How big is the Arecibo telescope, the former world's largest telescope?

When it comes to radio telescopes, the most well-known one should be the 500-meter-diameter "Sky Eye" telescope located in Guizhou Province, my country. However, some people who pay attention to the "Sky Eye" telescope will also know another telescope that is often compared with it - the Arecibo radio telescope in Puerto Rico, USA.

The "Sky Eye" telescope and the Arecibo telescope are both fixed telescopes, and they look similar in appearance. But in terms of aperture, the former "Sky Eye" is the new world's largest single-aperture radio telescope, while the latter Arecibo is the former world's largest single-aperture radio telescope. This is why they are often compared together.

Arecibo radio telescope (left; image source: Space)

With the "Sky Eye" radio telescope (right; Image source: China Daily)

So, how big is Arecibo, the world's largest telescope?

This shouldn't be a difficult question to answer, but when you use various search engines to search for information, you will find that some people say the diameter of the Arecibo telescope is 305 meters, some say 350 meters, and some say 366 meters. It is difficult to judge which statement is correct.

Let me first state the correct statement: The Arecibo Telescope in the United States has a diameter of 305 meters and was built using a natural karst depression. This statement comes from the latest version of the user manual provided by the Arecibo Observatory’s official website, updated on August 4, 2020 [1].

So what are the various "rumors" that have been circulating in China? Let's find out and see where the problem lies.

Rumor 1

The Arecibo telescope has been upgraded to a diameter of 350 meters.

This is the most widely circulated and influential "false rumor" in China. Many news media have used this statement when reporting related news, or directly said that the diameter of the Arecibo telescope is 350 meters.

Regarding the claim that the Arecibo telescope was upgraded to 350 meters, the earliest source found on the domestic Internet was a netizen in June 2006 [2] quoting the Wikipedia entry "Radio Telescope" which stated that "the Arecibo telescope located on the island of Puerto Rico in the United States is a single-aperture spherical antenna fixed in a valley with an aperture of 305 meters, which was later expanded to 350 meters." By looking through the historical versions of the Wikipedia entry, it was found that this claim was already in place when the entry was created in March 2005. Unfortunately, the Wikipedia entry does not provide a reference, and no earlier version has been found, so it is impossible to know why this rumor appeared.

Rumor 2

The Arecibo telescope was upgraded to 350 meters in 1974.

This is a very tempting "upgraded version" of rumor 1. The reason why it is very tempting is that from official data, we can know that the Arecibo telescope did replace the reflective surface panel in 1974, which improved the telescope's observation capabilities. However, replacing the panel does not mean that the telescope's aperture will be increased! In fact, this replacement of the panel only improved the panel's accuracy, thereby increasing the telescope's observation frequency, and did not increase the telescope's aperture.

The Arecibo telescope is replacing the new reflector panel.

The old panels have high light transmittance, and you can see the ground underneath them through them; this is not the case with the new panels.

(Image source: Reference 9)

The earliest source of information that can be found that links the increase in the diameter of the Arecibo telescope with the replacement of the reflector panel in 1974 is a travel note written by a netizen in March 2013 [3]: "On November 16, 1974, to celebrate the extension of the diameter of the radio telescope from 1,000 feet (305 meters) to 350 meters, the Arecibo radio telescope sent a total of 1,679 binary digits of radio information to the globular cluster M13, 25,000 light-years away from Earth. This is called the Arecibo message."

This description is in double quotes in the travel notes, which should be a reference to information from elsewhere, but no earlier source of information can be found. The transmission of the Arecibo message is indeed related to the 1974 upgrade, because the equipment used to transmit this message is the new radar equipped in this upgrade, but the conclusion that the "caliber was extended" is definitely inaccurate.

Rumor 3

The Arecibo telescope was later upgraded to 366 meters

In recent years, the rumor that the Arecibo telescope has been expanded to 366 meters has been spreading and getting more and more intense. The reasons are that some netizens made a mistake in quoting it, and some popular science articles mentioned that "after 1980, it was rebuilt again and the diameter of the antenna was expanded to 366 meters."

Rumors like the latter, which give a certain time for transformation, are equally persuasive. Why does this rumor sound plausible? Let's first take a look at the facts:

In 1984, American scientists suggested adding a metal fence around the Arecibo telescope's reflective surface to shield radio interference from the surrounding area.[6] In 1993, a 16-meter-high (some sources, including the Cornell University website, say 50 feet high, or 15.24 meters) stainless steel mesh fence (called a "ground screen") was added around the Arecibo main reflective surface to reduce electromagnetic radiation interference from the ground and the surrounding environment. There is no evidence that this fence is used as part of the telescope's reflective surface.

