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4.1. OCRA

OCRA (Browne 2000) was conceived as a 100-element cm-wave radiometer array suitable for mounting at the prime or secondary focus of a large single-dish telescope. A two-beam prototype (OCRA-p) is now operational on the Torun 32-m antenna, with a larger array (OCRA-F) in the final stages of completion. The sensitivity of the full OCRA is such that it could map 100 deg2 of sky to the confusion limit in a few months, and should generate a sample of clusters of galaxies at all redshifts where they have significant atmospheres, by virtue of the redshift-independence of the brightness temperature change caused by SZ effects.