Fringe Tracking at the MROI

We report on the opto-mechanical design of the MROI fringe tracker. This instrument, currently under development in collaboration with the University of Cambridge, will be a dedicated fringe tracking beam combiner and spectrographs. I will utilize the "Y" geometry of the array to stabilize fringes on shorter "nearest neighbor" baselines, and thus allow for increased integration times on the longer baselines and the buildup of signal to noise. The beam combiner has been designed to accommodate light from a maximum of ten telescopes (three in each array arm, one at the "Y" vertex), but can operate with fewer without having to change the overall layout. A single spectrograph will multiplex up to five nearest neighbor combinations onto a single detector. Identical spectrographs are located at opposite sides of the combiner outputs to simultaneously sample combination pairs that are π radians out of phase with respect to one another.