Image Processing Reference
motion associated with tracking errors of the telescope mount. Correcting for
mount movement is often referred to as autoguiding. In general, the more band-
width needed for the correction, the more challenging the system is to develop.
The image-stabilization system for an astronomical telescope is usually de-
signed to use as little light as possible, so it is common for the system to reside in the
converging beam of the telescope, between the telescope exit port and the imaging
camera. This removes the need for additional lenses or reflective surfaces to
collimate and refocus the beam. The overall layout of an image-stabilization sys-
tem for use in an astronomical telescope is shown in Fig. 5.1.
Figure 5.1 also shows that the effect of introducing an image-stabilization sys-
tem on the light path is to displace the optical path by the amount needed for the
tip-tilt mirror and beamsplitter. The beamsplitter is used to share the light between
the optical sensor and the imaging camera. By placing the imaging camera after the
reflection from the beamsplitter, the overall effect is that the imaging camera is
shifted off the telescope's optical axis by several centimeters. In this version, the
optical sensor sees the same image as the imaging camera and so is sensitive to any
motion of the spot. Any movement of the position of the star, whether caused by
tracking errors from the telescope or from the atmosphere, is seen and converted
into an electrical signal that is monitored. It is also possible to magnify the image
sent to the sensor in order to increase its sensitivity to motion at the expense of
some light reaching the sensor.
Figure 5.1 An image-stabilization system for use on an astronomical telescope. The
light from the telescope is reflected off a tip-tilt mirror and shared between the imag-
ing camera and the optical sensor. Movement of the focused spot on the optical
sensor is detected and the tip-tilt mirror adjusted to keep the spot fixed in the imager.