How does very long baseline interferometry allow us to see a black hole?




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HD 163296 is representative of a typical protoplanetary disk seen by the DSHARP collaboration. It has a central protoplanetary disk, external emission rings and spaces between them. There should be multiple planets in this system, and one can identify a strange artifact inside the second outermost ring that can be a telltale sign of a disturbing planet. The scale bar in the lower right is 10 AU, which corresponds to a resolution of only a few milliarcseconds. This can only be achieved through VLBI.

S. M. Andrews et al. and the collaboration DSHARP, arXiv: 1812.04040

The Event Horizon telescope has achieved what no other telescope or set of telescopes has ever done: photographed the event horizon of a black hole directly. A team of more than 200 scientists who used data from eight independent telescope facilities on five continents came together to achieve this monumental triumph. While there are many contributions and collaborators that deserve to be highlighted, there is a fundamental physical technique on which everything depends: Very long baseline interferometry, or VLBI. Patreon supporter Ken Blackman wants to know how that works and how he allowed this remarkable feat, asking:

[The Event Horizon Telescope] uses VLBI. So, what is interferometry and how was it used by [the Event Horizon Telescope]? It seems that it was a key ingredient in the production of the M87 image but I have no idea how or why. Care to elucidate?

You are in; let's do it.

Any reflector telescope is based on the principle of reflecting incoming light rays through a large primary mirror that focuses that light to a point, where it is then decomposed into data and recorded or used to construct an image. This specific diagram illustrates the light trajectories for a Herschel-Lomonosov telescope system.

User of Wikimedia Commons Eudjinnius

For a single telescope, everything is relatively simple. Light enters as a series of parallel rays, all originating from the same distant source. The light hits the primary mirror of the telescope and focuses on a single point. If you put an additional mirror (or a set of mirrors) along the path of light, they do not change that story; they simply change where that light ends up converging to a point.

All these rays of light arrive at that end point at the same time, where they can be combined in an image or saved as unprocessed data, to be processed in an image later on. That's the ultra-basic version of a telescope: light comes from a source, focuses on a small region and registers.

A small section of Karl Jansky Very Large Array, one of the largest and most powerful radio telescope sets in the world. Unless the individual dishes are correctly synchronized, they will not achieve a higher resolution than a single dish.

John Fowler

But what happens if you do not have a single telescope, but multiple telescopes that are networked in some kind of array? You might think that you could approach the problem in a similar way and focus the light of each telescope in the same way you would for a single-plate telescope. The light would still come in parallel rays; each primary mirror would still focus that light towards a single point; the light rays of each telescope reach the end point at the same time; All that data can be collected and stored.

You could do that, of course. But that would only give you two independent images. You could combine them, but that would only average the data. It would be like observing your objective with a single telescope at two different times and adding the data.

The Square Kilometer Array, when completed, will consist of a series of thousands of radio telescopes, capable of seeing farther into the Universe than any observatory that has measured any type of star or galaxy.

Project Development Office SKA and Swinburne Astronomy Productions

That does not help you with your big problem, and that is that you need the improved critical resolution that comes with the use of a network of telescopes connected together with VLBI. When you successfully connect several telescopes with the VLBI technique, you can provide an image that has the light pickup power of the individual telescope plates added, but (optimally) with the resolution of the distance between the telescope plates.

This technique has been used many times, not only to obtain images of a black hole and not even with radio telescopes. In fact, perhaps the most spectacular example of VLBI was used by the Large binocular telescope, which has two 8-meter telescopes that are mounted together, behaving with the resolution of a ~ 23 meter telescope. As a result, it can solve features that no 8-meter dish can do, such as the eruption of volcanoes in Io while experiencing an eclipse of another of Jupiter's moons.

The concealment of Jupiter's moon, Io, with its erupting volcanoes, Loki and Pele, hidden by Europe, which is invisible in this infrared image. The Big Binocular Telescope was able to do this due to the interferometry technique.

LBTO

The key to unlocking this type of power is that you need to be able to unite your observations at the same moments. & Nbsp; The light signals that reach the telescopes are arriving after slightly different light travel times, due to the variable distance, at the speed of light, which takes the signal to travel from the source object to the variable detectors / telescopes on earth.

You must know the time of arrival of the signals in the different locations of telescopes around the world to be able to combine them into a single image. Only by combining the data that corresponds to seeing the same source simultaneously can we achieve the maximum resolution that a network of telescopes is capable of offering.

This diagram shows the location of all the telescopes and array of telescopes used in the observations of the Horizon Telescope of the 2017 Event of M87. Only the South Pole Telescope could not visualize the M87 image, since it is located in the wrong part of the Earth to see the center of the galaxy. Each of these places is equipped with an atomic clock, among other equipment.

