Arecibo ¡que lástima!

Hello, Astronomy video class. I’ve made room in the syllabus schedule for a quick talk from someone with a personal connection to a timely topic. You may know we’ve lost one of Astronomy’s premier radio telescopes, Puerto Rico’s Arecibo Observatory. I’ve asked Maria to fill us in on the what and the why. If you have a question, type it into your chat window and I’ll relay it to her. Maria, you’re on.”

“Thank you, profesora. Yes, I do know Arecibo because I have worked there. I grew up in Hatillo, a small city on the north coast about half an hour away from the Observatory. My teacher of science in high school, somehow he got me a summer job there. Sometimes I worked in the gift shop, sometimes I helped the guided tours, but my best thing was running errands because then I could visit the science offices and chat with people about what they were doing. There I fell in love with Astronomy and that is why I came here to study.

“When people think of Arecibo they think of the big 300 meter dish, about 1000 feet across. Sharing my screen for you… there. This picture I got from Wikipedia:

The Arecibo Observatory
photo by JidoBG, licensed under the Creative Commons Attribution-Share Alike 4.0 International

“The installation sits in very rough mountains. They are so rough because they are mostly limestone that slowly dissolves in water. The water seeps in through cracks to attack the rock and make cliffs and holes and caves. The Arecibo observatory is where it is because water eroded a cavern close to the surface. The topmost material fell into the empty space to make a huge round sinkhole like very few other places in the world.”

Question from the chat, Maria. Did the rock actually dissolve into that convenient smooth reflector shape?

“¡Por Dios no! The circular shape, yes, but the sinkhole floor is nearly flat. The dish itself is many aluminum panels fixed to a floating steel grid. Here is a picture Mr Phil Perillat took from beneath the dish. I don’t know Mr Perillat’s title but he is always very busy keeping things running.

“Above you see the grid, five meters or more above the ground. The grid is supported by concrete all around the edges. Coming down from the grid you see cables leading to those round concrete piers. These cables pull the grid down into its curved shape which is actually a piece of a sphere.”

A sphere, not a parabola?

“No, profesora, and that is important. A fixed dish with a parabola shape like most telescope mirrors always would aim straight up. It would see targets at the top of the sky but for only a few minutes as the Earth turns through the day. With a sphere‑shaped dish and the antennas mounted where the center of the sphere would be, then the whole sky is in focus. The scientists aim the telescope by moving the antennas to point at different parts of the dish like you look at different parts of one of those funny mirrors in, sorry I don’t know the word, una casa de la diversión.”

A funhouse.”

“Thank you. The antenna carriage is so complicated because it must look at different parts of the dish. Here you see the carriage:

The Arecibo receiver mounting and dome
Photo by Phil Perillat, National Astronomy and Ionosphere Center

“The antennas point downward from inside that dome. When motors swing the dome along that crescent‑shaped arc, the antennas scan along an arc of the dish. More motors can rotate the arc around that circular track. By swinging and rotating together, the antennas can follow the reflection of any object that moves through the sky.”

All those motors and tracks and antennas must be heavy.”

“Yes, 900 tons hanging 500 feet above the grid. Eighteen cables hold it up. Each is many strands of steel braided together. Compressed air blows through the braids to prevent corrosion, but the storms won out in the end. Three cables have failed and it is too dangerous for repair. So sad.”

~~ Rich Olcott

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