The first part of this tutorial text discussed many of the standard approaches to image stabilization, focusing on optical correction for optical systems. Image-stabilization technology continues to grow and expand and new devices are constantly evolving. Two devices of particular interest are liquid crystal (LC) spatial light modulators and orthogonal transfer CCDs. These devices are being introduced into systems and provide novel, compact means for image stabilization. Microelectro-mechanical systems (MEMS) are now available for motion and vibration detection and can be used to provide mechanical sensing and compensation in optical systems.
8.2 Liquid Crystal Spatial Light Modulators
The idea of using LC as corrective elements dates back to the early 1980s. However, at that time LC technology was not developed enough to produce usable devices. Things started to change dramatically in the early 1990s, thanks especially to display-technology research and investments. Now, LC devices are available for use in laboratory setups and first telescope demonstrations.
To understand how LCs can be used as phase correctors, consider a very simple, single-element device. The LC material is sandwiched between two glass plates. Spacers maintain the separation of the glass plates. On the glass plates is deposited a thin film of material that is a transparent electrode, usually indium-tin oxide (ITO). The last layer is the alignment layer, used to anchor the molecules as shown in Fig. 8.1. In conventional display technology, the two faceplates, with ITO and alignment films, are mounted perpendicularly from each other. The net result is that the spatial arrangement of the molecules forms a spiral going from one extreme (the first faceplate) to the orthogonal one on the other side. Because of this spiral arrangement, these are called twisted nematic devices. Phase modulation requires untwisted arrangements, where the faceplates are parallel. Normal display technology is inadequate for phase correction applications for other reasons as well. First, the optical quality of the faceplates is not very high (Love et al. 1995). The single elements (pixels) are not controllable individually.