Arecibo Observatory

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Arecibo radio telescope
The cable-supported platform (above) with a rotatable azimuth arm (center) and feed antennas that can be displaced on it below: the Gregory cathedral on the left, the 430 MHz feed line on the right (2006)
The cable-supported platform (above) with a rotatable azimuth arm (center) and feed antennas that can be displaced on it below: the Gregory cathedral on the left, the 430 MHz feed line on the right (2006)
Type Radio telescope
Location approx. 15 km south of Arecibo ( Puerto Rico )

height 497 m (center of the sphere)
Geographic coordinates 18 ° 20 '39 "  N , 66 ° 45' 10"  W Coordinates: 18 ° 20 '39 "  N , 66 ° 45' 10"  W
wavelength 1 m to 3 cm
Aperture 305 m

construction time 1960 to 1963
Installation November 1, 1963
Specialty long the world's largest telescope

The Arecibo Observatory was an observatory with various telescopes located 15 kilometers south of the port city of Arecibo in Puerto Rico . It was known for its now destroyed large radio telescope , which was officially named William E. Gordon Telescope (1963-2020). Other instruments at the Arecibo Observatory include optical instruments for atmospheric research, a LIDAR , a smaller radio telescope and an ionospheric heater 30 kilometers away .

The radio telescope had an immovable primary mirror 305 meters in diameter made of adjustable facets. With instruments movably mounted above on a platform, almost 20 degrees around the zenith could be observed. The observatory was planned to study the ionosphere . For this purpose, the telescope was equipped with transmitters from the start, the radio waves of which are scattered back from the ionosphere. Radar astronomy was later operated with more powerful transmitters . In passive mode, radiation was received from remote radio sources . With the large reflector surface and after having been upgraded several times, the telescope was particularly suitable for surveys and the detection of weak, narrow-band or intermittent sources such as HI areas or pulsars , also in conjunction with other radio telescopes ( VLBI ).

About 140 people were employed at the observatory, which was in operation around the clock. (As of 2019) An independent committee distributed observation time according to scientific criteria to around 200 astronomers around the world each year , who were mostly able to perceive them from a distance. The observatory's visitor center has around 100,000 visitors a year.

On August 20 and November 6, 2020, broken steel cables of the platform suspension severely damaged the telescopic reflector. After the first rope break, a repair was sought. However, the strength analysis revealed that the risk of tearing was too high for repair work. The National Science Foundation therefore decided to decommission the large telescope. On December 1, 2020, further ropes failed and the 900-ton instrument platform fell 137 meters onto the reflector shell, which led to the complete destruction of the telescope.


William E. Gordon had the idea for ionospheric research with a large vertical radar and the will to implement it . The observatory was designed and built from summer 1960 to November 1963 for $ 9 million from ARPA funds . The facility was initially called Arecibo Ionospheric Observatory (AIO) and was subordinate to the US Department of Defense . In October 1969 it was transferred to the National Science Foundation (NSF) and renamed the National Astronomy and Ionosphere Center (NAIC) in September 1971 . The telescope was made suitable for astronomers from 1972 to 1974 for nine million dollars and significantly improved again from 1992 to 1998 for 25 million dollars.

The NAIC was managed on behalf of the NSF from 1969 to 2011 by Cornell University . In 2006 the NSF announced the gradual reduction of its share of the financing of the operating costs, so that it was threatened with closure in 2011. In 2011 a five-year cooperation was agreed with SRI International , which secured the financing for this period. After its expiry, the NSF was again looking for funding partners to keep the observatory running, especially after the million dollar damage caused by Hurricane Maria in 2017. In 2018, management went to a consortium of universities led by the University of Central Florida over. Associated with this was a steadily increasing financial contribution from the universities, which they had previously made as part of the student education, while the share of the NSF was to decrease gradually to 2 million dollars by October 2022.

A larger part of the sky is accessible to the Chinese FAST similar to the design with an adaptive primary mirror measuring 500 meters. The Square Kilometer Array would be a far superior competitor . However, FAST has no transmitter and is therefore not set up for radar astronomy.

