Stop the twinkling

For most of us, a clear, dark sky filled with twinkling stars is one of the most beautiful sights in nature.

Astronomers have a more nuanced view.

The turbulent atmosphere that makes the stars twinkle can degrade or obliterate astronomical observations. We see the sky through a turbulent flow of bubbles of air with different temperatures, humidities and pressures, blowing past on the wind.

The problem arises in the troposphere, the lowest few kilometres of atmosphere.

Imagine light radiated by a distant star or other astronomical object. The light expands into space as a smooth sphere, expanding at the speed of light. It is a three-dimensional version of the expanding ripples we get when we drop a stone into a pond.

This sphere, also referred to as a wavefront, is caught by the telescope’s objective lens and mirror, and formed into an image. However, when passing through the troposphere, that smooth wavefront gets corrupted.

The speed light travels in air depends on its temperature, density and humidity. So the part of the wavefront about to be captured by the telescope is corrupted by some parts being slowed down more than others due to having passed through different bubbles of air.

The result is that when they are captured by the telescope, the waves on the different parts of the wavefront are no longer in step. They arrive at different times, and the result is a distorted image that flickers and shakes as the bubbles of turbulent air blow past in front of the telescope.

Since atmospheric conditions vary from day to day, so does the degree of distortion and twinkling. Astronomers refer to this as “seeing.” When “seeing” is good, the atmosphere is steady and we can see fine detail on distant objects.

When the atmosphere is turbulent the “seeing” is bad and there may be no point in observing at all.
One way to reduce the problem is to put telescopes on high mountains, above the most troublesome part of the atmosphere.

The best solution is to put telescopes in space, completely above the atmosphere and its challenges.

However, space telescopes are expensive, take years to build and are hard to upgrade and not easy to adapt them quickly to work on new discoveries. These cannot be predicted, so high-speed adaptability is essential.

Hence, despite the value of space telescopes, we continue to complement them with advanced instruments on the ground. Luckily, we now have developed a technique that can make images from ground-based telescopes sometimes as good, often nearly as good as the best images made using telescopes in space.

It is known as “adaptive optics."

Basically, we analyze the wavefront distortions as they happen, and then, using a flexible mirror, flexed by a large number of computer-controlled actuators, we correct the distortions, restoring the wavefront to its original condition, giving us a good image.

This might sound like magic, but the principle is simple, although making it happen has taken years.

Most astronomical images contain stars, and we know that even through the largest telescopes, the stars are still just dots, or due to the limitations of the telescope, tiny round discs.

Therefore, a computer can adjust that “rubber mirror” until the star images are right, in which case we can assume the rest of the image is right too. If there is no convenient star for this image-correction process, we shine a laser into the sky to make a dot in the sky several kilometres up, and use that as our image reference star.

This technique has provided a new lease of life for many ground-based telescopes. It also means astronomers can enjoy twinkling starry skies without complaining.
Venus remains prominent in the west after sunset.

• Jupiter, Mars and Saturn are clustered low in the southeast before dawn
• Mercury lies very low in the dawn glow.
• The moon will be new on the 24th. 

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.