LGSAO glossary and acronym guide

AO: Adaptive optics!
DAR: Differential Amtospheric Refraction. Due to the wavelength-dependent refractive index of air, the apparent angle between an object's near-IR and visible images varies with elevation. This effect must be mechanically compensated (by tracking the TSS in LGSAO) to keep the near-IR image fixed on the NIRC2 detector. See paper by H. Roe for more information.
Dither: A coordinated move of the telescope, tip-tilt sensor stage (TSS), M3 laser pointing mirror, and field steering mirrors (FSMs) which moves the target on the NIRC2 focal plane and leaves the LGS pointed at the target.
FSM: Field steering mirror. The set of 2 articulated flat mirrors used to position the LGS or NGS on the WFS.
Image sharpening: Another term for optimizing the WFS centroids to correct for the varying extention of the LGS spot across the telescope pupil.
LBWFS: The low-bandwidth wavefront sensor.
LGS: Laser guide star, which consists of 589 nm light resonantly backscattered by sodium atoms in a column approximately 1.5m diameter and 5-10 km long in the Earth's mesosphere.
LTCS: Laser traffic control system, which monitors the pointing direction of each telescope on Mauna Kea to avoid potential interference between the projection of the Keck II laser and visible-light observations.
NGS: Natural guide star.
Nod: A coordinated move of the telescope and TSS which moves the target on the NIRC2 focal plane while leaving the LGS fixed with respect to the telescope's optical axis.
on-axis: The science object is the tip-tilt star.
PA mode: Position angle mode of the AO image rotator. The sky orientation is kept fixed on NIRC2.
STRAP: A 4-quadrant avalanche photo-diode tip-tilt sensor and controller.
Strehl ratio: The ratio of the peak brightness of a stellar image to that produced by an ideal optical system.
TSS: Tip-tilt positioning stage. An x-y-z stage which translates the STRAP TT controller and LBWFS across the focal plane for TT reference acquisition and positioning control.
TT: Tip and tilt, the angle-of-arrival variations in the light of the science target due to atmospheric turbulence.
TT reference: A star or other astronomical object which provides tip and tilt information to the wavefront controler.
VA mode: Vertical angle mode of the AO image rotator. The pupil orientation is kept fixed on the waverfront sensor (and thus the sky rotates on NIRC2).
WFS: Wavefront sensor (observes the LGS during LGSAO operation).