Implementing multiwavelength fringe tracking for the Large Binocular Telescope Interferometer’s phase sensor, PHASECam

PHASECam is the fringe tracker for the Large Binocular Telescope Interferometer (LBTI). It is a near-infrared camera that is used to measure both tip/tilt and fringe phase variations between the two adaptive optics-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, but only the K-band phase telemetry is used to send corrections to the system in order to maintain fringe coherence and visibility. However, due to the cyclic nature of the fringe phase, only the phase, modulo 360 deg, can be measured. PHASECam’s phase unwrapping algorithm, which attempts to mitigate this issue, occasionally fails in cases of fast, large phase variations or low signal-to-noise ratio. This can cause a fringe jump in which case the optical path difference correction will be incorrect by a wavelength. This can currently be manually corrected by the operator. However, as the LBTI commissions further modes that require robust, active phase control and for which fringe jumps are harder to detect, including multiaxial (Fizeau) interferometry and dual-aperture nonredundant aperture masking interferometry, a more reliable and automated solution is desired. We present a multiwavelength method of fringe jump capture and correction that involves direct comparison between the K-band and H-band phase telemetry. We demonstrate the method utilizing archival PHASECam telemetry, showing it provides a robust, reliable way of detecting fringe jumps that can potentially recover a significant fraction of the data lost to them.