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Astronomers want to plant telescopes on the Moon. The lunar surface offers advantages for infrared and radio astronomy, despite the challenges. By Ramin Skibba, Inside Science | Published: Tuesday, January 19, 2021 Astronomers want to plant telescopes on the Moon | Astronomy.com (See the link there to a journal special issue exploring the idea.) I speculated about the possibility of detecting exo-civilizations optically in this Kerbal forum post: How large a space telescope do we need to see exo-civilizations? - Science & Spaceflight - Kerbal Space Program Forums In the discussion in that thread, someone suggested we would need a telescope 1.6 km across to see a visible disk of an Earth-sized planet at the nearest stars. But we might not need to be able to resolve a visible disk to be able to observe illumination of the exoplanet beyond that which would be expected on its nightside. In any case quite large telescopes could be made on the Moon if you used rotating liquid mirrors. This proposal is for one 100 meters across: Texans Want to Put a Big Ol' Liquid Mirror Telescope on the Moon But can they do it? And why even try? BY TIM CHILDERS NOV 18, 2020 https://www.popularmechanics.com/space/moon-mars/a34714863/liquid-mirror-telescope-on-the-moon/ On Earth there are limits to the size you can make a liquid scope because the rotating mirror surface and containment vessel creates wind currents that distorts the liquid mirror surface. But this would not be a problem on the airless Moon. So that raises a question: is there a limit on the size you can make such a mirror on the Moon? Another possibility would be to do the detection through radio telescopes on the Moon. The advantage of radio telescopes is they don't have to have a solid surface but can consist of a set of grid wires, as was done with the Arecibo telescope. And this is the approach taken for one plan for a radio telescope on the Moon: Apr 7, 2020 Lunar Crater Radio Telescope (LCRT) on the Far-Side of the Moon. Saptarshi Bandyopadhyay NASA Jet Propulsion Laboratory https://www.nasa.gov/directorates/spacetech/niac/2020_Phase_I_Phase_II/lunar_crater_radio_telescope/ So how big would a radio telescope have to be on the Moon to detect Earth-like radio emissions from a near-by star like Alpha Centauri? Note this is a different question than that studied for example by SETI. With SETI they assumed such a civilization was beaming radio emissions directed at us. Such searches have been negative. But in the scenario I'm considering, an advanced civilization is creating omnidirectional radio emissions just as a byproduct of conducting its advanced civilization. How large a radio telescope would we need to detect those? Robert Clark