PHASECam is the Large Binocular Telescope Interferometer's (LBTI) phase
sensor, a near-infrared camera which is used to measure tip/tilt and phase
variations between the two AO-corrected apertures of the Large Binocular
Telescope (LBT). Tip/tilt and phase sensing are currently performed in the H
(1.65 μm) and K (2.2 μm) bands at 1 kHz, and the K band phase telemetry
is used to send tip/tilt and Optical Path Difference (OPD) corrections to the
system. However, phase variations outside the range [-π, π] are not
sensed, and thus are not fully corrected during closed-loop operation.
PHASECam's phase unwrapping algorithm, which attempts to mitigate this issue,
still occasionally fails in the case of fast, large phase variations. This can
cause a fringe jump, in which case the unwrapped phase will be incorrect by a
wavelength or more. This can currently be manually corrected by the observer,
but this is inefficient. A more reliable and automated solution is desired,
especially as the LBTI begins to commission further modes which require robust,
active phase control, including controlled multi-axial (Fizeau) interferometry
and dual-aperture non-redundant aperture masking interferometry. We present a
multi-wavelength method of fringe jump capture and correction which involves
direct comparison between the K band and currently unused H band phase
telemetry.Comment: 17 pages, 10 figure