Inside the Event Horizon Telescope

Mini-Circuits Products Support Historic Black Hole Image

In coordinated press conferences around the world on April 10, 2019 scientists announced that they had successfully captured the first-ever direct visual image of a black hole.  The image of the super-massive black hole at the center of the elliptical galaxy known as Messier 87 (or M87) was captured by a team of 200 researchers using the Event Horizon Telescope (EHT), a precisely coordinated array of eight powerful radio telescopes scattered across six countries and Antarctica working in concert to create a giant virtual telescope.

Black holes have long occupied the imaginations of scientists and science-fiction writers alike, raising questions about the nature of space and time that defy intuitive understanding of the physical world. They have remained elusive to direct observation because, by definition, their gravitational pull is so strong that they consume everything around them, even light. Until now, astronomers have only inferred their presence from the behavior of nearby stars. Photographing an astronomical object at such distance and scale, let alone one from which no light can escape, would be impossible using an optical telescope, even one as powerful as the Hubble Space Telescope. Fortunately, astronomers now have a sophisticated set of instruments that have redefined the frontiers of discovery.

Radio telescopes like EHT, detect radio light or emissions in the radio frequency spectrum (3 kHz to 300 GHz) with lower frequencies and longer wavelengths than those of visible light (430 to 770 THz). Because of their unique physical properties, radio waves pass through all the cosmic debris that obstructs visible light, allowing astronomers to observe phenomena at astonishing distances in astonishing detail.

These telescopes rely on a technique called radio interferometry to capture images of distant objects from cosmic radio emissions. The details require a technical understanding of concepts from physics and math which include wave interference and Fourier transforms, but in essence, interferometers consist of one or more pairs of radio antennas. The distance between the two antennas is known as the baseline. The longer the baseline, the greater the power of the telescope. The more antenna pairs in the array, the clearer the picture. The EHT, which comprises eight receiver stations as far apart as Greenland and the South Pole has an effective aperture roughly equal to the diameter of Earth and a resolution as fine as 20 micro-arcseconds, which the EHT website describes as “enough to read a newspaper in New York from a sidewalk café in Paris.”

The most powerful telescope in the EHT collaboration is the Atacama Large Millimeter/submillimeter Array (ALMA) in the Atacama Desert of northern Chile, built and operated by the National Radio Astronomy Observatory (NRAO) in partnership with the European Southern Observatory (ESO) and the National Astronomical Observatory of Japan (NAOJ). The NRAO is a longstanding Mini-Circuits customer and partner. In 2015, the two organizations entered into an exclusive patent licensing agreement to commercialize reflectionless filters, a novel type of RF filter circuit first developed by NRAO scientist and research engineer, Dr. Matthew Morgan.

“Six different component modules [in ALMA] use Mini-Circuits parts,” Morgan commented. “One of them was the receiver cartridge for Band 6… Another was a Band 6 beam scanner used for testing and characterization.”

Band 6 is the designation for the frequency range from 211 to 275 GHz which was used to capture the Pōwehi black hole image. Most of the Mini-Circuits parts on the list were used in the local oscillator and timing reference distribution systems or in test sets used to characterize the equipment. These components are designed into the Intermediate Frequency (IF) and baseband paths of the system, which means they operate at lower frequencies than the signal coming in from outer space.

ALMA ultimately became a lynchpin of the EHT collaboration, but its construction was undertaken independently by the international scientific community to support a wide range of research into the origins of the universe.  Morgan explained that when the initial hardware was being built, years of work lay ahead before ALMA would be complete and years more still before the EHT project would yield results. “At the time, EHT was just a gleam in someone’s eye,” he said. “The hardware was delivered to the ALMA site in Chile and had to be integrated with 66 antennas.  That took a number of years. Once ALMA was complete, the EHT consortium had to deliver duplicate hardware to all the sites around the world, develop software to join all the antennas, and forge interagency agreements.”

The inauguration of ALMA was held in March of 2013. It would take another six years of planning and development before the stars aligned for EHT to capture the historic image last April.

“Some of the parts [on the list] may seem like older models,” Morgan said. “That’s because most of the builds represented here are more than 10 years old… We’ve been waiting for this result for a long time.”

EHT researchers have already set their sights on their next target: the super-massive black hole at the center of our own Milky Way galaxy know as Sagittarius A*. The work of EHT scientists and researchers is rapidly advancing humanity’s understanding of the universe.  It’s inspiring to know that our efforts at Mini-Circuits are helping make such discoveries possible.

Mini-Circuits parts used on the ALMA telescope in Chile. Most of these parts were used in the local oscillator and timing reference distribution systems.

Mini-Circuits parts used in test sets for characterization of the critical band 6 (211-275 GHz) receiver which captured the image on most of the telescopes in the EHT collaboration.

Mini-Circuits wishes to thank Matt Morgan at the NRAO for his generosity in sharing the information in this blog post for the benefit and enjoyment of our readers.