A powerful set of radio transmitters called klystrons also live Inside the Gregorian dome. Klystrons are like light bulbs for radio waves: they are used for creating and amplifying radio signals for everything from weather radar to particle accelerators (like at the Stanford Linear Accelerator Center, SLAC). Unlike measly 100-watt light bulbs, our klystrons generate 500 kilowatts each in the radio part of the spectrum. They require vacuum tubes and magnets to keep their electron beams aligned. So, a little more complicated than light bulbs.

Klystrons transmit linearly polarized radio waves, which need to be turned into circularly polarized waves. Received echoes are turned from circularly polarized light back into linearly polarized light. The devices responsible for this radio frequency engineering magic are known as septum polarizers and turnstile junctions. Seriously, take a look at these things, they're pure black magic.

The process of observing, which we refer to as "experimenting," is very different at Arecibo than it is for an optical telescope. We observe in the rain, in the fog, and during the daytime. Radar's like the honey badger, it simply doesn't care.

Radar is also active: we transmit radio waves at our targets, then wait for the return echoes. This is unlike passive astronomy, where you wait for light that originated hundreds, thousands, millions, or billions of years ago, or observe photons from the Sun reflected off of objects in the solar system. Instead, we create the photons that we ultimately collect.

A lot of time with optical telescopes is spent taking calibration data. With radar, however, we know exactly what our signal is like that we transmit, which makes the calibration process part of transmitting and receiving, so we don't have to spend extra time observing comparison stars or solar analogs.

We send a powerful pulse of radio waves from our klystrons in the Gregorian dome that then reflects off of the dish and bounces off of passing asteroids. The echoes reflect off the dish, into the Gregorian dome, and just before they come back, we turn off our transmitter and use a sensitive radio receiver to collect the radio echoes from the asteroid.

Nights, or technically "observing runs," start with switching on diesel generators on the other side of the site. The telescope operator takes a 1972 diesel Jeep CJ, shoves in the sticky clutch, and drives over to the maintenance side of site to start the generators. Diesel fuel is the biggest single cost toward running the planetary radar system at Arecibo. The generators create 60,000 volts at 33 amps that travel along a high voltage cable to the Gregorian dome and the klystrons. That high voltage turns into two megawatts of power, one megawatt that we dump as waste heat via 300 gallons a minute of water pumped through cooling tubes from a homebrew setup.