<blockquote>
<p><strong>Reference request</strong> (explaining) how optical correlators combine light from multiple telescopes to produce ultra-high resolution interferometric images?</p>
</blockquote>
<p>There's some good information to be found in the following:</p>
<p>GRAVITY Instrument Description:
<a href="https://www.eso.org/sci/facilities/paranal/instruments/gravity/inst.html" rel="nofollow noreferrer">https://www.eso.org/sci/facilities/paranal/instruments/gravity/inst.html</a></p>
<p>GRAVITY Documentation (including user manuals):
<a href="https://www.eso.org/sci/facilities/paranal/instruments/gravity/doc.html" rel="nofollow noreferrer">https://www.eso.org/sci/facilities/paranal/instruments/gravity/doc.html</a></p>
<p>And a nice animation of the path of a light ray through GRAVITY: <a href="https://www.eso.org/public/videos/eso1622b/" rel="nofollow noreferrer">https://www.eso.org/public/videos/eso1622b/</a></p>
<p>Other files:
<a href="https://www.eso.org/sci/facilities/paranal/instruments/gravity.html#par_title" rel="nofollow noreferrer">https://www.eso.org/sci/facilities/paranal/instruments/gravity.html#par_title</a></p>
<hr />
<p><div class="youtube-embed"><div>
                <iframe width="640px" height="395px" src="https://www.youtube.com/embed/Zc2oMnTB08E?start=0"></iframe>
            </div></div></p>

CC BY-SA 4.0
2022-09-03T04:18:27.317Z
These are excellent; just what I was looking for. *Welcome to Astronomy SE!*
uhoh
CC BY-SA 4.0
2022-09-03T05:45:11.427Z
@antimony Well done, you did the impossible. You made uhoh happy. ;)
Ingolifs
CC BY-SA 4.0
2022-09-03T15:44:33.177Z
@Ingolifs [*uhoh!*](https://youtu.be/j6p2MoVtt8o)
uhoh
CC BY-SA 4.0
2022-09-03T04:15:34.083Z
2022-09-03T04:17:19.260Z
2022-09-03T04:17:19.260Z
antimony
<p>This is a reference or resource-request because it may be too challenging to explain in an answer post, but if you'd like to attempt a short summary as well, that will be great!</p>
<p>I have a <em>basic</em> understanding how correlators take down-converted radio signals from multiple dishes and build an image. Each dish delivers a 1 dimensional time-series of amplitude, and each pixel in the final image is somehow obtained through correlation of pairs of those signals (rathe than just summing all of them in simple interferometry) with specific time delays introduced for each dish corresponding to the position in the sky that that pixel will represents.</p>
<p>These correlator computers are huge because the data is already massive and to keep up with the yearly throughput of the telescope they need to process it at near-real-time speed even if it could be recorded.</p>
<p>The only exception I know of is the Event Horizon Telescope which collects data in short campaigns then allows for extended periods of time for analysis (and of course it takes a while to collect all the boxes of hard drives with the data)</p>
<p>For more on radio telescope correlators see questions and answers:</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/28666/7982">How does ALMA produce stable, mutually coherent ~THz local oscillators for all of their dishes?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/32014/7982">How would a Fast Fourier Transform Telescope work without a mirror, dish or lenses?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/31655/7982">Which techniques are used to convert radio signals received by antenna to images?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/36776/7982">How is the field of view of a radio telescope determined?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/14351/7982">Could mirrors be replaced with CCDs?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/20082/7982">How does the Event Horizon Telescope implement the interferometry?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/30252/7982">Is the delay to correlate the signal from one dish to another on a VLA radio telescope longer for red shifted objects?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/34566/7982">Would &quot;layered&quot; radio interferometry work?</a></li>
</ul>
<hr />
<h3>But there are big, mysterious-looking optical boxes</h3>
<p>that can <em>combine light from several telescopes</em> and produce images, and I haven't a clue how these work because of the potential added dimensionality: each telescope can potentially provide the amplitude from a small 2D section of their focal plane as a function of time.</p>
<p><strong>Question:</strong> I want to understand how optical correlators combine light from multiple telescopes interferometrically to produce ultra-high resolution images. I would like to read a few resources that explain the basics using mathematics, diagrams, simulations, etc.</p>
<p>If one is up to it, a short summary, perhaps addressing the <em>potentially</em> higher dimensionality of the focal plane amplitude per telescope vs single-feed amplitude per dish of a radio telescope correlator would be great!</p>
<hr />
<p>From <a href="https://astronomy.stackexchange.com/q/43253/7982">Was GRAVITY built to look at one star?</a></p>
<p><a href="https://i.sstatic.net/vO5M2.jpg" rel="noreferrer"><img src="https://i.sstatic.net/vO5M2.jpg" alt="GRAVITY is a second generation instrument for the VLT Interferometer and will allows the measurement of the positions and motions of astronomical objects on scales far smaller than (was) currently possible." /></a></p>
<p><a href="https://commons.wikimedia.org/wiki/File:GRAVITY_A_powerful_new_probe_of_black_holes.jpg" rel="noreferrer">Source</a></p>
<blockquote>
<p>A new instrument called GRAVITY has been shipped to Chile and successfully assembled and tested at the Paranal Observatory. GRAVITY is a second generation instrument for the VLT Interferometer and <s>will</s> allow(s) the measurement of the positions and motions of astronomical objects on scales far smaller than <s>is</s> was <s>currently</s> previously possible. The picture shows the instrument under test at the Paranal Observatory in July 2015.</p>
</blockquote>
<p>As mysterious-looking as a <a href="https://en.wikipedia.org/wiki/Guild_Navigator" rel="noreferrer">Guild Navigator</a> from David Lynch's <a href="https://en.wikipedia.org/wiki/Dune_(1984_film)" rel="noreferrer">Dune</a>:</p>
<p><a href="https://i.sstatic.net/XU9Ol.jpg" rel="noreferrer"><img src="https://i.sstatic.net/XU9Ol.jpg" alt="enter image description here" /></a></p>
<p><a href="https://www.youtube.com/watch?v=VSV9S2Ol3bc" rel="noreferrer">sharpened screenshot</a></p>

