Image Processing Reference
In-Depth Information
90
Chapter 8
transfer CCD is good to moderate astronomical seeing provided up to a 20% im-
provement in the image diameter of a star.
Orthogonal transfer CCDs have been successfully used on astronomical tele-
scopes for long-exposure images. Tonry reports that on integration times of 100 s
with a stabilization bandwidth of 100 Hz, only about 3% of the light was found to
be in the star image halo. This is after approximately 10,000 corrections to the
image position.
The modification of the gate structure of the device lowers its overall quantum ef-
ficiency. This also raises the question of whether such devices will be usable for
high-precision photometric measurements. This issue was addressed by Howell et al.
(2003), who determined in a series of experiments at the University of Hawaii's 2.2-m
telescope that orthogonal transfer CCDs could be used for precision photometry.
8.4 Microelectromechanical Systems
When using an optical system, it is common to think about image stabilization in
terms of keeping the light fixed on the camera. However, in many circumstances,
the dominant blurring effect in an optical system is mechanical motion or system
vibration. This is particularly true of optical systems used as part of a moving plat-
form or operating near vibration sources. This mechanical vibration can be com-
pensated for in the optical system by sensing the optical movement, but it can also
be compensated by directly sensing mechanical motion and using it to drive an op-
tical corrector. This is particularly valuable in low-light situations, as incoming
light can go directly to the camera rather than being shared with a sensor.
The idea of compensating for mechanical-vibration-induced blurring has been
successfully used in a number of commercial systems, perhaps the most popular be-
ing image-stabilizing binoculars and cameras. This has proven an excellent ap-
proach for developing higher-magnification optical instruments than could nor-
mally be used as hand-held or on a moving platform. Using a mechanical system to
remove predominant mechanically induced blurring has the additional advantage
of lowering the bandwidth of an optically driven stabilization system.
Vibration measurements are conveniently made using an accelerometer, which
measures acceleration and tilt. The effect of mechanical vibration is to introduce
both acceleration and tilt. MEMS accelerometers usually use a cantilever spring
that converts force to a displacement that can be measured as a change in capaci-
tance, as seen in Fig. 8.6. The advantage to MEMS devices is that they are small,
lightweight, and have been incorporated into a number of commercial applications,
so they are relatively inexpensive.
In optical telescope system applications, two accelerometers are mounted onto
the optical tube, since the optical tube movement causes the image blurring and
must be sensed. The sensors are mounted orthogonal to each other to provide mea-
surements in the two axes of motion. The output of the accelerometers is processed
electronically and fed into a two-axis tilt mirror to provide correction (figure) to the
camera. The result is an image with the vibration blurring removed.
