-0.34       0.22            13.08
  ---------------- --------------- -------------- ------------ ------------- -------- ------------------------

\[tab:coord\]

OCRA DATA
=========

Observing Strategy {#sec:obs}
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The clusters were observed in good weather periods between September 2005 and January 2009. We aimed to achieve a uniform noise level $<0.5$mJy, which required $\sim 220$ minutes of data at each pointing centre. Trail fields, blank patches of sky separated from the target fields by 15–20m in RA, were observed over the same range of hour angle after each cluster observation, using the same position switching routine (described below) as for the cluster fields. The integration times for the target and trail fields are identical.

For each observation, whether the cluster or its corresponding trail field, we employ a position switching strategy to remove atmospheric contamination from our data. The feeds are positioned such that one beam, beam $B$, is coincident with the cluster centre and the other, beam $A$, provides a measure of the blank sky signal. (For extended sources, beam $A$ may measure a small signal itself. This must be properly accounted for - see Section \[sec:beta\].) We switch between the $A$ and $B$ beams at a rate of 277kHz, recording the $A-B$ difference every second. We integrate in 