<p>Supplementary graphical representation for the <a href="https://astronomy.stackexchange.com/a/20086/7982">accepted answer</a>, from <a href="https://twitter.com/MIT_CSAIL/status/1116007460039483392/photo/1" rel="nofollow noreferrer">tweet</a>:</p>

<p><img src="https://i.sstatic.net/4ugoo.jpg" alt="enter image description here"></p>

CC BY-SA 4.0
2019-07-24T02:48:41.193Z
нαηαzσησ lαη∂ commented: "at the end of the data collection there were over 5 petabytes of data stored in over 100 stacks of hard drives like the ones on the picture." Also, Peter Telford "You'd think black hole data would compress really well ."
Keith McClary
CC BY-SA 4.0
2019-07-24T02:56:03.627Z
@KeithMcClary hah!
uhoh
CC BY-SA 4.0
2022-05-15T01:54:30.923Z
@KeithMcClary okay now I am curious, where are these quotes from exactly?
uhoh
CC BY-SA 4.0
2019-07-20T00:32:30.537Z
2019-07-20T00:32:30.537Z
uhoh
<p>The BBC article <a href="http://www.bbc.com/news/science-environment-38937141" rel="nofollow noreferrer">Event Horizon Telescope ready to image black hole</a> describes the <a href="https://en.wikipedia.org/wiki/Event_Horizon_Telescope" rel="nofollow noreferrer">Event Horizon Telescope</a>, a coordinated observing technique with several radio telescope arrays across the globe forming a synthetic aperture with an Earth-sized baseline.</p>
<p><span class="math-container">$$\frac{\lambda}{r_{Earth}} \sim \frac{r_{Sag A*}}{D_{Sag A*}} \sim  10^{-11}$$</span></p>
<p>...when plugging in 1 millimeter for <span class="math-container">$\lambda$</span>, and with <span class="math-container">$r_{Sag A*}$</span> and <span class="math-container">$D_{Sag A*}$</span> the radius of, and distance from Earth to Sagittarius A*, the black hole in the center of the Milky Way Galaxy of 20 million km and 26,000 light years, respectively. (values taken from the article).</p>
<p>The equation I've written shows that millimeter wavelength interferometry with an Earth-sized baseline has the possibility to resolve the existence of some structure with scale of the black hole's event horizon.</p>
<p>My question is <em>How does the Event Horizon Telescope implement the interferometry?</em> It would certainly be impossible to bring all signals together to a central site and perform the interference there in real time as down-converted analog signals, and quite difficult/expensive to do it with dedicated, synchronized digital optical fiber lines. Is the massive amounts of data sent as IP packets over the internet to a central correlator (numerical interferometer)?</p>
<p>The article mentions atomic clocks and lots of hard drives, and I have a hunch these have something to do with it.</p>
<p><a href="https://i.sstatic.net/Ndfbs.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Ndfbs.png" alt="The eventual EHT array will have 12 widely spaced participating radio facilities&quot;. From The BBC's February 16, 2017 &quot;Event Horizon Telescope ready to image black hole&quot; http://www.bbc.com/news/science-environment-38937141" /></a></p>
<p><strong>above:</strong> &quot;The eventual EHT array will have 12 widely spaced participating radio facilities&quot;. From The BBC's February 16, 2017 <a href="http://www.bbc.com/news/science-environment-38937141" rel="nofollow noreferrer">Event Horizon Telescope ready to image black hole</a>.</p>

CC BY-SA 3.0
2017-02-17T09:44:10.717Z
If I had to guess - since you can't synchronize these telescopes in real time - it is that each telescope is programmed to take measurements at predefined points in time on the atomic clock and then the interference is computed on a central server (or rather several servers) for all the data recorded. Would be a very complex setup that involves some pretty nasty calculations, but could actually be possible.
Adwaenyth
<black-hole><radio-astronomy><sagittarius-a>
How does the Event Horizon Telescope implement the interferometry?
uhoh
# How does the Event Horizon Telescope implement the interferometry?

(Asked by: username_0 on 2017-02-17)

The BBC article [Event Horizon Telescope ready to image black hole](http://www.bbc.com/news/science-environment-38937141) describes the [Event Horizon Telescope](https://en.wikipedia.org/wiki/Event_Horizon_Telescope), a coordinated observing technique with several radio telescope arrays across the globe forming a synthetic aperture with an Earth-sized baseline.

$$\frac{\lambda}{r\_{Earth}} \sim \frac{r\_{Sag A\*}}{D\_{Sag A\*}} \sim 10^{-11}$$

...when plugging in 1 millimeter for $\lambda$, and with $r\_{Sag A\*}$ and $D\_{Sag A\*}$ the radius of, and distance from Earth to Sagittarius A\*, the black hole in the center of the Milky Way Galaxy of 20 million km and 26,000 light years, respectively. (values taken from the article).

The equation I've written shows that millimeter wavelength interferometry with an Earth-sized baseline has the possibility to resolve the existence of some structure with scale of the black hole's event horizon.

My question is *How does the Event Horizon Telescope implement the interferometry?* It would certainly be impossible to bring all signals together to a central site and perform the interference there in real time as down-converted analog signals, and quite difficult/expensive to do it with dedicated, synchronized digital optical fiber lines. Is the massive amounts of data sent as IP packets over the internet to a central correlator (numerical interferometer)?

The article mentions atomic clocks and lots of hard drives, and I have a hunch these have something to do with it.

[![The eventual EHT array will have 12 widely spaced participating radio facilities](https://i.sstatic.net/Ndfbs.png)](https://i.sstatic.net/Ndfbs.png)

**above:** "The eventual EHT array will have 12 widely spaced participating radio facilities". From The BBC's February 16, 2017 [Event Horizon Telescope ready to image black hole](http://www.bbc.com/news/science-environment-38937141).

**Tags:** black-hole, radio-astronomy, sagittarius-a
> **Comments:**
> **username_1**: If I had to guess - since you can't synchronize these telescopes in real time - it is that each telescope is programmed to take measurements at predefined points in time on the atomic clock and then the interference is computed on a central server (or rather several servers) for all the data recorded. Would be a very complex setup that involves some pretty nasty calculations, but could actually be possible.  

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## 1 answers

### Answer by username_0 on 2019-07-20. Score: 1

Supplementary graphical representation for the [accepted answer](https://astronomy.stackexchange.com/a/20086/7982), from [tweet](https://twitter.com/MIT_CSAIL/status/1116007460039483392/photo/1):

![enter image description here](https://i.sstatic.net/4ugoo.jpg)

> **Comments:**
> **username_2**: нαηαzσησ lαη∂ commented: "at the end of the data collection there were over 5 petabytes of data stored in over 100 stacks of hard drives like the ones on the picture." Also, Peter Telford "You'd think black hole data would compress really well ."  
> **username_0**: @KeithMcClary hah!  
> **username_0**: @KeithMcClary okay now I am curious, where are these quotes from exactly?  

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