The NOVA Fringe Tracker (NFT) is a proposed solution to the call by ESO for a second generation fringe tracking facility.
This instrument at the VLTI will enable the cophasing of up to 6 telescopes simultaneously. Using broad band optics with
detection from 1.2 to 2.4 microns, a unique configuration is employed that eliminates so-called “photometric crosstalk.”
This refers to imbalance in the beam combiner which results in fluctuations of the incoming wavefronts and the proportion
of power accepted by a spatial filter masquerading as a visibility, a common problem afflicting previous interferometric
instruments and fringe trackers. Also proposed for use in “science instruments” (for the measurement of visibility), the
“Polarization-Based Collimated Beam Combiner,” with its achievement of photometric symmetry in hardware, is particularly
suited for combined use of the smaller AT (1.8 meter) telescopes with the UT (8 meter) telescopes involving a 20:1
intensity ratio of the interfering beams, and also for fringe tracking using highly resolved sources having a very small visibility.
Recent enhancements to the proposed fringe tracker include selectable modes which detect only a single quadrature
phase, both quadrature phases, or an uneven combination of the two. Optimization of partial spatial filtering using pinholes
has been performed using a wavefront simulator and simulated tracking loop. Aiming for an instrument achieving the
best limiting sensitivity, analysis and simulations predict that reliable cophasing will be obtained using the 1.8 meter AT
telescopes tracking on an unresolved reference star with a K magnitude of 10