New Focus 8783 GuideStar, Руководство пользователя

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4
System Operation
4.1
System Overview
GuideStar II is a closed-loop beam stabilization system in which cameras measure
beam position at two points along the beam path, while two preceding mirrors
actuated by Picomotors keep these two positions steady. The centroid information
from each camera is processed and fed back to the X and Y control of each mirror
mount in a generalized manner via a 16-element array (called the “Motion
Matrix”). This approach accounts for all direct- and cross- couplings between the
centroid position and the mirror axes, allowing for nominally perfect control of
the beam with arbitrary optical systems. An automated calibration routine
provides the needed stimulus to each motor to determine the Motion Matrix
values.
The control loop functions by making adjustments to the mirrors only when the
beam has drifted out of a user-defined target area on one or both of the camera
sensors. When this happens, the beam is moved back inside the target area on
both sensors. To provide control over how often the beam is moved, an Inner
Target and an Outer Target are defined for the sensors. When the beam drifts
outside of one or both of the Outer Targets, it is moved back inside the smaller
Inner Target on both sensors. Then it is not moved again until it drifts outside of
one or both of the Outer Targets. Suitable choice of the target sizes can result in
relatively infrequent motor activity to maintain beam pointing and position within
a user-defined range. Thus the beam can be left largely unperturbed.
Dynamic performance of the control loop is determined by several things. Two
settings in particular allow one to control the dynamic behavior – the
Loop Gain
and Frames to Average Over . These provide the flexibility to optimize the loop
response.
The Model 8784 cameras are shipped with an installed lens and neutral density
filters. The lens provides beam size reduction, allowing ~1-cm beams to be used
with the 5.3 mm sensors. The cameras’ OD-1,-2, and -4 filters provide coarse
optical attenuation, while the exposure time setting of the cameras allows for fine
exposure control over a range of roughly 1 – 1000 ms.
In addition, operation with low (<500 Hz) repetition rate lasers is possible with
the Model 8783-T Trigger Kit.