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9 July 2018 A two-band approach to nλ phase error corrections with LBTI's PHASECam
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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 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.
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E. R. Maier, P. M. Hinz, D. Defrère, S. Ertel, and E. Downey "A two-band approach to nλ phase error corrections with LBTI's PHASECam", Proc. SPIE 10701, Optical and Infrared Interferometry and Imaging VI, 107011M (9 July 2018); https://doi.org/10.1117/12.2313627
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