The Arecibo Observatory – Puerto Rico

The Arecibo Observatory – Past, Present and Future

The Arecibo Observatory may well be the world’s most famous telescope. Because of its enormous size and rather unique design it has appeared in all kinds of movies. That being said its primary purpose is to do radio astronomy and this is something it does very well. The observatory has a fascinating history but its future is uncertain. Reductions in spending on science and a change in research priorities have left Arecibo underfunded. If money can’t be found soon this great telescope may be shut down.

The Arecibo Observatory is a radio telescope located in Puerto Rico. It is operated by Cornell University in partnership with the National Science Foundation. It is by far the largest single aperture telescope ever built with a diameter of 305 meters. The receiver is located on a platform 150 meters above the dish. The Arecibo telescope uses a spherical reflector that is built into a giant sinkhole. The reason a spherical reflector is used rather than a parabolic reflector which is used on most telescopes is because of the large size of the dish. Since it can’t be moved to track objects like a regular telescope the receiver has to be moved instead. If a parabolic reflector was used every time the receiver was moved out of the center of the dish there would be a refractive error. This isn’t an issue with a spherical reflector.

arecibo observatory

Originally the Arecibo Observatory was intended to be used for the study of the upper atmosphere, the result being that the design would provide very limited use when it came to things like radio astronomy or planetary science. When it was pointed out by other scientists that if they were going to spend the money to build the telescope they might as well build one that would serve the needs of as many scientists as possible rather than the very limited design that was originally envisioned. The result was a redesign that proved to be much more useful. With most of the funding provided by the Department of Defence through the Advanced Research Projects Agency construction was started in 1960 and completed in 1963. The Arecibo telescope has maintained the same basic lay out ever since but there have been a number of upgrades over the years to improve its performance.

There were a number of reasons that the telescope was built in Puerto Rico, the biggest being because it was advantageous to build it as near to the equator as possible. This eliminated any locations in continental United States since they are all too far north. The reason you want to build near the equator is that the stars visible north of the equator and those visible to the south are different. A telescope at the equator can see both the northern and the southern sky. It is also a requirement that a radio telescope be located far away from any large population centers since the radio waves produced in cities would interfere with operations. It was also important to keep the Arecibo Observatory on American territory because of its military uses. This pretty much made it a requirement to build in Puerto Rico.


A radio telescope is simply a telescope that observes radio waves rather than visible light like an optical telescope would do. Because radio waves are so much larger than the waves of visible light a radio telescope must be quite large. This is why the Arecibo Observatory is so big. The reason that you would want to have a radio telescope is that many objects radiate waves in the radio wavelength but not visible light. This is because the shorter the wavelength the hotter the object must be to emit it. A lot of objects in space aren’t hot enough to emit visible light but are hot enough to emit radio waves. These objects would never be seen without a radio telescope. The other reason that you would want to use a radio telescope is that objects that are too far away to be observed by an optical telescope can be observed by a radio telescope. This is simply down to the fact that a radio telescope can be built so much bigger.

There have been many important astronomical discoveries made at the Arecibo Observatory over the years. The first major one, made almost as soon as the observatory opened was that the rotation of Mercury was much faster than anybody had imagined. For years it was believed that the rotation period of Mercury was 88 days, observations from Arecibo determined it was actually 59 days. Most of the major discovers that have been made with the Arecibo telescope are pulsars. A pulsar is a fast rotating neutron star that emits a beam of electromagnetic radiation. Since the beam can only be seen when it is directed toward the earth the star appears to pulse on and off. Pulsars have played an important role in determining the age, size and composition of the universe. The first planets outside of our solar system were also discovered using the Arecibo telescope in 1994.


Given that construction was largely funded by the military it should probably come as no surprise that the telescope also has military uses. There are a number of things the military can use it for but the most important was to find Soviet radar installations during the cold war. Since radars send out radio waves that go out in all directions some of them will eventually hit something and be reflected back to where they came from, this is the whole theory behind how radar works. However some of the radio waves will hit targets you weren’t aiming at, like the moon. The military figured out that they could use the Arecibo observatory to detect the radio waves sent out by Soviet radar when they bounced off the moon. From this they could determine where they were located.

In addition to its role in radio astronomy the Arecibo observatory also plays important roles in other areas of astronomy. The biggest is radar astronomy which involves bouncing microwaves off of objects in space. This is different from radio astronomy in that radio astronomy observes the radio waves being emitted by objects in space, radar astronomy actually sends out the radio waves and measures how long it takes for them to return. This allows astronomers to get very accurate measurements of an objects distance and speed. It also allows them to accurately determine the size and shape of an object. The telescope is also used for aeronomy, which was actually the reason it was built in the first place. Aeronomy is the study of the upper atmosphere.

Perhaps the most famous research project at Arecibo is SETI, the search for extra-terrestrial intelligence. This is the project in which astronomers send radio signals out into space in the hopes that the will be picked up by extra-terrestrials who will then respond to them. A number of radio telescopes have been used for this project, including Arecibo. One of the projects of SETI connects the data coming from the telescopes to peoples home computers through the internet so that when your home computer isn’t being used SETI can use it to analyze the signals. This project is based at the Arecibo Observatory.

Despite its decades of useful scientific research the future of the Arecibo Observatory is very unclear as the result of funding cuts. In 2006 the National Science Foundation recommended that the budget for the telescope be significantly cut and this has resulted in a much reduced observing program. There have been various attempts to replace the funding; the best option seems to be to have the government of Puerto Rico provide the money. To a certain extent this has happened as they put up the money to do maintenance on the telescope but they haven’t as of yet put any money into the operation. There is a very real danger the observatory may be shut down if funding can’t be found.

One possible hope to keep the observatory open in the future might be to use it to detect asteroids. In recent years concern about an asteroid hitting the earth and wiping out human life has increased and the government have actually started putting money into finding asteroids that could be a threat. Since a radio telescope is the ideal instrument to do this there is hope that the Arecibo Observatory will receive government funding to find asteroids. This has already happened on a small scale, the government have made NASA responsible for detecting these asteroids and they have put some money into Arecibo as part of the project. So far the money hasn’t been enough to keep the telescope in full scale operation and there is no long term commitment to its use for this purpose but it is a start. With few if any other prospects for funding on the horizon it looks like if the Arecibo Observatory is going to survive into the future it will be to find asteroids.