According to this historical fact, if you believed the rumor that Arecibo was upgraded to 350 meters in 1974, and then found out that its edge had increased by 16 meters when you looked up the information, then wouldn’t 350 plus 16 equal 366 meters?! (Of course, readers who are slightly better at math will immediately find that this calculation method itself is wrong)

(Image source: NAIC)

The Arecibo telescope feed platform fell and hit the main reflector. It can be seen that the "panel" on the edge of the main reflector is meshed, which is different from the panel in the middle. This part of the "panel" is not used to gather electromagnetic waves and is not counted in the telescope aperture. Comparing with Figure 2, it can be found that the mesh "panel" on the outer edge was originally not there.

It is worth mentioning that FAST once considered adding a similar fence to reduce interference from the surrounding area. For FAST, such a fence needs to be 45 meters high[7]. I hope that there will be no false rumors in the future because of the addition of a fence to FAST.

Rumor 4

Arecibo telescope built on a volcano crater

In addition to the "diameter mystery", another rumor that has been widely circulated in recent years is that the Arecibo telescope was built in a crater.

We know that the "Sky Eye" telescope was built using a natural karst depression, which can reduce the amount of soil and rock excavation, greatly reducing the difficulty and budget of the project. Admittedly, the crater is also in the form of a depression, but from the perspective of geological conditions and drainage, it is not a good choice. Furthermore, the Arecibo telescope user manual clearly mentions that Arecibo was built using a karst depression.

The karst depression where the Arecibo telescope is located, photographed in 1959

(Image source: Reference 9)

The earliest information about the Arecibo telescope being located in the crater of a volcano is a report from October 2008[8], which mentioned "Cornell University's Arecibo Observatory located in the crater of a volcano in Puerto Rico."

Later, on August 8, 2009, in a revision of the "radio telescope" entry on Wikipedia, the original statement "The Arecibo telescope on the island of Puerto Rico, USA, is a single-aperture spherical antenna fixed in a valley, with an aperture of 305 meters, which was later expanded to 350 meters" was changed to "The Arecibo telescope on the island of Puerto Rico, USA, is a single-aperture spherical antenna fixed in a natural crater, with an aperture of 305 meters, which was later expanded to 350 meters." Given the influence of Wikipedia, perhaps this entry is the main source of the spread of the "crater rumor."

Conclusion

Although the above rumors about the Arecibo telescope have spread widely, they are limited to Chinese documents. If you search by English keywords, you can hardly find any related rumors. There are very few English documents that write the diameter of the Arecibo telescope as 350 meters, but there is no mention of upgrading. It should be just a typo that 305 meters is written as 350 meters.

Pursuing science is also pursuing the truth. In the Internet age where information is widely circulated, information distortion often occurs, and people often lose their way to find the truth in the vast ocean of information. Rumors are not terrible, and the way to break them is to tell the truth to the public. I hope that through this article, some rumors can be put to an end.

References:

[1] Arecibo Telescope User Guide. https://www.naic.edu/ao/scientist-user-portal/astronomy/astronomer-guide

[2] A related post on Tianya Community in 2006 quoted an incorrect statement from Wikipedia: http://bbs.tianya.cn/post-no04-553330-1.shtml

[3] 2013 Sina blog by a netizen: http://blog.sina.com.cn/s/blog_48dcdda20102e45z.html

[4] Answer from Zhihu user: https://www.zhihu.com/question/33911076/answer/84207405

[5] Related popular science articles by Big Eye Guest: https://mp.weixin.qq.com/s/5ntKK9Wu_ZimBdaE87xVZg

[6] Some historical materials about the Arecibo telescope on NASA’s official website: https://www.history.nasa.gov/SP-4218/ch9.htm

[7] Jin C, Zhu K, Fan J, et al. The optics of the five-hundred-meter aperture sphericalradio telescope. IEEE, 2013.

[8] Harbin News Network report on crop circles https://harbin.dbw.cn/system/2008/10/12/051540377.shtml

[9] Materials written by Professor Altschuler www.naic.edu/~astro/School/Talks/large_pdf/altschuler_ao_2.pdf

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