Towards controlled Fizeau observations with the Large Binocular Telescope

The Large Binocular Telescope Interferometer (LBTI) can perform Fizeau interferometry in the focal plane, which accesses spatial information out to the LBT's full 22.7-m edge-to-edge baseline. This mode has previously been used to obtain science data, but has been limited to observations where the optical path difference (OPD) between the two beams is not controlled, resulting in unstable fringes on the science detectors. To maximize the science return, we are endeavoring to stabilize the OPD and tip-tilt variations and make the LBTI Fizeau mode optimized and routine. Here we outline the optical configuration of LBTI's Fizeau mode and our strategy for commissioning this observing mode.
Comment: 15 pages, 7 figures, to appear in the Proceedings of the SPIE (SPIE Astronomical Telescopes + Instrumentation 2018; Optical and Infrared Interferometry and Imaging VI)