Under a blanket of snow

How long can you stand outside looking at the Universe when it snows? No need to try it out, because you won’t be able to compete with the ALMA antennas in this Picture of the Week. At the Chajnantor plateau in Chile, ESO operates the Atacama Large Millimeter/submillimeter Array (ALMA), together with its international partners. Temperatures here can drop as low as -20°C on winter nights, but ALMA’s radio antennas are usually unbothered by these chilling conditions.

Compared to optical telescopes, radio antennas are pretty tough. Their gray metallic reflectors are exposed to biting winds and fluctuating temperatures. For a telescope’s mirror or dish to properly reflect the light it collects, the imperfections on its surface need to be smaller than the wavelength of the light. Optical telescope mirrors need to be perfectly smooth because they observe short wavelengths, like visible light, with wavelengths shorter than one micron. But ALMA observes at longer millimeter wavelengths, so sturdy metallic panels do the trick.

ALMA antennas can also work around the clock, because radio telescopes don’t need dark skies to capture signals from space. Day in, day out. From the snow here on Earth, the antennas are peering into the cold, hidden places of the cosmos.