the atmosphere and the ground, and since the noises are not correlated the covariance matrix for this component is diagonal. Experimental error in the SZ signal also arises from the presence of intrinsic fluctuations in the CMB that obscure the cluster. Since these fluctuations are inherent in the sky brightness, they are correlated in the visibilities, and so result in a non-diagonal covariance matrix. The uncertainty due to the CMB is largest on the shortest CBI2 baselines and dominates the error in the SZ signal at $l \la 1000 $ [see e.g. @Udomprasert:2004].

The remaining principal source of experimental error is the presence of bright radio point sources, especially near to the centre of the field, where there will be less attenuation from the primary beam and more confusion with components of the SZ signal. This error can be treated either by identifying the point sources in the long-baseline data or by simultaneously observing with higher resolution instruments. If one is confident that source variability is not an issue, then the source fluxes can be subtracted directly from the data and any residual flux errors incorporated into the covariance matrix. Alternatively, contamination due to point sources can be treated during 