CC BY-SA 4.0
2021-06-28T18:39:46.547Z
Maybe [these lectures](http://www.jmmc.fr/twiki/bin/view/Jmmc/VltiSchool2018/SchoolLectures)  from 2018 VLTI School of Interferometry might be useful ? Covers basics and the GRAVITY and MATISSE instruments.
astrosnapper
CC BY-SA 4.0
2021-06-28T21:20:28.257Z
Have you seen the documentary about black holes? Featuring Hawking, Strominger, and the Event Horizon team? Nothing technical is said but it took a supercomputer to combine pentabytes of information from the different sites.
Deschele Schilder
CC BY-SA 4.0
2021-06-28T21:37:53.713Z
@astrosnapper It's a reference-request question and the three PDFs together (those for the GRAVITY and MATISSE instruments) are *exactly answer I need*, what could be better than lecture notes from a "VLTI School of Interferometry"? These are excellent! Please consider reposting your comment as an answer so that I can accept it thereby notifying future readers that this question has an accepted answer. *Thank you!*
uhoh
<observational-astronomy><resource-request><interferometry><gravity-vlt-interferometer>
Reference request (explaining) how optical correlators combine light from multiple telescopes to produce ultra-high resolution interferometric images?
uhoh
# Reference request (explaining) how optical correlators combine light from multiple telescopes to produce ultra-high resolution interferometric images?

(Asked by: username_0 on 2021-06-27)

This is a reference or resource-request because it may be too challenging to explain in an answer post, but if you'd like to attempt a short summary as well, that will be great!

I have a *basic* understanding how correlators take down-converted radio signals from multiple dishes and build an image. Each dish delivers a 1 dimensional time-series of amplitude, and each pixel in the final image is somehow obtained through correlation of pairs of those signals (rathe than just summing all of them in simple interferometry) with specific time delays introduced for each dish corresponding to the position in the sky that that pixel will represents.

These correlator computers are huge because the data is already massive and to keep up with the yearly throughput of the telescope they need to process it at near-real-time speed even if it could be recorded.

The only exception I know of is the Event Horizon Telescope which collects data in short campaigns then allows for extended periods of time for analysis (and of course it takes a while to collect all the boxes of hard drives with the data)

For more on radio telescope correlators see questions and answers:

* [How does ALMA produce stable, mutually coherent ~THz local oscillators for all of their dishes?](https://astronomy.stackexchange.com/q/28666/7982)
* [How would a Fast Fourier Transform Telescope work without a mirror, dish or lenses?](https://astronomy.stackexchange.com/q/32014/7982)
* [Which techniques are used to convert radio signals received by antenna to images?](https://astronomy.stackexchange.com/q/31655/7982)
* [How is the field of view of a radio telescope determined?](https://astronomy.stackexchange.com/q/36776/7982)
* [Could mirrors be replaced with CCDs?](https://astronomy.stackexchange.com/q/14351/7982)
* [How does the Event Horizon Telescope implement the interferometry?](https://astronomy.stackexchange.com/q/20082/7982)
* [Is the delay to correlate the signal from one dish to another on a VLA radio telescope longer for red shifted objects?](https://astronomy.stackexchange.com/q/30252/7982)
* [Would "layered" radio interferometry work?](https://astronomy.stackexchange.com/q/34566/7982)

