ALMA has 66 antennas: fifty-four 12-meter diameter antennas and twelve 7-meter diameter antennas. The radio telescope combines signals from each, working as an interferometer, in other words, a single giant telescope equal in size to the total array.
With perfect parabolic shapes and a precision equivalent to within a fraction of the thickness of a human hair, this is one of the most precise antennas in the world.
Unlike a telescope that is built in place and remains there, these antennas are solid enough to be moved between different pads without damaging their high-precision mechanisms. They are moved with transporter trucks that have been designed especially for these antennas and are able to reposition them to adapt to the required observation needs.
They do not need a dome or cupola to protect them. Although their reflectors are exposed to extreme conditions at an altitude of 5,000 meters above sea level in the Chajnantor Plateau, facing gusty winds and temperatures that vary from 20 to -20 degrees Celsius, and even snow, the antennas are designed to withstand these conditions.
The most visible part of an antenna is the reflector. Each reflector performs the same function as a mirror in an optical telescope: capturing radiation from distant astronomical objects and directing it towards a Front End that measures the levels of that radiation. The distinguishing feature of all telescopes is the wavelength of the radiation absorbed. Visible light captured by optical telescopes, with wavelengths between 380 and 750 nanometers, makes up only a small fraction of the spectrum of electromagnetic radiation. When it has all its receiver bands enabled, ALMA will capture radiation from space in longer wavelengths, from a few hundred micrometers to approximately 1 millimeter (close to one thousand times longer than visible light waves). These wavelengths, as can be expected, are known as millimeter and submillimeter radiation, and can be found in the electromagnetic spectrum in radio waves.
The fact that the ALMA reflectors are metallic panels and not mirrors is due to the wavelength for which they are designed. The reflective surfaces of any telescope must be practically perfect, as any imperfection that is slightly greater than the wavelength it captures will prevent the antenna from obtaining accurate data. The ALMA antennas detect wavelengths that are longer than visible light, with a precision of up to 25 micrometers (much finer that the thickness of a sheet of paper), which means that they do not need mirror reflectors. Therefore, although the ALMA reflectors look like giant satellite receptors, for a photon with submillimetric wavelength (a particle carrying electromagnetic radiation), they are almost perfect and very precise reflective surfaces.
On the one hand, the reflector surfaces are rigorously controlled and on the other, the antennas can be moved and pointed with an angular precision of 0.6 arcseconds (an arcsecond is equal to 1/3,600 parts of a degree), enough to distinguish a golf ball at a distance of 15 kilometers.