Image Processing Reference
4.4 Use of Tip-Tilt Correction with Laser Guide Stars
Several advanced laser guide star systems use an artificial star projected high in the
atmosphere. These systems eliminate the need for a bright reference star to sample
the atmosphere; however, there is still a need for a reference star. A natural guide
star is still required to stabilize the systems to the star field. This requires only a
low-order image-stabilization system, so a significantly fainter star than is needed
for high-order correction can be used, allowing more of the sky to be imaged. The
configuration of the laser-guide star system is shown in Fig. 4.6.
4.5 Mechanical Operation of Tip-Tilt Stages
Tip-tilt stages, often called steering mirrors, are made by several manufacturers and
come with a wide range of features that improve the mirror's open and closed loop
performance. Mirrors that use actuators based on piezo materials have distinct per-
formance differences as compared to mirrors that use voice-coil or audio-speaker
Piezo actuators are made from plumbum zirconate titanate (PZT), a polycry-
stalline ceramic that changes length with applied voltage. These actuators, fitted to
a platform, provide deflection that can be controlled.
Voice-coil actuators (or, more commonly, audio speakers) are electromagnetic
coils that move in a magnetic field. By applying a current to the electromagnet, the
Figure 4.6 Laser-guide stars require a natural guide star to provide tip-tilt correction.
The artificially generated star is projected above the turbulence layer and must be
close to the position in the sky of the natural star for good correction.