To optimize dark current, sensitivity, and cosmetics, the OmegaCAM detectors need to be operated at a temperature of 155 K. The detector mosaic, with a total area of 630 cm~2 directly facing the dewar entrance window, is exposed to a considerable heat load. This optimization can only be achieved with a high performance cooling system. This paper describes the cooling system, which is built to make the most efficient use of the cooling power of liquid nitrogen. This is done by forcing nitrogen through a series of well designed and strategically distributed heat exchangers. Results and performance of the system recorded during the past months of system testing are presented.
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