Important General Considerations

Generally SSB is Better for Spectral Lines

For spectral line observations it's generally better to tune for SSB mode - the calibration doesn't depend upon the sideband gain ratio and T(sys) is generally lower because the image sideband termination is colder than the sky/ambient termination in DSB mode. In SSB T(Rx) will be roughly twice the DSB value, but it's T(sys) that reflects the sensitivity.

Only One Channel for Continuum Observations

Because there is only one lock-in amplifier for continuum observations only one mixer can be used at a time for continuum pointing, focusing and beam mapping even though you may be tuned in dual polarity mode. The mixer is selected by the total power switch on the IF2 unit in the control room. The output of mixer A is on channel 1 and that of mixer B on channel 2. There is typically about an arcsecond squint in pointing between the two channels. Generally it is better to choose Mixer B for all continuum observations, since the second-stage (HEMT) amplifier has a broader passband. If you see or know that one channel is less stable than the other use the channel with better stability for pointing etc. If you are tuned in single polarity mode, be sure that the Channel selected on the IF2 unit is the one in use. Otherwise you'll see no continuum signal (the other channel's mixer will be turned off).

DSB for Continuum Observations

For continuum observations, when sensitivity is a factor, it is generally better to observe in DSB mode because the signal is then present in both sidebands. To change from SSB to DSB mode and back is quick - just issue a new fe] command, changing only the sideband mode. It takes only a few seconds to adjust the DBI and junction currents.