Peering through the fog
to interact with the upper atmosphere – is measured by a wavefront sensor to calculate the turbulent distortion, 11 and the data is fed to a software-controlled deformable mirror, which rapidly reshapes itself to counteract the error. This technology, which was theorized long before it was made possible by modern computer systems, 12 has already proven itself as capable and is now relatively commonly used. Currently, the Very Large Telescope (VLT) – specifically the Yepun telescope – features the largest adaptive mirror, which is 1.1m in diameter and 1.96mm thick and deformed by 1170 MEMS actuators. 13 However, the ESA’s Extremely Large Telescope (ELT) is set to surpass this with a mirror of diameter 2.4m levitated by 5000 magnets. It is planned to be completed in 2027. 14
Figure 4 - First direct images of a
Figure 5 - Neptune photographed by the VLT
planetary family - used adaptive optics
Atmospheric extinction Observations are further complicated by the fact that not all light is able to get through the atmosphere in the first place. Much of it is either absorbed or scattered by a plethora of atmospheric molecules such as water, carbon dioxide, diatomic oxygen or ozone. 15 The combination of these effects lead to a constriction of the sections of the electromagnetic spectrum in which observation of extraterrestrial sources are possible – the optical window and the radio window. 16 The atmosphere is opaque for UV light below 30onm due to the ozone layer; infrared (IR) light is absorbed by water vapour and oxygen (it is important to note that this effect is not absolute at all IR wavelengths) and finally the ionosphere is opaque to low-frequency radio waves. This is a severe hindrance, since these wavelengths contain useful information about the structure of the universe, but unfortunately there is very little that a ground-based observatory can do to mitigate these effects. Remote telescope placements can help: deserts are often picked for their lack of atmospheric moisture, and high-altitude observatories are 11 Cowen, R. ‘Getting a Clear View’, Science News 157.10: 156. 12 Babcock, H. (1953) ‘The possibility of compensating astronomical seeing’, Astronomical Society of the Pacific 65: 386 13 ESO. ‘Cutting-edge Adaptive Optics Facility Sees First Light’, https://www.eso.org/public/news/eso1724/. Consulted: 26/08/2025. 14 ESO. ‘M4 Mirror’, https://elt.eso.org/mirror/M4/ Consulted: 26/08/2025. 15 Rowan-Robinson, M. (2003) Cosmology. Oxford. 16 NASA. ‘Introduction to the electromagnetic spectrum’, https://science.nasa.gov/ems/01_intro/. Consulted: 26/08/2025.
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