<p>A 130m baseline operating at 2 microns gives a theoretical resolution of <span class="math-container">$2\times 10^{-6}/130$</span> radians. At a distance of 400,000 km this translates to 6m.</p>

<p>My guess is that Genzel is referring to the accuracy with which the position of a point source of light can be measured. This not the same thing as the smallest thing you can image. The centroid of a Gaussian can be measured much more precisely than its FWHM.</p>

<p>The moon landing sites have been photographed by the lunar reconnaissance orbiter. If the 'foil hat brigade don't believe that, they aren't going to believe a reconstructed interferometric image in any case.</p>

CC BY-SA 4.0
2020-04-16T11:51:37.053Z
Yes this makes absolute sense, thanks! just fyi [I'm a card-carrying member](https://space.stackexchange.com/questions/26889/why-is-sentinel-3bs-dish-antenna-overwrapped-with-metallized-film/31650#comment79813_26889) of the foil hat brigade.
uhoh
CC BY-SA 4.0
2020-04-16T10:25:53.430Z
2020-04-16T10:33:59.407Z
2020-04-16T10:33:59.407Z
ProfRob
<p>Phys.org's <a href="https://phys.org/news/2020-04-eso-telescope-star-supermassive-black.html" rel="nofollow noreferrer">Very Large Telescope sees star dance around supermassive black hole, proves Einstein right</a> links to several ESO videos, including <a href="https://youtu.be/OgwrnLOXhNA?t=531" rel="nofollow noreferrer">Interview with Reinhard Genzel (in English)</a>. After <code>08:51</code> in the video professor Genzel says:</p>
<blockquote>
<p>So we take four 8 meter class telescopes of the European Southern Observatory, the Very Large Telescope in Chile, and combined it optically to an equivalent 130 meter telescope, and the bigger the telescope, the better the resolution.</p>
<p><strong>So with this interferometer, called Gravity, we can measure basically a few centimeters on the Moon if you like</strong>, and that gives us the precision to really you know <em>nail the motion</em> of a star around a black hole.</p>
</blockquote>
<p>To the question <em>Is seeing the apollo moon Landers via earth telescope that hard?</em> I recently wrote <a href="https://astronomy.stackexchange.com/a/33256/7982">Yes, it still is.</a> and then asked <a href="https://astronomy.stackexchange.com/q/33914/7982">Will the Magdalena Ridge Optical Interferometer be able to image extended objects like the surface of the Moon?</a>, but if Gravity can resolve a few centimeters on the Moon then it should be capable of photographing all of the Moon landing sites.</p>
<p><strong>Question:</strong> Did professor Genzel mis-speak, perhaps meaning that the resolution is a few centimeters at the distance of the International Space Station, or does Gravity at least theoretically have a resolution of a few centimeters at the distance of the Moon?</p>
<p>I know that three orders of magnitude is fairly small for astrophysicists, but since the reference is <em>The Moon</em> and there's stuff on it that we'd like to see, in this case <em>enquiring minds want to know!</em></p>
<p>FYI there is also <a href="https://youtu.be/m4dZskdhA98" rel="nofollow noreferrer">Interview with Reinhard Genzel (<strong>in German</strong>)</a>; it might be interesting to compare.</p>

CC BY-SA 4.0
2023-07-13T11:45:57.150Z
Just sat through a talk by Genzel in which he said more or less the same thing and indeed he does mean the accuracy with which the position of something that can be measured rather than the angular size of something that can be resolved.
ProfRob
CC BY-SA 4.0
2023-07-14T00:57:11.200Z
@ProfRob thanks, and lucky you! I wonder if it included anything that might be helpful here: [Reference request (explaining) how optical correlators combine light from multiple telescopes to produce ultra-high resolution interferometric images?](https://astronomy.stackexchange.com/q/44591/7982) It's still literally a black box to me. I haven't a clue so anything will likely be helpful.
uhoh
<observational-astronomy><telescope><optics><interferometry><angular-resolution>
Can the interferometer called "Gravity" measure "a few centimeters on the Moon"?
uhoh
# Can the interferometer called "Gravity" measure "a few centimeters on the Moon"?

(Asked by: username_0 on 2020-04-16)

Phys.org's [Very Large Telescope sees star dance around supermassive black hole, proves Einstein right](https://phys.org/news/2020-04-eso-telescope-star-supermassive-black.html) links to several ESO videos, including [Interview with Reinhard Genzel (in English)](https://youtu.be/OgwrnLOXhNA?t=531). After `08:51` in the video professor Genzel says:

> So we take four 8 meter class telescopes of the European Southern Observatory, the Very Large Telescope in Chile, and combined it optically to an equivalent 130 meter telescope, and the bigger the telescope, the better the resolution.
>
> **So with this interferometer, called Gravity, we can measure basically a few centimeters on the Moon if you like**, and that gives us the precision to really you know *nail the motion* of a star around a black hole.

To the question *Is seeing the apollo moon Landers via earth telescope that hard?* I recently wrote [Yes, it still is.](https://astronomy.stackexchange.com/a/33256/7982) and then asked [Will the Magdalena Ridge Optical Interferometer be able to image extended objects like the surface of the Moon?](https://astronomy.stackexchange.com/q/33914/7982), but if Gravity can resolve a few centimeters on the Moon then it should be capable of photographing all of the Moon landing sites.

**Question:** Did professor Genzel mis-speak, perhaps meaning that the resolution is a few centimeters at the distance of the International Space Station, or does Gravity at least theoretically have a resolution of a few centimeters at the distance of the Moon?

I know that three orders of magnitude is fairly small for astrophysicists, but since the reference is *The Moon* and there's stuff on it that we'd like to see, in this case *enquiring minds want to know!*

FYI there is also [Interview with Reinhard Genzel (**in German**)](https://youtu.be/m4dZskdhA98); it might be interesting to compare.

**Tags:** observational-astronomy, telescope, optics, interferometry, angular-resolution

> **Comments:**
> **username_1**: Just sat through a talk by Genzel in which he said more or less the same thing and indeed he does mean the accuracy with which the position of something that can be measured rather than the angular size of something that can be resolved.  
> **username_0**: @username_1 thanks, and lucky you! I wonder if it included anything that might be helpful here: [Reference request (explaining) how optical correlators combine light from multiple telescopes to produce ultra-high resolution interferometric images?](https://astronomy.stackexchange.com/q/44591/7982) It's still literally a black box to me. I haven't a clue so anything will likely be helpful.  

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## 1 answers

### Answer by username_1 on 2020-04-16. Score: 7 | Accepted answer

A 130m baseline operating at 2 microns gives a theoretical resolution of $2\times 10^{-6}/130$ radians. At a distance of 400,000 km this translates to 6m.

My guess is that Genzel is referring to the accuracy with which the position of a point source of light can be measured. This not the same thing as the smallest thing you can image. The centroid of a Gaussian can be measured much more precisely than its FWHM.

The moon landing sites have been photographed by the lunar reconnaissance orbiter. If the 'foil hat brigade don't believe that, they aren't going to believe a reconstructed interferometric image in any case.

> **Comments:**
> **username_0**: Yes this makes absolute sense, thanks! just fyi [I'm a card-carrying member](https://space.stackexchange.com/questions/26889/why-is-sentinel-3bs-dish-antenna-overwrapped-with-metallized-film/31650#comment79813_26889) of the foil hat brigade.  

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