Damage 2020 and shutdown

On August 10, 2020, one of the eight-centimeter-thick steel cables, which act as an auxiliary cable to stabilize the height of the receiver platform, tore from its end sleeve. It damaged the Gregory Dome and left a 30-meter hole in the main mirror. The operation of the telescope was stopped - initially for the time being. On November 6, 2020, one of the main load-bearing ropes on which the receiving unit is suspended broke and caused further damage to the system. Due to the timely sequence of the two rope breaks, material fatigue could not be ruled out. Since the remaining stability of the construction was uncertain, another cascading rope break and a crash of the 900-ton instrument platform were feared. The National Science Foundation considered the repair too dangerous and decided to permanently shut down and demolish the observatory. The abrupt end of the radio telescope, which had been in scientifically productive operation for decades, was received with dismay by the global astronomers and astrophysicists community. On December 1, 2020, the instrument platform crashed after more ropes broke. Considerable parts of the mirror were destroyed, the three reinforced concrete pillars lost their upper segments and other buildings were damaged. Nobody got hurt.


The instrument platform

Primary mirror

The main mirror of the radio telescope is carried by a grid of wire ropes , which is orthogonal when projected into the horizontal plane, above the floor of a carved karst hollow. Until 1971, this grid was only covered by a wire network - too wavy to achieve diffraction-limited resolution at the reception frequencies of 318, 430 and 611 MHz at that time, and too wide-meshed ( 12 inch) for higher frequencies. During the initial upgrade of the telescope, this wire mesh was replaced by 38,778 individually adjustable, perforated aluminum panels. The deviations of the surface from the desired spherical shape were thus only 2 mm ( RMS ), which extended the usable frequency range to 10 GHz. During the second upgrade, a fine-meshed fence was built around the main mirror to shield it from ambient thermal radiation.

Instrument platform

When the viewing direction is varied, the primary focus moves on a spherical shell with half the radius ( focal length for paraxial rays ). The instruments must be moved accordingly, with a precision in the millimeter range. A triangular framework hung on rigid wire ropes as the base . Six ropes each led to three reinforced concrete pillars outside the main mirror, which in turn were braced to the outside with seven ropes each. A lattice beam rotated on a ring of rails on the underside of the platform to adjust the azimuth . The underside of this azimuth arm was curved in a circle and provided with rails on which two antenna carriers could move independently of one another. This set the zenith angle . During the initial upgrade, outriggers were fitted to the corners of the platform that protrude beyond the azimuth arm and are connected by pairs of cables to anchorages under the primary mirror to stabilize the height of the platform.

Correction of the caustic

Caustic due to spherical aberration (sunlight falls obliquely into a gold ring that is cylindrical on the inside)

The coupling of the radiation field from the main mirror to the waveguides of the transmitting and receiving devices is complex because of the spherical aberration to be corrected . One solution that only succeeded at the second attempt uses a so-called line feed (see waveguide and slot antenna ) on the optical axis (the straight line in the direction of view through the center of the sphere). On the optical axis, rays from the edge of the main mirror cross at a lower height than rays close to the axis. In addition, they meet “earlier”, on a shorter path (from the radio source or from the wavefront level, before reflection). The place at which a certain wave front hits the optical axis travels upwards faster than the speed of light. The phase velocity of the wave in the waveguide is also faster than the speed of light. The cross-section of the waveguide, which varies over the length, adapts the speeds to one another so that they interfere positively at the upper end . This adaptation is sensitive to the free space wavelength, so that a high antenna gain was only achieved over a small bandwidth of around 10 MHz. First line feeds for frequencies of 318, 430 and 611 MHz were built. Only the line feed of 430 MHz is still in use, both for sending and receiving. It is 29 m long and illuminates the entire main mirror (aperture at the zenith 305 m). As the zenith angle increases, the antenna gain and thermal noise (from the environment next to the main mirror) quickly deteriorate.

