M.A. Holdaway , F.N. Owen , Darrel T. Emerson 
September 20, 1995
Keywords: total power, continuum, atmospheric emission fluctuations, phase errors, observing modes, fast switching, on the fly observing, OTF, slew rate
We use the Chajnantor, Chile phase monitor data to determine the total power error made in subtracting the atmospheric emission for beam switched and On-The-Fly (OTF) continuum observations. When the error in atmospheric removal is less than the system noise per switching interval, the total power observations will not be limited by errors in atmospheric subtraction. If the errors in the atmospheric removal are larger than the system noise per switching interval, they will dominate the total power noise but should average down if a systematic atmospheric trend is removed by double beam switching or by OTF scanning.
On the Chajnantor site, switching on time scales of 1 s will usually increase the continuum total power noise over the optimum value by about 50%. If switching can be done on time scales of 0.2 s, the atmosphere will only rarely limit the noise. Using the ``fast switching'' capabilities which appear to be required for phase calibration, it should be possible to switch the primary by a few arcminutes on 1 s time scales. However, with slew speeds of about one degree per second, On-The-Fly (OTF) continuum single dish observations of sources up to a few degrees across will usually not be limited by atmospheric subtraction.
These calculations indicate that a nutating subreflector may not be required for the MMA if fast OTF mapping can be used for measuring the total power. Scanning faster than about 1 degree per second will not improve the atmospheric subtraction since the errors will then be dominated by the differing paths through the atmosphere at opposite ends of the extended target source. In order to accommodate 1 degree per second scanning, the correlator will need to record integration times as short as 0.003 s, which may be possible for total power data alone, but will likely not be possible for the visibility data. This will require that the total power and interferometric data be taken separately for many continuum observations.
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