min scans. If splot repeatedly prompts you q...quit try decreasing nav.