Abstract: The Gemini Adaptive Optics System, under construction at the Dominium Astrophysical Observatory of the National Research Council of Canada's Herzberg Institute of Astrophysics is unique among AO systems. Altair is designed with its deformable mirror (DM) conjugate to high altitude. This concept is only practical at an observatory where extraordinary measures have been taken to reduce local seeing degradation. We summarize these measures. We then describe Altair. Both the wavefront sensor foreoptics and control system are unconventional, because the guide star footprint on an altitude-conjugated DM moves as the guide star position varies. During a typical nodding sequence, where the telescope moves 10 arcseconds between exposures, this footprint moves by half an actuator and/or WFS lenslet. The advantages of altitude conjugation include increased isoplanatic patch size, which improves sky coverage, and improved uniformity of the corrected field. Although the initial installation of Altair will use natural guide stars, it will include features to use a laser guide star with minimal rework. Altitude conjugation also reduces focal anisoplanatism with laser beacons. The infrastructure of Gemini observatory provides a variety of wavefront sensor and nested control loops that together permit some unique design concepts of Altair. !7
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