Images showing Neptune captured using the new adaptive optics mode, and another with it switched off
One of the greatest problems faced by conventional ground-based telescopes is the disturbance caused by the particles that populate our planet’s dense atmosphere, which disrupt and scatter the light emitted by distant bodies such as planets and massive galaxies. This turbulent effect, known as astronomical seeing, is the reason that stars seem to twinkle in the night sky.
Scientists and engineers are able to mitigate some of the disruptive mixing of Earth’s atmosphere with the help of advanced instrumentation and software, known collectively as adaptive optics.
The ESO has just released the first images taken using the new mode of its Ground Atmospheric Layer Adaptive Corrector for Spectroscopic Imaging (GALACSI) adaptive optics module. GALACSI, which works in conjunction with the Multi-unit Spectroscopic Explorer (MUSE) instrument, is mounted on the fourth unit of the VLT, known as YEPUN.
GALACSI had previously been able to work with the MUSE spectrograph to obtain wide field-of-view observations of the cosmos that were relatively free from atmospheric interference. The new mode, which makes use of a technique called laser tomography, allows GALACSI to work with MUSE in its Narrow-Field setting, in which it can capture a more detailed image of a smaller patch of sky. The narrow-field mode is much better suited to capture sharp images of planets, black holes, or any other of a host of celestial objects than the wide-field equivalent.
Here’s how it works – four 30 cm (12 inch)-wide, 22-watt laser beams are projected into the sky above the Paranal Observatory, stimulating sodium atoms high in the atmosphere, which essentially act like persistent artificial stars, revealing turbulence at varying altitudes. GALACSI observes disturbances in the light, and calculates corrections roughly one thousand times per second. These corrections are used to constantly manipulate the shape of the VLT unit’s flexible secondary mirror, which rests on a mount holding 1,170 actuators.
By shifting the mirror to account for disturbances in the atmosphere as they occur, the telescope is able to capture images significantly sharper than those taken by the Hubble Space Telescope at visible wavelengths. The images captured in the first light observations include a shot of Neptune, a globular star cluster and other cosmic objects.
(For the balance of this article, plus a very interesting video presentation, see: https://newatlas.com/eso-vlt-adaptive-optics-first-light-neptune/55517/)