on timescales of a few days. By performing regular calibration ($\approx$daily) we reduce the effects of the larger fluctuations. In addition, we adopt a conservative approach by comparing the total powers for the NGC7027 and cluster observations and rejecting any data for which they differ significantly, although a study suggests the calibration often stays virtually the same as the total power drifts. Residual uncertainties in the calibration of the system at the level of 5% may be expected.

The telescope pointing is calibrated via azimuth and elevation scans across a bright source close to each cluster. Where an acceptable pointing calibration does not exist within 60minutes of a cluster observation, the cluster data are rejected.

Statistical Data Analysis
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After combination of the second-by-second average data into double–differenced measurements of the brightness of the sky, and calibration, the data are examined for periods of increased noise (which might arise from receiver instabilities or bad weather conditions) or individual anomalous points. The combination of the data into final averages was performed including statistical tests for outlier data. The fractions of the data points rejected by $3\sigma$ or $5\sigma$ cuts were small in all cases, and no cut-dependent changes larger than a 