The other solution, installed during the second upgrade, uses a secondary mirror behind the focus of the main mirror (as with a Gregory telescope ), where the paraxial rays also diverge again. The wave front is made spherical again via the shape of the secondary mirror. An even smaller third mirror also contributes to this, but its main task is to shorten the effective focal length after the second mirror. In this way more power is coupled into the subsequent horn antenna . The Gregory optics can be used over the entire bandwidth of the various receivers, 0.3 to 10 GHz, which can be turned into focus remotely as required, together with their horn antennas and frozen mixers / preamplifiers. The Gregory optics illuminate an oval area of ​​the main mirror (213 m × 237 m). Therefore the antenna gain is slightly lower than with the line feeds (with the same wavelength, in the zenith). It is housed in a cathedral that protects it from the weather .

Spatial and polarization resolution

The angular resolution is frequency dependent. The product of the frequency with the full angle, within which half the flux of a point source is received, is about 5 arc minutes · GHz, regardless of the line feed or Gregory optics.

The line feeds are only suitable for one pixel image resolution, and the Gregory optics are also used in some frequency bands with one pixel (a horn antenna). This is not uncommon in radio astronomy, since many radio sources cannot be resolved with a single telescope anyway. It is often panned periodically between an object and the neighboring sky background. The 7-pixel horn antenna array ALFA (Arecibo L-band Feed Array) has been available at Gregory-Optik since 2004, which improves the angular resolution only slightly , but has accelerated surveys enormously.

While the first line feed was designed for only one linear polarization direction , the successor models and the Gregory optics including horn antennas are transparent for any polarization. The analysis of the polarization is made possible by waveguide polarization switches and two or four receiver channels per pixel.

Coherent signal paths

Depending on the frequency band and the age of the equipment which are pre-amplified signals before or after conversion to a lower intermediate frequency supplied to a control room next to the telescope over coaxial cable or analog -powered fiber-optic connections . There is one fiber for each signal component, and ALFA needs one of 14. Local oscillators are partly located in the Gregory dome, in the cabins that can be moved on the azimuth arm and in the control room. For coherent signal processing, the local oscillators are not free-running, but are controlled by frequency synthesis, from a system of mutually monitoring atomic clocks in the control room via optical fibers. The connection to external clocks is done via GPS . Low phase noise and low frequency drift are important for the interconnection of several telescopes (VLA, VLBI), for planetary radar measurements and for the observation of pulsars.

Technical specifications

Primary mirror
  • Aperture: 305 m
  • Radius of curvature: 265 m
  • Surface accuracy: 2.2mm (RMS)
Antenna platform
  • Weight: 800 t
  • Span of the azimuth arm: 100 m (± 19.7 ° zenith angle, declination −1.3 ° to + 38 °)
  • Distance of its arc from the main mirror: 137 m
  • Travel speed on the arm (zenith angle): max. 2.4 ° / min
  • Rotation speed of the arm (azimuth angle): max. 24 ° / min
  • Positioning accuracy : 3 mm (5 pointing accuracy )
  • Transmission power: 1 MW, more pulsed (1998-2020)
  • Reception range: 300 MHz to 10 GHz



  • In 1974 a more powerful radar transmitter was put into operation with the broadcast of the Arecibo message .
  • In 1985 an asteroid, (4337) Arecibo , was named after the observatory.
  • In the 1990s, the system became known to a wider public through the films GoldenEye , Species and Contact, as well as an episode of the television series The X-Files .
  • Signals from the telescope are sometimes also searched for signs of extraterrestrial intelligence ( SETI ).
  • The computer game The Moment of Silence is set in the Arecibo telescope, among other places.
  • The multiplayer map "Rogue Transmission" of the computer game Battlefield 4 is based on the Arecibo observatory.

