Progress on Altair: The Gemini North Adaptive Optics System

The Gemini Adaptive Optics System, (Altair), under construction at 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. We summarize construction progress. We then describe Altair in more detail. 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. Altitude conjugation also reduces focal anisoplanatism with laser beacons. Although the initial installation of Altair will use natural guide stars, it will be fully ready to use a laser guide star (LGS). The infrastructure of Gemini observatory provides a variety of wavefront sensors and nested control loops that together permit some unique design concepts for Altair.