<p>The resolution of an interferometer at a wavelength <span class="math-container">$\lambda$</span> is <span class="math-container">$\theta\sim\lambda/b$</span>, where <span class="math-container">$b$</span> is the longest baseline (distance between two telescopes) in the array. <a href="https://iopscience.iop.org/article/10.3847/2041-8213/ab0ec7#apjlab0ec7s3" rel="nofollow noreferrer">According to the first of this week's six papers</a>, with observations at <span class="math-container">$\lambda=1.3\text{ mm}$</span>, the longest baseline was <span class="math-container">$b\approx10700\text{ km}$</span>, giving us <span class="math-container">$\theta\sim25.1\mu\text{as}$</span>. <a href="https://eventhorizontelescope.org/building-larger-array" rel="nofollow noreferrer">The team expects this limit to be lowered even further</a> with the addition of more observatories.</p>

<p>The picture, of course, seems much finer. The above limit can be lowered a bit by algorithms like <a href="https://iopscience.iop.org/article/10.3847/2041-8213/ab0e85" rel="nofollow noreferrer">CLEAN or regularized maximum likelihood</a>; this brings the resolution down to about <span class="math-container">$20\mu\text{as}$</span>. This still doesn't explain why the image seems even finer than that figure; that comes from image synthesis techniques that smooth out the data, rather than leaving it in a coarser form.</p>

CC BY-SA 4.0
2019-04-12T19:16:30.940Z
2019-04-12T19:16:30.940Z
HDE 226868
<p>I am quite impressed with the recently published image of M87* black hole. It appears that this image achieves spatial resolution well above and beyond what was available until today. But how high this resolution is, exactly?</p>

<p>The article "<a href="https://iopscience.iop.org/article/10.3847/2041-8213/ab0ec7/meta" rel="nofollow noreferrer">First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole</a>" states that Event Horizon Telescope has theoretical diffraction-limit resolution of ~25 μas.</p>

<p><a href="https://en.wikipedia.org/wiki/Messier_87#Supermassive_black_hole" rel="nofollow noreferrer">Another wiki article</a> states that M87* has Swartzschild radius of 5.9×10<sup>-4</sup> parsecs. Combined with the distance of 16.4 million parsecs, this gives me angular size of 14.8 microarceconds.</p>

<p>Even with the shadow of the black hole being 2.5 bigger that its radius, the resolution of a widely distributed picture seems to be much higher than 25 microarcseconds stated above. Is the image actually blown up well beyond its original resolution (i.e. becoming more a less an "artistic impression"), or Event Horizon Telescope is actually capable of higher resolutions (10 microarcseconds or less)?</p>

<p><a href="https://i.sstatic.net/YIdm6.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/YIdm6.png" alt="M87* image"></a></p>

CC BY-SA 4.0
2019-04-12T18:50:22.523Z
$\lambda$/R$_{Earth}$ in radians
eshaya
<supermassive-black-hole><angular-resolution><event-horizon-telescope>
What is the actual resolution of Event Horizon Telescope?
Alexander
# What is the actual resolution of Event Horizon Telescope?

(Asked by: username_0 on 2019-04-12)

I am quite impressed with the recently published image of M87\* black hole. It appears that this image achieves spatial resolution well above and beyond what was available until today. But how high this resolution is, exactly?

The article "[First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole](https://iopscience.iop.org/article/10.3847/2041-8213/ab0ec7/meta)" states that Event Horizon Telescope has theoretical diffraction-limit resolution of ~25 μas.

[Another wiki article](https://en.wikipedia.org/wiki/Messier_87#Supermassive_black_hole) states that M87\* has Swartzschild radius of 5.9×10-4 parsecs. Combined with the distance of 16.4 million parsecs, this gives me angular size of 14.8 microarceconds.

Even with the shadow of the black hole being 2.5 bigger that its radius, the resolution of a widely distributed picture seems to be much higher than 25 microarcseconds stated above. Is the image actually blown up well beyond its original resolution (i.e. becoming more a less an "artistic impression"), or Event Horizon Telescope is actually capable of higher resolutions (10 microarcseconds or less)?

[![M87* image](https://i.sstatic.net/YIdm6.png)](https://i.sstatic.net/YIdm6.png)

**Tags:** supermassive-black-hole, angular-resolution, event-horizon-telescope

> **Comments:**
> **username_1**: $\lambda$/R$_{Earth}$ in radians  

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## 1 answers

### Answer by username_2 on 2019-04-12. Score: 2

The resolution of an interferometer at a wavelength $\lambda$ is $\theta\sim\lambda/b$, where $b$ is the longest baseline (distance between two telescopes) in the array. [According to the first of this week's six papers](https://iopscience.iop.org/article/10.3847/2041-8213/ab0ec7#apjlab0ec7s3), with observations at $\lambda=1.3\text{ mm}$, the longest baseline was $b\approx10700\text{ km}$, giving us $\theta\sim25.1\mu\text{as}$. [The team expects this limit to be lowered even further](https://eventhorizontelescope.org/building-larger-array) with the addition of more observatories.

The picture, of course, seems much finer. The above limit can be lowered a bit by algorithms like [CLEAN or regularized maximum likelihood](https://iopscience.iop.org/article/10.3847/2041-8213/ab0e85); this brings the resolution down to about $20\mu\text{as}$. This still doesn't explain why the image seems even finer than that figure; that comes from image synthesis techniques that smooth out the data, rather than leaving it in a coarser form.


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