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arXiv · 1807.04786

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

E. R. Maier·P. M. Hinz·D. Defrère·S. Ertel·E. Downey
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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.

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E. R. Maier, P. M. Hinz, D. Defrère, S. Ertel, E. Downey. 2018-07-12. A two-band approach to n$\lambda$ phase error corrections with LBTI's PHASECam. https://doi.org/10.1117/12.2313627

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