In 1610, Galileo used a telescope to discover the four largest moons of Jupiter, and for more than two hundred years the only way to get a better look at the night sky was to build a bigger telescope. In the 19th century, astronomers began hauling telescopes up mountains to reduce the amount of atmospheric interference blurring their observations. By 1968, astronomers achieved the ultimate high-altitude telescope by successfully launching the first space telescope into orbit, the Orbiting Astronomical Observatory 2 (OAO-2).

Scientists working on the OAO missions may have thought that they had finally discovered the best kind of telescope, one high above the distorting air of the atmosphere. But using a new technology called adaptive optics—a deformable mirror system with lasers that can correct for atmospheric interference in real time—astronomers have managed to snap a sharper picture of Neptune with a ground-based telescope than is possible with even the mighty Hubble Space Telescope.

The image of the planet Neptune on the left was taken with an adaptive optics instrument on the ESO’s Very Large Telescope. The image on the right is a comparable image from the NASA/ESA Hubble Space Telescope. Note that the two images were not taken at the same time so do not show identical surface features. ESO/P. Weibacher (AIP)/NASA, ESA, and M.H. Wong and J. Tollefson (UC Berkeley)

The Very Large Telescope (VLT) in the Atacama Desert of Chile, operated by the European Southern Observatory (ESO), has been steadily improving its adaptive optics for almost two years. The GALACSI (Ground Atmospheric Layer Adaptive Corrector for Spectroscopic Imaging) system works with a variety of instruments on the telescope—which is actually four 8.2-meter telescopes—to take observations across a wide range of the electromagnetic spectrum (different wavelengths or colors of light).

The latest upgrade pairs GALACSI with the MUSE (Multi Unit Spectroscopic Explorer) instrument, working with the VLT's Unit Telescope 4. MUSE is capable of imaging the night sky in both wide-field and narrow-field modes. Previously, in wide field mode, GALACSI and MUSE were used to correct for atmospheric interference up to one kilometer above the telescope.

These images of the planet Neptune were obtained during the testing of the Narrow-Field adaptive optics mode of the MUSE/GALACSI instrument on ESO’s Very Large Telescope. ESO/P. Weibacher (AIP)

Now, in the narrow-field mode, GALACSI and MUSE used a new adaptive optics technology called laser tomography to correct for nearly all of the atmospheric interference impeding the telescope, although for a smaller area of the sky. The technology works by shooting four orange lasers up into the sky, each 30 centimeters in diameter, to serve as artificial guide stars. The thick lasers excite sodium atoms in the high atmosphere, revealing the amount of turbulence in the air. The system can then measure the atmospheric interference and use actuators to warp a thin, flexible secondary mirror up to one thousand times per second to correct for the blurred light.

The resulting images are nearly as crisp as if the telescope were in space, and given that the VLT telescopes are more than three times the size of Hubble, the space pictures are even more fantastic. The new adaptive optics technology was used to image Neptune, as well as star clusters and other objects.

These images of the globular star cluster NGC 6388 were obtained during the testing of the Narrow-Field adaptive optics mode of the MUSE instrument on ESO’s Very Large Telescope. The image on the left is from MUSE in Wide Field Mode, without the adaptive optics system in operation and the centre panel is an enlargement of a small part of this view. The image on the right is the view from the Narrow-Field Mode of MUSE when adaptive optics are switched on. ESO/S. Kammann (LJMU)

Adaptive optics was first dreamed up in the 1950s, but it was not until the 1990s that computing advanced enough to make it possible to warp a mirror with the required speed and precision to actually realize the technology. However, what was true for Galileo is still true for astronomers today—if you want to get a better look at the night sky, build a bigger telescope—which is why the ESO is working on a 39-meter monster telescope to dwarf even the VLT: The Extremely Large Telescope (ELT).

When we turn on the ELT, outfitted with adaptive optics technology just like the VLT, who knows what we will see.

The four Laser Guide Stars Facility points to the skies during the first observations using the adaptive optics-equipped MUSE instrument, August 2, 2017. Roland Bacon​

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