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The current Boeing/Spectra Technology visible FEL oscillator experiment requires an optical cavity of unusual size. Due to the long narrow gain medium provided by the wiggler and electron beam, a cavity of near concentric geometry is used. The sensitivity of such near concentric FEL cavities to misalignment increases with increasing size and decreasing wavelength. As a result, an actively stabilized set of cavity mirrors is required. In addition, the micropulse temporal structure of an RF linac-driven FEL requires that the absolute length of the cavity be maintained to ensure that the light within the cavity overlaps individual micropulses. This paper describes the 55.4 m cavity with its associated stabilization and control system being used in these experiments.