A Clearer Picture from the Stars
Since its invention over four centuries ago, the astronomical telescope has changed dramatically. No longer simply a small tube with a series of lenses to magnify the heavens; the modern day telescope has evolved to a large and sophisticated computer-controlled instrument with full digital output ‘read’ by powerful computers.
Until recently, the astronomical telescope has essentially remained a ‘passive’ instrument, without any in-built corrective devices to improve the quality of star images during observations.
One technique that was developed for overcoming atmospheric blurring is called ‘speckle interferometry’, in which hundreds of very short exposures (‘specklegrams’) are later analysed to reconstruct the unblurred image. However, because the specklegrams must be short exposures and at the same time have good signal-to-noise, speckle interferometry is limited to imaging very bright objects. And furthermore, results can only be seen following a lengthy reconstruction process.
Although it was thought that atmospheric distortions could not be avoided, mechanical improvements have been made to minimise telescope errors. Improvements in the construction of the mirrors, and stiffer structures used to minimise gravitationally-induced deformations have also been introduced. Low-expansion glass is now used to avoid mirror distortions as temperature varies; and to reduce local temperature effects, heat dissipation from motors and electronic equipment has been improved, and the observatory dome, which in addition shields the telescope from the effects of wind buffeting, is cooled during the day. In such properly designed and well-manufactured medium size telescopes, image quality is limited mainly by atmospheric distortions.
The problem came as plans were developed in the 1980s to enhance light-collecting power by building telescopes with primary mirrors well above 4m in diameter. It soon became clear that conventional methods of maintaining image quality were ruled out by cost and structure weight limitations. As a result, a new technique called ‘Active Optics’ (AO) was developed for medium or large telescopes, with image quality optimised automatically by means of constant adjustments by in-built corrective optical elements.
Adaptive optics leads to appreciably sharper images with an additional gain in contrast, and for astronomy, where light levels are often very low, this means fainter objects can be detected and studied.
All AO systems work by determining the shape of the distorted wavefront from the stars, and using an ‘adaptive’ optical element - usually a deformable mirror - to restore the uniform wavefront by applying an opposite cancelling distortion.
The most basic systems use a point source of light, such as a bright star, as a reference beacon, whose light is used to probe the shape of the stars’ wavefronts. Light from this reference source is analysed by a wavefront sensor, and then commands are sent to actuators which change the surface of a deformable mirror to provide the necessary compensations. For the system to work well, it must respond to wavefront changes while they are still small; for the earth's atmosphere, this can mean updating the mirror's shape several hundred times a second.
Deformable mirrors and their control is one of the areas being tackled by experts at BAE Systems in America. These mirrors use an array of small devices that push and pull a glass or polished metal surface into the required shape. One of the latest designs has a gold coated mirror approximately 90 mm in diameter which can be reshaped up to 1000 times every second in order to correct for even the fastest movement of the atmosphere.
Adaptive Optics is also finding applications on a more human scale such as in eye surgery. Here, just like the atmosphere, the eye’s internal fluid is constantly in motion, and a surgeon looking into a patient's eye gets a blurred image of the retina. Using AO a much clearer image of the retina can be obtained, enabling major improvements in eye surgery. (It has even been suggested that, in the longer term, AO could be used to endow people with supernormal vision.)
AO is effectively the only option for studying living retinal tissue and a full adaptive optics system can compensate for micro-fluctuations in eye muscles, which means that the eye does not have to be temporarily paralysed while under examination.
BAE Systems have interests in many areas which could benefit from AO, including laser and imaging systems. If, for example, a laser can be more efficiently focussed onto a target, then a smaller device can be used, leading to size, energy and cost savings. BAE Systems' strategy is to develop AO systems which are compact and relatively inexpensive, but which nevertheless provide a useful improvement in performance.