Translated material
Telescope in a sieve
It was proposed to make a lunar telescope mirror from liquid salt
Someday in the future, perhaps not so distant, people will build a base on the Moon - manned or automatic. Even the Japanese, whose space program is still moving in very leisurely steps, have similar plans, not to mention the Americans and Chinese. And if a lunar base is created, then an optical telescope will probably be built there - one cannot miss such a unique opportunity. And it is possible that the main mirror of this telescope will be liquid and will be delivered into orbit in an ordinary barrel or canister. Canadian optics are trying to develop such a system for the Ames Research Center, part of NASA.A classic reflecting telescope uses a large concave parabolic mirror to concentrate light rays at one point. This reflector is most often made of special polished glass or some other similar material. And the larger the diameter of the mirror, the better the results, so powerful telescopes have very respectable dimensions. However, delivering such a structure into space is extremely difficult: you have to somehow manage to fit a huge mirror on a standard launch vehicle, which is not always possible. Compromises have to be made: for example, the James Webb infrared telescope, which is planned to be sent into low-Earth orbit in 2013, will apparently be folded several times and deployed in airless space. However, an optical telescope mirror with this approach is unlikely to give astronomers the required accuracy.
Ermanno Borra and his colleagues from Quebec's Université Laval proposed that NASA make a mirror for a lunar telescope liquid. In principle, there is nothing revolutionary in this; similar mirrors have already been designed, built and used. For example, the University of British Columbia telescope uses a reflector with a diameter of 6 meters ( pictured ). The liquid (usually mercury, which is highly reflective) is poured into a wide, round container with low walls, after which it begins to rotate. Under the influence of centrifugal force, the contents of this “basin” are collected closer to the walls, forming a parabolic surface, and the mirror is ready for use.
However, not all terrestrial know-how can be easily transferred to the Moon. You can’t make a mirror there from the usual mercury: this metal freezes at 39 degrees below zero, and on the Moon the temperature can drop to minus 150. This means that you need to look for a replacement: “If you want to get a liquid mirror for a telescope on the Moon, you need the right liquid . And if you don’t have such a liquid, give it up,” Borra said. As a “correct” material, the Canadian proposed using “ionic salts” - very unusual compounds that can not freeze even at very low temperatures.
The second difficulty is evaporation. The liquid on the mirror must somehow be protected from evaporation, otherwise its molecules will soon scatter throughout the cislunar space. In principle, ionic salts are not very volatile anyway, but to guarantee this, engineers propose covering the liquid with a kind of double-layer insulation. The first layer is expected to consist of tiny particles of chromium, the second - the outer layer - of silver dust. This amalgam should not only retain liquid on the surface of the mirror, but also improve the reflectivity of this salt “pancake”. With the latter, by the way, opticians have encountered certain problems: ionic salts are not yet very good as an optical mirror.
Interestingly, scientists suggest pouring liquid salt literally into a sieve. The bottom of the moon mirror container will be riddled with small holes. However, as the developers assure, this is not scary: surface tension forces will be able to hold the liquid inside the container, and not a drop of ionic salt will spill onto the lunar soil.
As NASA suggests, a liquid mirror in lunar conditions will make it possible to create an optical telescope of truly cyclopean dimensions - with an aperture from 20 to 100 meters. This, combined with the airless space, should ensure the highest image quality. "Telescopes like these, perhaps up to 100 meters in diameter, could see the early phases of the universe after the Big Bang," said Ames Center director Pete Worden. In this case, apparently, what is meant is the ability of such instruments to capture the light of galaxies extremely distant from us. But this miracle of technology will not be able to be sent to the Moon tomorrow or the day after tomorrow - not earlier than 2020, as engineers believe. So far, the construction of lunar bases is only in the long-term plans of comic agencies. And in the coming years, astronomers will have to limit themselves to ground-based telescopes and space laboratories.