Web links

Commons : Arecibo Observatory  - collection of images and audio files

Individual evidence

  1. ^ A b J. D. Mathews: A short history of geophysical radar at Arecibo Observatory . Hist. Geo Space Sci. 4, 2013, doi: 10.5194 / hgss-4-19-2013 (free full text).
  2. Amelie Saintonge et al .: The Arecibo Legacy Fast ALFA Survey: V. HI Source Catalog of the Anti-Virgo Region at dec = 27 degrees . Astronomical Journal 135, 2008, doi: 10.1088 / 0004-6256 / 135/2/588 , arxiv : 0711.0545 .
  3. JM Cordes, et al .: Arecibo Pulsar Survey Using ALFA. I. Survey Strategy and First Discoveries . Astrophys. J. 637, 2006, doi: 10.1086 / 498335 , arxiv : astro-ph / 0509732 .
  4. Jessica L. Rosenberg, Stephen E. Schneider: The Arecibo Dual-Beam Survey: Arecibo and VLA Observations . Astrophys. J. Suppl. 130, 2000, doi: 10.1086 / 317347 , arxiv : astro-ph / 0004205 .
  5. a b Mark Zastrow: Famed Arecibo radio telescope to be decommissioned after cable failures. In: Astronomy, November 19, 2020, accessed November 19, 2020 .
  6. a b Martin Holland: Can't be repaired: Arecibo telescope is taken out of service. In: heise online , November 20, 2020, accessed on November 21, 2020 .
  7. Puerto Rico: Iconic Arecibo Observatory telescope collapses. BBC News, December 1, 2020, accessed December 2, 2020 .
  8. Thorsten Dambeck: USA make all listening devices deaf and mute . Der Spiegel, November 21, 2006.
  9. ^ NSF: Dear Colleague Letter: Concepts for Future Operation of the Arecibo Observatory . October 2015.
  10. ^ Sarah Kaplan: Hurricane-battered Arecibo telescope will keep studying the skies . Washington Post, November 16, 2017.
  11. ^ Daniel Clery, Adrian Cho: Iconic Arecibo radio telescope saved by university consortium . Science 2018, doi: 10.1126 / science.aat4027 .
  12. JS Deneva, et al .: Pulsar surveys present and future: The Arecibo pulsar-ALFA survey and projected SKA survey . Proc. 363. WE-Heraeus Seminar on: Neutron Stars and Pulsars, Bad Honnef, May 2006, arxiv : astro-ph / 0701181 .
  13. ^ Morgan McFall-Johnson: A broken cable smashed a hole 100 feet wide in the Arecibo Observatory, which searches for aliens and tracks dangerous asteroids. In: August 12, 2020, accessed on August 16, 2020 (English, the photo here shows the potted end of the spiral rope.).
  14. ^ Daniel Clery: Arecibo radio telescope goes dark after snapped cable shreds dish. Science, August 12, 2020, doi: 10.1126 / science.abe3033 .
  15. Zenaida Gonzalez Kotala: Broken Cable Damages Arecibo Observatory | University of Central Florida News. In: - UCF Today . August 11, 2020, accessed August 12, 2020 (American English).
  16. Martin Holland: Arecibo observatory badly damaged by a broken wire. In: heise online , August 12, 2020, accessed on August 13, 2020 .
  17. ^ Nadja Podbregar: Second cable break on the Arecibo radio telescope. In: November 10, 2020, accessed November 10, 2020 .
  18. Martin Holland: Arecibo Observatory: Feared of total collapse. In: heise online , November 10, 2020, accessed on November 13, 2020 .
  20. Alexandra Witze: Legendary Arecibo telescope will close forever - scientists are reeling . In: Nature Communications . November 19, 2020, doi : 10.1038 / d41586-020-03270-9 (English).
  21. ORF at / Agencies red: Famous Arecibo radio telescope collapsed in Puerto Rico. December 1, 2020, accessed December 1, 2020 .
  23. Martin Holland: Instrument platform crashed: The Arecibo telescope is destroyed. In: heise online , accessed on December 2, 2020 .
  24. ^ NAIC: ALFA: Arecibo L-Band Feed Array .
  25. ^ NAIC: Surveys and Data Products (from ALFA).
  26. ^ NAIC: Local Oscillators (LOs) and the Intermediate Frequency (IF) Chain .
  27. ^ NAIC: Time Transfer Issues at Arecibo Observatory - 2000 .
  28. Cornell University press release - Cornell News, November 12, 1999: It's the 25th anniversary of Earth's first (and only) attempt to phone ET ( Memento from August 2, 2008 in the Internet Archive ), In: edu, accessed on August 13, 2020