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:

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?

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 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 from David Lynch's Dune:

enter image description here

sharpened screenshot

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    $\begingroup$ Maybe these lectures from 2018 VLTI School of Interferometry might be useful ? Covers basics and the GRAVITY and MATISSE instruments. $\endgroup$ Jun 28, 2021 at 18:39
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    $\begingroup$ 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. $\endgroup$ Jun 28, 2021 at 21:20
  • $\begingroup$ @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! $\endgroup$
    – uhoh
    Jun 28, 2021 at 21:37

1 Answer 1


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

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    $\begingroup$ These are excellent; just what I was looking for. Welcome to Astronomy SE! $\endgroup$
    – uhoh
    Sep 3, 2022 at 4:18
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    $\begingroup$ @antimony Well done, you did the impossible. You made uhoh happy. ;) $\endgroup$
    – Ingolifs
    Sep 3, 2022 at 5:45
  • $\begingroup$ @Ingolifs uhoh! $\endgroup$
    – uhoh
    Sep 3, 2022 at 15:44

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