similar projects and reported in the literature. The list of 58 contaminating sources is presented in Table \[table:sources\].

GBT observations
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We observed the 58 sources over the course of several sessions at the Green Bank Telescope (GBT) in January 2008 (observations limited due to poor weather conditions), then January and May 2009 (re-scheduled time, again compromised in part by non-ideal conditions). We used the GBT Ka–band receiver and the Caltech Continuum Backend (CCB). For a detailed description of this configuration of equipment and the observing methods used, see [@Mason2009]; we will describe the salient points.

### Calibration

Every observing session included at least one flux calibration observation, using the most accessible of the GBT standard calibrators 3C 147, 3C 48 or 3C 286. We observed sources cluster by cluster, and thus they tend to lie in small regions. The GBT pointing is stable for up to 40minutes for night time observing, but we rarely required re-pointing near a field because slews tended to occur after only a few source integrations. We chose one pointing calibrator per cluster (and hence its associated sources). We took pointing calibrators from the GBT database, choosing the brightest available within 10degrees of the cluster 