NRAO

The way we do this, practically, is through the use of atomic clocks. In each of the 8 locations around the world where the Horizon Telescope of the Event receives data is an atomic clock, which allows us to maintain the accuracy time of a few attoseconds (10).-18 s). There was also a need to install specialized computational equipment (hardware and software) to allow observations to be correlated and synchronized between different stations around the world.

You must observe the same object at the same time with the same frequency, while correcting things such as atmospheric noise with a properly calibrated telescope. It is a laborious task that requires enormous precision. But when you get there, the reward is amazing.

The protoplanetary disk around the young star, HL Tauri, photographed by ALMA. The holes in the disk indicate the presence of new planets. This system is already hundreds of millions of years old, and it is likely that the planets are reaching their final stages and their orbits. This resolution is only possible due to the use of VLBI by ALMA.

SOUL (ESO / NAOJ / NRAO)

The image above may seem to have nothing to do with a black hole, but it is actually one of the most famous images of the most powerful variety of radio telescopes that exist: ALMA. ALMA means Atacama's large millimeter / submillimeter matrix, and consists of 66 independent radio antennas that can be adjusted to have a separation of 150 meters to 16 kilometers.

The power to capture light is only determined by the area of ​​the individual plates all together; That does not change. But the resolution you can achieve is determined by the distance between the plates. This is how you can achieve resolutions of only a few milliseconds of arc, or resolutions of 1 / 300,000 degree.

The Atacama Large Millimeter / submillimeter Array (ALMA) is one of the most powerful radio telescopes on Earth. These telescopes can measure long-wavelength signatures of atoms, molecules and ions that are inaccessible to shorter-wavelength telescopes such as Hubble, but can also measure details of protoplanetary systems and, potentially, even strange signals that neither telescopes infrared can see. It was the most important addition to the Horizon Event Telescope.

ESO / C. Malin

But as impressive as ALMA is, the Event Horizon telescope goes even further. With baselines between stations that approach the diameter of the Earth & nbsp; – more than 10,000 km & nbsp; – Can solve objects as small as around 15 micro-seconds of arc. This incredible improvement in resolution is what allowed him to visualize the event horizon of the black hole (which has 42 micro-arc-seconds in diameter) at the center of the galaxy M87.

The key to obtaining that image, and to make these observations of high resolution in general, is to synchronize each of the telescopes with observations that are absolutely coincidental in time. For this to happen it is conceptually simple, but it requires & nbsp;a monumental innovation To put this into practice.

In VLBI, radio signals are recorded on each of the individual telescopes before being sent to a central location. Each data point that is received is marked with an extremely accurate high frequency atomic clock along with the data to help scientists obtain the correct synchronization of observations.

Public domain / Wikipedia user Rnt20

The key breakthrough came in 1958, when scientist Roger Jennison wrote a now famous paper: & nbsp; A phase-sensitive interferometer technique for the measurement of Fourier transforms of spatial brightness distributions of small angular extent. That sounds like a mouthful, but here is how you can understand it in a direct way.

  1. Imagine that you have three antennas (or radio telescopes) all connected and separated by particular distances.
  2. These antennas will receive signals from a distant source, where the relative arrival times of the different signals can be calculated.
  3. When the different signals are mixed, they interfere with each other, both by real effects and by errors.
  4. What Jennison was a pioneer & nbsp; and what is still used today in the form of self-calibration & nbsp; It was the technique to properly combine the real effects and ignore the errors.

This is known today as opening synthesis, and the basic principle has remained the same for more than 60 years.

In April 2017, the 8 telescopes / arrays of telescopes badociated with the Event Horizon Telescope pointed to Messier 87. This is what a supermbadive black hole looks like, where the event horizon is clearly visible. Only through VLBI could we achieve the resolution necessary to build an image like this.

Event Horizon Telescope collaboration et al.

The great thing about this technique is that it can be applied literally to any wavelength range. At this time, the Event Horizon telescope measures radio waves of a particular frequency, but theoretically could operate at a frequency between three and five times higher. Because the resolution of your telescope depends on the number of waves that can fit through the diameter (or baseline) of your telescope, going to higher frequencies results in shorter wavelengths and a higher resolution. We could get five times the resolution without having to build a single new dish.

The first black hole may have arrived a few days ago, but we are already looking to the future. The first event horizon is really just the beginning. On top of that, Event Telescope Event Horizon should one day be able to solve the characteristics of distant blazars and other bright radio sources, allowing us to understand them like never before. Welcome to the world of VLBI, where if you want a higher resolution telescope, you only need to move the ones that are furthest away!


