LGSAO glossary and acronym guide
AO: Adaptive optics!
DAR: Differential Atmospheric 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 mode) to keep the near-IR
image fixed on the science detector.
DM: Deformable Mirror.
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: Daytime calibration routine used to achieve the highest Strehl ratio on the science detector by finding the optimal shape of the deformable mirror. The wavefront sensor centroid offsets for nighttime operation are recorded for this image sharpened deformable mirror shape.
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 science camera 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 the science instrument.
STRAP: System for Tip-tilt Removal with Avalanche Photodiodes. 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.
TBAD: Transponder Based Aircraft Detection system.
TRICK: Tilt Removal with IR compensation at Keck. The near-infrared tip-tilt sensor based on a H2RG detector.
TSS: Tip-tilt Sensor 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 controller.
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 the science instrument).
WFC: Wavefront Controller. The real-time control system.
WFS: Wavefront sensor (observes the LGS during LGSAO