A very important parameter that should be carefully set is the Representative Frequency (RF). This is done by selecting one of the spectral windows to be the Representative Window and this automatically sets the RF to the centre frequency, as measured in the rest frame of the source. Using a rest frequency is important as this is common to all sources, even if they have different velocities. For actual application, the rest frequencies are converted to their sky equivalents using the source velocity. If desired, the RF can be changed to another rest frequency within the spectral window.
This parameter is important because it is used when determining the time required to achieve the requested sensitivity. If the spectral window frequencies correspond to relatively uniform atmospheric transmission, this being especially the case in bands 3, 4 and 6, the exact choice of the RF is usually not crucial. However, where the transmission is changing rapidly, which spectral window is chosen to set the RF can make a huge difference to how much time the OT will calculate.
The Representative Frequency should therefore usually be placed at the centre of the line that one is most interested in, and the requested sensitivity usually also applies to the RF. This is not the case for continuum projects because the sensitivity is usually being measured over multiple spectral windows. Here, one should probably select the spectral window with the poorest atmospheric transmission, but continuum measurements are usually made at frequencies where the transmission curve is fairly flat.
The Representative Frequency also sets the antenna beamsize and therefore the spacing parameter for mosaics (Section 5.3.2.5). For this reason, with all other things being equal it can be a good idea to use the highest-frequency spectral window for setting the RF, and single continuum setups default to this. If the lowest were used, the sampling of the spectral windows at higher frequencies (smaller beam) would be less than that requested.