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Laser Guide Star Adaptive Optics (LGS AO) systems play a fundamental role in correcting atmospheric distortions for astronomical observations. Adaptive Optics requires at least one bright star near the scientific object of interest as a reference for measuring the distorted wavefront. Requirements on brightness and proximity restrict Natural Guide Star (NGS) AO systems to a small fraction (< 10%) of the sky. Since the inclusion of Laser Guide Stars (LGS), the sky coverage has risen dramatically to ~85%. However, the tip-tilt indetermination problem remains unsolved: the tip-tilt uplink component is dominant over the tip-tilt downlink component of the LGS and cannot be distinguished from one another using only the light from the artificial star. This means that the LGS cannot measure tip-tilt modes, preventing 100% sky coverage. NGSs are required to be sufficiently bright, being currently the main limitation for AO systems. This is an issue for targets at the galactic poles where the density of stars is lower, high redshift targets, and for targets of opportunity operations, responding to transient events such as gravitational waves, gamma-ray bursts, and supernova explosions. This presentation explores the intricacies of tip-tilt determination in LGS AO, examining current methodologies, technological advancements, and unresolved issues. We have identified key research directions and potential solutions to enhance tip-tilt estimation in LGS AO systems, ultimately advancing the adaptive optics technology for groundbreaking astronomical observations with 100% sky coverage. |
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