The polarization-based collimated beam combiner and the proposed NOVA fringe tracker (NFT) for the VLTI

The Polarization-Based Collimated Beam Combiner efficiently produces pairwise interference between beams from multiple telescopes. An important feature is achieving "Photometric Symmetry" whereby interference measurements have no first-order sensitivity to wavefront perturbations (or photometric variations following spatial filtering) which otherwise entail visibility measurements with increased error, bias, and nonlinearity in phase determination. Among other proposed applications, this topology has been chosen as the basis for the design of the NOVA Fringe Tracker (NFT), a proposed 4 or 6 telescope second-generation fringe tracker for the VLTI. The NFT takes advantage of the photometric symmetry thus achieved making it capable of tracking on stars resolved beyond the first visibility null, as well as interfering a telescope beam with one which is 20 times brighter, a design goal set by ESO. By not requiring OPD modulation for interferometric detection, the detector exposure time can be increased without performance reduction due to time skew nor is sensitivity reduced by removing optical power for photometric monitoring, and use of two-phase interferometric detection saves one half of the photons being diverted for detection of the other two (mainly) unused quadrature phases. The topology is also proposed for visibility measuring interferometers with configurations proposed for the achievement of balanced quadrature or 3-phase interferometric detection. A laboratory demonstration confirms >>100:1 rejection of photometric crosstalk in a fringe tracking configuration where atmospheric OPD fluctuations were simulated using a hair dryer. Tracking with a 30:1 intensity ratio between the incoming beams was performed while rejecting large introduced photometric fluctuations.