Image Processing Reference
Image-Stabilization Systems Simplified
Figure 5.9 Information flow diagrams for image stabilization.
tion. This block diagram shows the important first step of calibrating the sensor and
mirror response before entering the main decision loop.
Two control loops, an inner and outer loop, are identified in the flow diagram.
The inner loop provides the rapidly updated mirror position loop. This inner loop is
responsible for taking the information from the sensor and using it to keep the im-
age fixed in the imaging camera. Over time, inaccuracies in the telescope tracking
system can lead to the tilt mirror being driven to one of its extremes, so the available
throw for correction is reduced.
When the tilt mirror begins to reach one of the extremes and can no longer re-
spond effectively, the optical system itself must be repositioned to restore the tilt
mirror to its neutral position. An outer loop, triggered off a soft limit (software
rather than a hardware switch), allows the necessary movement of the whole optical
system. In an astronomical telescope, this is equivalent to having the system repoint
the telescope on the sky.
5.6 Types of Controllers
The most common controller used for image-stabilization systems is known as the
proportional, integral, and derivative (PID) controller. The name describes the pro-
cessing activities within this closed-loop feedback system, which follows an error
signal generated from the difference between the actual and desired location of the