The verse “Twinkle, twinkle little star, how I wonder what you are” could, in fact, refer to the frustration felt by astronomers trying to view celestial objects obscured by turbulence in the earth’s atmosphere. It’s that turbulence that causes stars and other heavenly bodies to twinkle, and it’s one of the reasons that space-based telescopes like the Hubble can see those objects more clearly than telescopes down here on the ground. Recently, however, a team of astronomers from the University of Arizona developed a technique that allows them to effectively turn off the twinkling over a large field of view, allowing them to get Hubble-quality images in a fraction of the usual time.
The majority of atmospheric turbulence occurs less than half a mile above the ground, due to heat waves rising from the earth. The UA team’s system utilizes five green lasers, that are shot into the air above a telescope. A computer notes how the five points of laser light are affected by turbulence, then correspondingly adjusts the telescope’s flexible adaptive mirror – a thousand times a second, 336 magnetic actuators on the back of the mirror individually warp its surface to compensate for the turbulence-caused twinkling.
Because the five lasers are shot across a wide area, the amount of twinkle-free sky is much larger than that managed by previous adaptive mirror systems. The trade-off is that with a wider field of view comes lower resolution. For many applications, such as the observation of very old red-shift galaxies, that trade-off is worth it.
“To understand the evolution of those ancient galaxies, we have to observe thousands of them and study their spectral characteristics and chemical composition," said study leader Michael Hart . "Taking a spectrum of a high red-shift galaxy takes a long time because they are so faint... with our new adaptive optics technique, you can now observe dozens at a time. Sampling thousands of galaxies' spectra becomes feasible."
The Laser Adaptive Optics system is currently in use on the 6.5 meter telescope at Arizona’s MMT Observatory, although it is expected to be implemented into much larger telescopes, such as the gargantuan 24.5 meter Giant Magellan under construction in Chile.
The astronomers’ research was published this week in the journal Nature.