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 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.