Image Processing Reference
In-Depth Information
Low-Order Wavefront Compensation
43
) required to contain peak angular excur-
sions for up to three standard deviations is
The mechanical angular motion (
α
D
d
α
=±
15
.
σ
.
(4.3)
The ratio
D/d
is the magnification of the system from beam compression of the pri-
mary mirror,
D
, to the corrective beam diameter,
d
.
The most common type of mirror used in image stabilization and beam steering
is the fast-steering or tracking mirror. First used by Babcock in 1956 as part of a
spectrograph at the Mount Wilson Observatory, they kept a star fixed on the jaws of
the slit (Babcock 1956). Most systems in common use today are reflection based and
provide maximum angular travel with low jitter while providing high open-loop ac-
curacy and angular resolution.
4.3.1 Mirror correctors
The most common type of tip-tilt corrector, based on a moving mirror, has been
used in astronomical applications since the late 1950s. These correctors consist of a
high-quality, lightweight mirror mounted onto a tilt stage that can be electrically
controlled. The tilt stages are commonly driven by two to four actuators and can use
either force or displacement actuators.
In the case of a system with two degrees of freedom (DOF), the mirror typically
has a pivot positioned either at the mirror center or such that the pivot and the actua-
tors are 120 deg apart. A system with three DOF has three actuators located 120 deg
apart, while a four-actuator system has the actuators located 90 deg apart. All three
actuator orientations are shown in Fig. 4.3.
Figure 4.3
Three actuator configurations for use in a tilt mirror. The pivot point may be
a real, hard point or may just represent the center of movement, depending on the
mirror design.