Send your questions to Ethan to start with gang in gmail dot com!

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HD 163296 is representative of a typical protoplanetary disk seen by the DSHARP collaboration. It has a central protoplanetary disk, external emission rings and spaces between them. There should be multiple planets in this system, and one can identify a strange artifact inside the second outermost ring that can be a telltale sign of a disturbing planet. The scale bar in the lower right is 10 AU, which corresponds to a resolution of only a few milliarcseconds. This can only be achieved through VLBI.

S. M. Andrews et al. and the collaboration DSHARP, arXiv: 1812.04040

The Event Horizon telescope has achieved what no other telescope or set of telescopes has ever done: photograph the event horizon of a black hole directly. A team of more than 200 scientists who used data from eight independent telescope facilities on five continents came together to achieve this monumental triumph. While there are many contributions and contributors that deserve to be highlighted, there is a fundamental physical technique on which everything depended: very long baseline interferometry or VLBI. Patreon supporter, Ken Blackman, wants to know how that works and how he allowed this remarkable feat, by asking:

[The Event Horizon Telescope] uses VLBI. So, what is interferometry and how was it used by [the Event Horizon Telescope]? It seems that it was a key ingredient in the production of the M87 image but I have no idea how or why. Care to elucidate?

You are in; let's do it.

Any reflector telescope is based on the principle of reflecting incoming light rays through a large primary mirror that focuses that light to a point, where it is then decomposed into data and recorded or used to construct an image. This specific diagram illustrates the light trajectories for a Herschel-Lomonosov telescope system.

User of Wikimedia Commons Eudjinnius

For a single telescope, everything is relatively simple. Light enters as a series of parallel rays, all originating from the same distant source. The light hits the primary mirror of the telescope and focuses on a single point. If you put an additional mirror (or a set of mirrors) along the path of light, they do not change that story; they simply change where that light ends up converging to a point.

All these rays of light arrive at that end point at the same time, where they can be combined in an image or saved as unprocessed data, to be processed in an image later on. That's the ultra-basic version of a telescope: light comes from a source, focuses on a small region and registers.

A small section of Karl Jansky Very Large Array, one of the largest and most powerful radio telescope sets in the world. Unless the individual dishes are correctly synchronized, they will not achieve a higher resolution than a single dish.

John Fowler

But what happens if you do not have a single telescope, but multiple telescopes that are networked in some kind of array? You might think that you could approach the problem in a similar way and focus the light of each telescope in the same way you would for a single-plate telescope. The light would still come in parallel rays; each primary mirror would still focus that light towards a single point; the light rays of each telescope reach the end point at the same time; All that data can be collected and stored.

You could do that, of course. But that would only give you two independent images. You could combine them, but that would only average the data. It would be like observing your objective with a single telescope at two different times and adding the data.

The Square Kilometer Array, when completed, will consist of a series of thousands of radio telescopes, capable of seeing farther into the Universe than any observatory that has measured any type of star or galaxy.

Project Development Office SKA and Swinburne Astronomy Productions

That does not help you with your big problem, and that is that you need the improved critical resolution that comes with the use of a network of telescopes connected together with VLBI. When you successfully connect several telescopes with the VLBI technique, you can provide an image that has the light pickup power of the individual telescope plates added, but (optimally) with the resolution of the distance between the telescope plates.

This technique has been used many times, not only to obtain images of a black hole and not even with radio telescopes. In fact, perhaps the most spectacular example of VLBI was the Great Binocular Telescope, which has two 8-meter telescopes that are mounted together, behaving with the resolution of a ~ 23 meter telescope. As a result, it can solve features that no 8-meter dish can do, such as the eruption of volcanoes in Io while experiencing an eclipse of another of Jupiter's moons.

The concealment of Jupiter's moon, Io, with its erupting volcanoes, Loki and Pele, hidden by Europe, which is invisible in this infrared image. The Big Binocular Telescope was able to do this due to the interferometry technique.

LBTO

The key to unlocking this type of power is that you need to be able to gather your observations at the same moments. The light signals arriving at the telescopes arrive after slightly different light travel times, due to the variable distance, at the speed of light, which carries the signal to travel from the source object to the Earth-based variable detectors / telescopes. .

You must know the time of arrival of the signals in the different locations of telescopes around the world to be able to combine them into a single image. Only by combining the data that corresponds to seeing the same source simultaneously can we achieve the maximum resolution that a network of telescopes is capable of offering.

This diagram shows the location of all the telescopes and array of telescopes used in the observations of the Horizon Telescope of the 2017 Event of M87. Only the South Pole Telescope could not visualize the M87 image, since it is located in the wrong part of the Earth to see the center of the galaxy. Each of these places is equipped with an atomic clock, among other equipment.

