Could a telescope be vibrated with the right frequency to counter the atmospheric twinkle of stars?
Robert Byerly
Windsor, California
Yes, it can! This is one facet of adaptive optics, which astronomical observatories can use to compensate for distortions in astronomical images caused by our atmosphere.
Let’s set the stage first: Stars twinkle because on Earth, we are observing them through air. Air is a fluid, just like water, and its turbulent motions distort incoming light, much like the way objects at the bottom of a pool or a pond appear distorted when you peer down at them.
So, adaptive optics are designed to do just that: adapt to the changes in the atmosphere to essentially “undo” the turbulence and create sharp, undistorted images. Professional observatories do this not by vibrating the telescope, but by subtly deforming the surface of the telescope’s mirror in exactly the right way to correct or “straighten out” the jumbled, distorted images they receive through the atmosphere.
Deformable mirrors are built with many tiny actuators, which push or pull on the mirror’s surface, spread across the backside of the mirror — typically the secondary. A specialized computer system that incorporates a device called a wavefront sensor then detects the “shape” of incoming light waves and determines in real time how to deform the mirror to compensate. This is often done using a laser guide star, a strong laser beam shot into the sky that provides a “perfect” example star whose distortion is measured so that the system can correct for it. Essentially, the system knows what a laser spot on the sky should look like, so it measures the ways in which the observed spot differs and calculates how to change the mirror’s surface so it will reflect a perfect spot, as expected. Adaptive optics systems generally run at about 1,000 hertz, meaning measurements and corrections are made about 1,000 times a second.
The secondary can also be tilted or rotated to make additional adjustments as well — this is a more limited correction than deforming the mirror (though more like the vibrations you suggest) but is still often used in tandem with the deformable mirror for additional image correction.
Adaptive optics systems are now available for use on many major professional telescopes, though there are, as always, limitations. The region of the telescope’s field of view that can be corrected is usually much smaller than the entire field it is capable of seeing, although there are newer, more complex techniques that use multiple lasers or mirrors to improve this. These systems also historically operate in optical wavelengths, though their use in infrared observations is improving and increasing.
Additionally, unless a telescope can generate its own laser guide star, astronomers must rely on a bright field star near the object they want to observe to use as a guide, which is not always available. Observatories that do use laser guide stars typically have aircraft detection systems built in to ensure that the laser never crosses the path of any planes flying overhead; if the safety system is not working or there is an aircraft passing by, the laser guide star cannot be used.
Alison Klesman
Senior Editor
