Looking at the sky with telescopes sitting on the Earth is like looking through a somewhat primitive and dirty window. That hasn’t stopped astronomers from wanting and sometimes getting bigger and better optical telescopes. Even a somewhat distorted window on the universe is far better than human eyesight. Then along came rockets and eventually it was possible to put telescopes in space – the Hubble Space Telescope being the most famous example. That hasn’t stopped optical engineers from continuing to gnaw away on the problems of earth-based telescopes. Their efforts have not been in vain. A new breed of telescope with adaptive optics has been steadily growing mature and more powerful. A new kind of optics based on liquid mirrors is getting started, but shows equal promise.
The measurement of the promise is wrapped up in a concept unfamiliar to all but a few: the Strehl Ratio. It is the unit of measurement for the perfection of image quality with 100% being perfection. The Strehl Ratio of traditional ground-based optics has been around 1%. The application of adaptive optics changes the Strehl Ratio dramatically. Instead of using a single fixed mirror (or lens), many small mirrors are used together, controlled by computer so that their surfaces can be shaped and directed – deformed, as needed to overcome the optical distortions of the atmosphere. The electronics and especially the software algorithms are tricky, but by using special sensors that measure the atmospheric distortions, passing them to a computer that correlates the distortion with the movement of the many mirrors for compensation, adaptive optical systems have achieved Strehl Ratios between 30 and 50 percent.
In mid-June 2010 a new generation of adaptive optics presented a convincing demonstration. Based at the University of Arizona Steward Observatory (USA) and built with the collaboration of the Arcetri Observatory (Italy), and the Max Planck Institute (Germany) the Large Binocular Telescope (LBT) delivered an image quality three times sharper than the Hubble Space Telescope – and that was with using just one of the LBT’s twin 8.4 meter mirrors. With both mirrors working the image should be ten times sharper than that of the Hubble. The Strehl Ratios of the LBT have been in the range of 60-80%. The cost of the LBT was around $120 million. The Hubble has Strehl Ratios of 80-90% and cost $2.5 billion just to build it (i.e. it cost another 6-8 billion to launch, fix, and re-fix it).
LBT project members were delighted with the results, perhaps even more than they expected…
“This is an incredibly exciting time as this new adaptive optics system allows us to achieve our potential as the world’s most powerful optical telescope,” said Richard Green, Director of the LBT. “The successful results show that the next generation of astronomy has arrived, while providing a glimpse of the awesome potential the LBT will be capable of for years to come.”
Another approach to telescope mirrors comes from the notion of liquid mirrors, the simplest form being the element mercury spun in a bowl so that it uniformly coats the interior like a concave mirror. The problem with mercury, however, is when the bowl is anything but flat on the ground; the mercury wants to run out of the bowl. There are other ways to do liquid mirrors. A research team under Denis Brousseau at Laval University (Quebec, Canada) is developing a ferromagnetic liquid technique that combines low-cost with some of the capability of adaptive optics. They have made a tiny proof of principle mirror (5 cm in size) that uses a honeycomb of 91 actuators to deform the iron-like particles (which are coated with a highly reflective surface material) so that they can compensate for waveform (atmospheric) distortion. This liquid mirror system can be controlled by the same type of software as used for adaptive optic system. Obviously there is a way to go before this approach can demonstrate that it can be scaled to mirror sizes comparable to the current standard glass or adaptive optics systems, but if this can be done, the price will be a fraction of those systems (no expensive mirrors to grind).
Keep in mind that while adaptive optics and eventually liquid mirrors will provide much less expensive and high quality telescopic results, they cannot overcome some of the problems inherent in Earth-based astronomy: The day/night cycle, atmospheric glow, spectrum disruption in the atmosphere (a.k.a. atmospheric keyholes), and particulate interference (not all of which can be compensated). Telescopes in space have little or none of these problems; they do have one Big Problem – the cost of installation and maintenance.
As is often the case in the very small and the very distant, humanity’s ‘vision’ is often limited by the power of scientific instruments. The questions we ask of the universe, especially cosmology, tend to be limited by what we can observe – or in some cases like dark matter, what we fail to observe. The advent of powerful new optical technology has almost always led to new discoveries but also new mysteries – new questions. Problems aside, it may be fair to say that at least compared to the previous century this may be a golden age of astronomical telescopy with the power and diversity of scopes achieving views and insights that were once the dreams of theorists, if they were dreamt at all.