NRAO

The way we do this, practically, is through the use of atomic clocks. In each of the 8 locations around the world where the Horizon Telescope of the Event receives data is an atomic clock, which allows us to maintain the accuracy time of a few attoseconds (10).-18 s). There was also a need to install specialized computational equipment (hardware and software) to allow observations to be correlated and synchronized between different stations around the world.

You must observe the same object at the same time with the same frequency, while correcting things such as atmospheric noise with a properly calibrated telescope. It is a laborious task that requires enormous precision. But when you get there, the reward is amazing.

The protoplanetary disk around the young star, HL Tauri, photographed by ALMA. The holes in the disk indicate the presence of new planets. This system is already hundreds of millions of years old, and it is likely that the planets are reaching their final stages and their orbits. This resolution is only possible due to the use of VLBI by ALMA.

SOUL (ESO / NAOJ / NRAO)

The image above may seem to have nothing to do with a black hole, but it is actually one of the most famous images of the most powerful variety of radio telescopes that exist: ALMA. ALMA means Atacama's large millimeter / submillimeter matrix, and consists of 66 independent radio antennas that can be adjusted to have a separation of 150 meters to 16 kilometers.

The power to capture light is only determined by the area of ​​the individual plates all together; That does not change. But the resolution you can achieve is determined by the distance between the plates. This is how you can achieve resolutions of only a few milliseconds of arc, or resolutions of 1 / 300,000 degree.

The Atacama Large Millimeter / submillimeter Array (ALMA) is one of the most powerful radio telescopes on Earth. These telescopes can measure long-wavelength signatures of atoms, molecules and ions that are inaccessible to shorter-wavelength telescopes such as Hubble, but can also measure details of protoplanetary systems and, potentially, even strange signals that neither telescopes infrared can see. It was the most important addition to the Horizon Event Telescope.

ESO / C. Malin

But as impressive as ALMA is, the Event Horizon telescope goes even further. With baselines between stations that approach the diameter of the Earth (more than 10,000 km), you can solve objects as small as about 15 micro-seconds of arc. This incredible improvement in resolution is what allowed him to visualize the event horizon of the black hole (which has 42 micro-arc-seconds in diameter) at the center of the galaxy M87.

The key to obtaining that image, and to make these observations of high resolution in general, is to synchronize each of the telescopes with observations that are absolutely coincidental in time. For this to happen it is conceptually simple, but a monumental innovation is required to put this into practice.

In VLBI, radio signals are recorded on each of the individual telescopes before being sent to a central location. Each data point that is received is marked with an extremely accurate high frequency atomic clock along with the data to help scientists obtain the correct synchronization of observations.

Public domain / Wikipedia user Rnt20

The key breakthrough came in 1958, when scientist Roger Jennison wrote a now-famous article: A Phase-sensitive Interferometer Technique for Measuring Fourier Transforms of Spatial Brightness Spatial Distributions. That sounds like a mouthful, but here is how you can understand it in a direct way.

  1. Imagine that you have three antennas (or radio telescopes) all connected and separated by particular distances.
  2. These antennas will receive signals from a distant source, where the relative arrival times of the different signals can be calculated.
  3. When the different signals are mixed, they interfere with each other, both by real effects and by errors.
  4. What Jennison was a pioneer, and what is still used today in the form of self-calibration, was the technique to properly combine the real effects and ignore the errors.

This is known today as a synthesis of openness and the basic principle has remained the same for more than 60 years.

In April 2017, the 8 telescopes / arrays of telescopes badociated with the Event Horizon Telescope pointed to Messier 87. This is what a supermbadive black hole looks like, where the event horizon is clearly visible. Only through VLBI could we achieve the resolution necessary to build an image like this.

Event Horizon Telescope collaboration et al.

The great thing about this technique is that it can be applied literally to any wavelength range. At this time, the Event Horizon telescope measures radio waves of a particular frequency, but theoretically could operate at a frequency between three and five times higher. Because the resolution of your telescope depends on the number of waves that can fit through the diameter (or baseline) of your telescope, going to higher frequencies results in shorter wavelengths and a higher resolution. We could get five times the resolution without having to build a single new dish.

The first black hole may have arrived a few days ago, but we are already looking to the future. The first event horizon is really just the beginning. On top of that, Event Telescope Event Horizon should one day be able to solve the characteristics of distant blazars and other bright radio sources, allowing us to understand them like never before. Welcome to the world of VLBI, where if you want a higher resolution telescope, you only need to move the ones that are furthest away!


Send questions from Ask Ethan to startswithabang in gmail dot com!


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