May General Meeting
Syllabus Speaker Barry Adcock,
President of the Astronomical Society Victoria
on the subject “Untwinkling the Stars”

Twinkle, twinkle, little star,
How I wonder what you are!
Up above the world so high,
Like a diamond in the sky.
Twinkle, twinkle, little star,
How I wonder what you are!

Richard Dawkins, the renowned biologist / writer and father of the expression the “God Delusion” once wrote a book called Un-weaving the Rainbow. In it tells of the public’s shock reaction to Newton’s discovery of the nature of the rainbow. Interfering in Gods handiwork, they shouted, destroying our faith in God’s biblical promise, taking the wonder out of Nature’s most beautiful mystery. Will the twenty-first century’s ‘Untwinkling the Stars” cause a similar public reaction? Will we no longer be able to sing one of the most popular English nursery rhymes in faithful innocense, or claim that chocolate is better than reason?

Feed back, Barry Adcock said, feedback in a closed loop was the solution to a problem that had plagued earthbound astronomy from its infancy. On a clear night you can see the stars twinkle as their light passes through numerous layers in the Earth atmosphere on its way to us. Minute movements or density changes in the air above us causes the image of a pin-point star to wiggle or twinkle. It has always been so. But when, with the manufacture of larger telescopes, the measurement resolution reached the arc-seconds range, the consistency of air became a major hurdle. Turbulence in the atmosphere causes spatial and temporal anomalies in atmosphere's refractive index. It appeared to be an insurmountable barrier to going any further. Placing the telescope beyond Earth’s atmospheric envelope seemed to be the only answer. The Hubble Space Telescope provided the world with stunning pictures that earthbound telescopes could never hope to equal. Or could they? What if you could compensate for the distortion the light from the stars suffers as it passes through the atmosphere? Surely that would improve atmospheric seeing? Simple enough in principle; a water tap, garden hose and a lawn sprinkler were Barry’s props, and the means of feedback a computer to control the amount of water appropriate for the lawn. Adaptive Optics, as it became known in astronomy, is a technology to improve the performance of optical systems by taking a known pin-point object in the sky as a reference and creating a point image from it by compensating for the optical effects of atmospheric distortion. The measure of correction then becomes the feedback, much like Barry’s lawn sprinkler, but here the system uses a wavefront sensor, a deformable mirror in the optical path, and a computer that receives input from the detectors and sends instruction back to a deformable mirror. The wavefront sensor takes millisecond measurements of the distortion introduced by the atmosphere; the computer calculates the optimal mirror shape to correct for the distortions and the surface of the mirror is reshaped accordingly. While the technique has been theoretically understood for some time, it was only with advances in computer technology and number crunching capacity during the 1990s that the technique became practical. In its simplest form adaptive optics has only one segment mirror which can tip and tilt to re-shape the image. Modern large telescopes have an array of multiple segments which can tip and tilt independently, or have computer controlled material with variable refractive properties.
The reference point in the sky can be any reasonably bright star in the field of the telescope. The brighter the star and the closer it is to the object to be observed the more accurate the distortions can be measured and compensated for. To free the telescope from this restriction (many sections of the sky do not contain a bright star), and to make the measurement results more consistent, an artificial point is nowadays often created in the sky (a Laser Guide Star) by shining a powerful, tightly focussed yellow laser beam in the line of sight of the telescope. The yellow light of the laser is reflected from sodium molecules present in the high mesosphere layer, forming a pin-pint reference source about 90km above the Earth surface. Sometimes Laser beams are pulsed in an effort to ignore scattered light at ground level. One of the problems with Laser guide stars is that because the upward path of the light is virtually the same as the path of the back scattered light, the effects of the atmosphere on the `tilt' of the wavefront virtually cancel out. For best results light from a star is still needed. Barry showed pictures of futuristic telescopes in the making, 25m, 30m, even 100m, all made possible with adaptive optics untwinkling the stars.

As with most novel ideas, the military soon finds a use for it. In the current issue of Scientific American an article heralds the advent of the weapon of the future with the perfection of Adoptive Optics on a 100Kilowatt Laser cannon to keep the beam focussed on its way to the target. Beam me up, Scotty. AK