Objects in the sky appear to rise and set as the Earth spins beneath them. To collect enough interesting light from these distant objects, ALMA telescopes need to watch them for a long time. That means these telescopes need to slew, which means they track the moving objects smoothly for hours across the arc of the sky.
Gracefully slewing and suddenly stopping with a 115-ton telescope is an engineering art. The forked mount of the dish lets it swivel up and down, and the pedestal turns it side to side. To follow an arc on the sky, these two work together under computer control. ALMA's state-of-the-art combination of software and hardware allows each 115-ton telescope to slew and stop in mere seconds an important feature for a giant telescope that must battle a tiny foe.
Wiggling water molecules in the air give off radio waves just like the ones that our telescopes grab from objects in space. To give our telescopes a fighting chance, we have located ALMA in the Atacama Desert. But even in one of the driest places on Earth, some water lurks in the air to affect the super weak signals we capture from space.
If you've ever looked at something through a glass of water, you know that water can work like a lens. Water vapor in the air will also have a small effect on the radio waves coming from space. This can disrupt the ability of ALMA's telescopes to work together as a single telescope, so the telescopes are designed to battle this foe.
As the telescopes collect radio waves from a cosmic target, they also have a device that measures water vapor in the sky above them. Based on the amount found, computers can correct for the water's lens effect as a pre-emptive strike.
Also, every ten seconds, ALMA's dishes move slightly off-target to view a nearby, well known radio-bright object. By comparing the bright object's water-affected signal with what the signal should look like, computers will calculate a correction to apply to the target, further erasing the effects of water vapor on its signal. Every ALMA dish will perform this missile-tracking-fast maneuver, allowing us to remove the lensing above each before we combine all the signals.
Atmospheric signal degradation models courtesy of Nikolic/University of Cambridge and still shot of cloudy day at the OSF courtesy of Dr. Kartik Sheth.