Image Processing Reference
In-Depth Information
52
Chapter 5
5.4 Model Image Stabilizer
Many of the complications in an image-stabilization system are necessary because
the controlling signal (the light from a star) is not very bright and the atmosphere
introduces rapid movement of the star image, requiring very short integration times
on the wavefront sensor to keep up. Astronomical systems are usually referred to as
photon starved; that is, there is very little light to work with and nearly every pho-
ton must reach the imaging system in order to get a satisfactory image. Thus, astro-
nomical systems are often expensive to build and must use state-of-the-art compo-
nents, which usually requires a custom rather than a commercial instrument.
In our model tilt compensator, the challenging conditions of the astronomical
image-stabilization system is avoided by not designing for atmospheric turbulence
and using a laser rather than a telescope and star to drive the system. A moving mir-
ror in the optical path provides a slowly varying change in the position of the fo-
cused spot, to which the image-stabilization system can react.
The next sections introduce each piece of the model system and discusses how
it fits into the image-stabilization system. This image-stabilization system can be
constructed in nearly any college optical lab and can be used to demonstrate the
principles of image-stabilization systems and control.
5.4.1 Light source
The model image-stabilization system uses an artificial light source produced by a
laser beam in plane of a star. The specific laser type selected is not important; a la-
ser diode or gas HeNe laser work equally well. The primary beam of the laser is too
intense for the optical sensor and saturates the detector, making it unusable. To use
a laser source, it is necessary to expand the primary beam and then collimate the
light into a beam. The wavefront of the expanded laser beam does not usually have
a plane wavefront. To obtain a nearly plane wavefront and reduce the intensity of
the laser, the beam is expanded several times the diameter needed to feed the en-
trance lens of the optical system. By expanding the beam such a large amount,
nearly any commercial laser, ranging from a simple laser pointer to a commercial
HeNe laser, provides a usable wavefront. Usually, a HeNe laser wavefront is con-
siderably more uniform than that of a diode laser and would require less expansion
to obtain the same wavefront quality.
A spatial filter provides an alternative approach to expanding the beam beyond
the diameter needed to fill the optical system. The spatial filter is a pinhole on which
the laser light is focused to generate a spherical wavefront. If the laser is bright
enough, the first focusing lens is not needed, and the spatial filter, illuminated by the
primary beam of the laser, is sufficient to generate the nearly plane wave.
The beam-expanding and -collimating system selected for the model image-sta-
bilization system is shown in Fig. 5.2. It makes use of an inexpensive laser-diode
module, a microscope objective, a spatial filter, and a lens to collimate the beam.
The desired wavefront shape from the laser is a plane wave. Most lasers pro-
duce a Gaussian profile, as shown in Fig. 5.3. To obtain a nearly plane wavefront
