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
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
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
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