---

### But there are big, mysterious-looking optical boxes

that can *combine light from several telescopes* and produce images, and I haven't a clue how these work because of the potential added dimensionality: each telescope can potentially provide the amplitude from a small 2D section of their focal plane as a function of time.

**Question:** I want to understand how optical correlators combine light from multiple telescopes interferometrically to produce ultra-high resolution images. I would like to read a few resources that explain the basics using mathematics, diagrams, simulations, etc.

If one is up to it, a short summary, perhaps addressing the *potentially* higher dimensionality of the focal plane amplitude per telescope vs single-feed amplitude per dish of a radio telescope correlator would be great!

---

From [Was GRAVITY built to look at one star?](https://astronomy.stackexchange.com/q/43253/7982)

[![GRAVITY is a second generation instrument for the VLT Interferometer and will allows the measurement of the positions and motions of astronomical objects on scales far smaller than (was) currently possible.](https://i.sstatic.net/vO5M2.jpg)](https://i.sstatic.net/vO5M2.jpg)

[Source](https://commons.wikimedia.org/wiki/File:GRAVITY_A_powerful_new_probe_of_black_holes.jpg)

> A new instrument called GRAVITY has been shipped to Chile and successfully assembled and tested at the Paranal Observatory. GRAVITY is a second generation instrument for the VLT Interferometer and ~~will~~ allow(s) the measurement of the positions and motions of astronomical objects on scales far smaller than ~~is~~ was ~~currently~~ previously possible. The picture shows the instrument under test at the Paranal Observatory in July 2015.

As mysterious-looking as a [Guild Navigator](https://en.wikipedia.org/wiki/Guild_Navigator) from David Lynch's [Dune](https://en.wikipedia.org/wiki/Dune_(1984_film)):

[![enter image description here](https://i.sstatic.net/XU9Ol.jpg)](https://i.sstatic.net/XU9Ol.jpg)

[sharpened screenshot](https://www.youtube.com/watch?v=VSV9S2Ol3bc)

**Tags:** observational-astronomy, resource-request, interferometry, gravity-vlt-interferometer
> **Comments:**
> **username_1**: Maybe [these lectures](http://www.jmmc.fr/twiki/bin/view/Jmmc/VltiSchool2018/SchoolLectures)  from 2018 VLTI School of Interferometry might be useful ? Covers basics and the GRAVITY and MATISSE instruments.  
> **username_2**: Have you seen the documentary about black holes? Featuring Hawking, Strominger, and the Event Horizon team? Nothing technical is said but it took a supercomputer to combine pentabytes of information from the different sites.  
> **username_0**: @astrosnapper It's a reference-request question and the three PDFs together (those for the GRAVITY and MATISSE instruments) are *exactly answer I need*, what could be better than lecture notes from a "VLTI School of Interferometry"? These are excellent! Please consider reposting your comment as an answer so that I can accept it thereby notifying future readers that this question has an accepted answer. *Thank you!*  

---

## 1 answers

### Answer by username_3 on 2022-09-03. Score: 3 | Accepted answer

> **Reference request** (explaining) how optical correlators combine light from multiple telescopes to produce ultra-high resolution interferometric images?

There's some good information to be found in the following:

GRAVITY Instrument Description:
<https://www.eso.org/sci/facilities/paranal/instruments/gravity/inst.html>

GRAVITY Documentation (including user manuals):
<https://www.eso.org/sci/facilities/paranal/instruments/gravity/doc.html>

And a nice animation of the path of a light ray through GRAVITY: <https://www.eso.org/public/videos/eso1622b/>

Other files:
<https://www.eso.org/sci/facilities/paranal/instruments/gravity.html#par_title>

---

> **Comments:**
> **username_0**: These are excellent; just what I was looking for. *Welcome to Astronomy SE!*  
> **username_4**: @antimony Well done, you did the impossible. You made uhoh happy. ;)  
> **username_0**: @Ingolifs [*uhoh!*](https://youtu.be/j6p2MoVtt8o)  

---

