this position for 25 seconds. The telescope then slews to reposition the beams such that beam $A$ measures the cluster and beam $B$ measures the sky background, and the differencing is repeated, this time integrating for 50 seconds. We then return to the starting position and integrate for another 25 seconds. The differenced signals from the first and third positions, $(A_1-B_1)$ and $(A_3-B_3)$ are summed, and then subtracted from the differenced signal from the second position $(A_2-B_2)$. This recovers twice the cluster signal relative to the background regions. The symmetric nature of the switching pattern ensures proper subtraction of most atmospheric emission and other contaminants such as ground pick-up, provided that the target region is not rapidly changing in azimuth and elevation.

Calibration
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The data are calibrated against an internal noise source, which is itself calibrated via observations of the well–known bright radio source NGC7027 of flux density 5.37Jy. We observe significant changes in the level of the noise source in the form of: (a) sudden ‘jumps’, sometimes due to the telescope reconfiguration and (b) additional smaller amplitude fluctuations which appear random in nature. We note that other than the low–amplitude random fluctuations, the voltage level is generally stable 