A two-band approach to n$\lambda$ phase error corrections with LBTI's PHASECam

TL;DR

This paper introduces a two-band phase correction method for LBTI's PHASECam, improving the robustness of phase error correction by comparing H and K band telemetry to prevent fringe jumps during interferometry.

Contribution

The paper presents a novel multi-wavelength approach for phase error correction that enhances the reliability of fringe jump detection and correction in LBTI's PHASECam system.

Findings

Effective detection of fringe jumps using H and K band comparison

Improved phase correction stability during fast, large phase variations

Potential for automated, robust phase control in advanced interferometry modes

Abstract

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 $\mu$m) and K (2.2 $\mu$m) 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 [-$\pi$, $\pi$] 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.