Active optical fringe tracking at the NPOI

The Navy Prototype Optical Lnterferometer, NPOI, is routinely used to measure visibility amplitudes and closure phase for stellar objects at optical wavelengths (e.g. , Benson et al. ,' Hajian et al.2) . In this poster we describe the fringe data collection aspects and the real time algorithm that enables us to actively track fringes with the instrument. For a detailed description of the overall instrument see Armstrong et al. .