Earth’s Closed Eye

Question in the chat box, Maria, and I paraphrase to preserve anonymity — ‘So the Arecibo telescope won’t work any more. Why should we care? There’s lots of other telescopes that could so the same job.‘”

“But profesora, there aren’t. Arecibo is special in many ways. First, it is a very good telescope. That means it has high sensitivity and high resolution. Compare two radio telescopes with different‑size dishes and the same kinds of antennas and everything else. The one with the bigger dish is more sensitive because it can capture more photons. Arecibo’s 300‑meter dish used to be the largest in the world. China activated their FAST instrument five years ago. Its 500‑meter dish should make it more than 200 times as sensitive as Arecibo, but it doesn’t because neither telescope is designed to use the entire dish surface at once except for looking straight up. Their active areas are about the same.”

Is FAST another one of those goofy acronyms?

“Of course. It stands for ‘Five‑hundred‑meter Aperture Spherical radio Telescope‘ but in Chinese its name is Tianyan, which means ‘Heaven Eye.’ I think that is more pretty. FAST and Arecibo overlap their wavelength ranges, although FAST can receive some longer wavelengths and Arecibo can receive some shorter ones. Oh, there is also a big Russian radio telescope, RATAN‑600, with an even bigger diameter. But it is a ring, not a disk, so not as sensitive as Arecibo or FAST.”

A ring? Why did they build it that way?

“Because of the other thing you need in a good telescope, resolution. If you have good resolution in an image, you can see points that are very close together. The how‑close limit angle comes from dividing the light wavelength by the dish diameter. The diameter of RATAN’s ring is 600 meters, so RATAN’s resolving power is twice as good as Arecibo’s 300‑meter disk. RATAN doesn’t need to be sensitive, though, because it is used mostly for looking close at the Sun, not at stars and galaxies. That is OK because RATAN is so far north.”

What difference does that make?

“No telescope can see what is below its horizon. RATAN is at 43° north, almost 1400 miles north of Arecibo. It has a good view of the northern sky but cannot see down to the Equator where many asteroids and all the planets are.”

Sorry, Maria, that’s not quite correct. Earth is tilted relative to the orbital plane by 23° so even Arecibo only sees the northern portion of planetary orbits. While I’ve got the mic I’ll add some background on RATAN‑600. RATAN is the acronym for ‘Academy of Sciences Radio Telescope’ in Russian. It was built in the Cold War era when that part of the world was the USSR. Although I don’t believe it’s ever been publicly confirmed, many people think that RATAN‑600‘s original purpose was detection of ICBMs coming in over the North Pole. However, over the decades it has been a productive source of information for the solar physics community. Back to you, Maria.

“That is good to know, profesora. Thank you. So, Arecibo is — was —special because of its sensitivity and its resolution. It is also about 500 miles further south than FAST. But Arecibo has one additional feature that FAST cannot have — radar. Arecibo has high-powered transmitters that can send out terawatt pulses to things in the Solar System that are closer than Saturn. The dish gathers echoes that give us detailed knowledge of those objects. For instance, Arecibo’s radar echoes from Mercury showed us that the planet is not tidally locked to the Sun. We used to think Mercury’s day was 88 days long, like its year, but now we know it rotates in only 59 days.”

Why can’t the Chinese just add transmitters to FAST?

“The Chinese designers gave FAST a light‑weight antenna carriage to hang over its dish. Arecibo’s 900‑ton carriage can handle massive transmitters, but FAST’s cannot. There is one other radio telescope with radar, at Goldstone, California, but it has less than one‑millionth the power of Arecibo’s transmitters. Without Arecibo’s sensitivity, resolution, location and high-powered radar capability we cannot find near‑Earth asteroids on track to hit us.”

~~ Rich Olcott

Author’s note — Early in the morning of 1 December, after I completed last week’s and this post, the National Science Foundation reported that Arecibo’s central instrument platform has fallen onto the dish as a result of further cable failures.
“Vale, nostri servi boni et fidelis”
Farewell, our good and faithful servant.

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