<p>This is a really good question in that there are good answers to it.</p>
<p>Alas I'm no expert, but I will get the ball rolling.</p>
<p>About the &quot;only four days&quot;, this is almost certainly only the period for the data that ended up in the final image. There is probably a much larger body of observing time spent in technology development. The EHT really pushed VLBI radio Astronomy to new limits, one of which is processing the recorded data offline to discover the exact distances and orientations of each telescope dish with respect to the other to sub-millimeter <em>accuracy</em>. For more on that see Physics SE's <a href="https://physics.stackexchange.com/q/473320/83380">Precise Event Horizon Telescope locations?</a></p>
<h3>The &quot;light&quot; (radio waves) in the image comes from swirling, changing hot gas in the accretion disk</h3>
<p>Now for the data contributing to the image, I can't state with certainty that this is the exact and complete answer, but it is certainly a limiting factor for exposure time.</p>
<p>The goal is to produce a static image, not a movie. To use interferometry to build up a clear &quot;snapshot&quot; requires the exposure time to be fast enough that the object doesn't blur.</p>
<p>The algorithms they used at the time to generate that image from that pretty sparsely populated array of telescopes, for which at any given moment a significant fraction can't see the target, assumed that the the target was static, that it didn't change. See</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/30311/7982">How can the Event Horizon Telescope image Sgr A* when it&#39;s not visible from all sites at one time?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/20082/7982">How does the Event Horizon Telescope implement the interferometry?</a></li>
</ul>
<p><a href="https://i.sstatic.net/Ndfbs.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Ndfbsm.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> (click for larger) &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>
<p>It's the same way we assume, or at least hope that our targets don't move much during our photograph exposure times of a fraction of a second, but for the EHT's image of M87* that shutter time was four days.</p>
<p>The problem of the motion of the material in the accretion disk is one reason that M87* was chosen instead of our own black hole SgrA* in the center of our own Milky Way galaxy. M87's black hole is much much larger than ours and the accretion disk is much brighter and more active at radio wavelengths, offsetting to some extent its larger distance, but because its event horizon and accretion disk are so much larger than ours, it changes much more slowly.</p>
<p>To my understanding that's why it was chosen as the EHT's first target, rather than SrgA*. For more on that see answers to:</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/39354/7982">How will they know when to start taking the picture of the black hole at the center of the Milky Way?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/39352/7982">Will the first Event Horizon Telescope image of the Milky Way&#39;s black hole Sgr A* just be another lumpy orange donut?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/30313/7982">Why didn&#39;t the Event Horizon Telescope team mention Sagittarius A*?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/30339/7982">Why not take a picture of a closer black hole?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/39340/7982">Why doesn&#39;t the black hole in the center of the Milky Way glow similarly to the famous M87 image?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/30441/7982">Why does the Event Horizon Telescope (EHT) not include telescopes from Africa, Asia or Australia?</a></li>
</ul>
<hr />
<p>Illustration of what photographing a spinning object might look like where the exposure time (shutter speed) is too long and the motion too fast:</p>
<p><a href="https://i.sstatic.net/OHuc0.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/OHuc0m.jpg" alt="clothes blurred in photograph of a spinning washing machine with the top open from https://jerrysappliancerepair.com/washer-repair/why-wont-my-washing-machine-stop-spinning/" /></a></p>
<p><a href="https://jerrysappliancerepair.com/washer-repair/why-wont-my-washing-machine-stop-spinning/" rel="nofollow noreferrer">Source</a></p>

CC BY-SA 4.0
2022-07-10T10:24:35.383Z
Another factor is the fact that the telescopes making up the EHT are used for many different thing in addition to EHT observations, so the EHT team did not have (potentially) unlimited time for their observations.
Peter Erwin
CC BY-SA 4.0
2022-07-10T11:12:44.690Z
@PeterErwin Presumably the whole EHT collaboration and effort is based on some serious long-term commitment by each of them to make it happen and and it's quite possible EHT publications provide a level of visibility and funding leveraging that might not have been possible for each instrument on its own. So I think that's not at all a limitation - every group in the collaboration will do whatever it will take to make the whole project a success. Everybody wins. The whole is larger than the sum of its parts, etc.
uhoh
CC BY-SA 4.0
2022-07-10T13:35:45.797Z
The EHT collaboration are not the people in charge of the telescopes (there may be overlap individuals). While the collaboration may get some special privileges in the scheduling (as it needs all telescopes to be scheduled to be online at the same time). Requests for telescope time get reviewed by observation panels. While the science outcome is high impact, that does not mean they would be given unlimited telescope time to make it happen. They would have had to put in their request how many hours of observations they want.
Rob
CC BY-SA 4.0
2022-07-10T20:55:20.523Z
@Rob I'm not seeing "unlimited" mentioned anywhere, and I'm still having a difficult time believing that if X days of observing time are necessary or the the project fails that the review committee at one of the observatories will say "Tough noodles, you only get X/2." I have a hunch there was some substantial commitments made at administrative levels of each observatory from the beginning. But instead of folks leaving comments to me, perhaps someone should add an additional answer for the benefit fo the OP and future readers to the effect of "four days is all they could get"?
uhoh
CC BY-SA 4.0
2022-07-09T01:02:21.877Z
2022-07-09T01:02:21.877Z
uhoh
<p>I wonder why only 4 days of data were used in the M87 Event Horizon Telescope (EHT) observation. What is preventing you from obtaining a more detailed and clearer image over a longer period of time?</p>

CC BY-SA 4.0
2022-07-08T20:41:36.153Z
would be nice, if you could quote the source of your knowledge
planetmaker
CC BY-SA 4.0
2022-07-08T22:46:06.900Z
Generally speaking, time on the scopes are very valuable, and observations are limited to what is strictly necessary to collect only the data they're after.
Greg Miller
CC BY-SA 4.0
2022-07-12T00:05:16.890Z
@planetmaker "First M87 Event Horizon Telescope Results. I.
The Shadow of the Supermassive Black Hole"
Aegean
<observational-astronomy><supermassive-black-hole><m87>
Why were only 4 days of data used in the M87 EHT observation?
Aegean
# Why were only 4 days of data used in the M87 EHT observation?

(Asked by: username_0 on 2022-07-08)

I wonder why only 4 days of data were used in the M87 Event Horizon Telescope (EHT) observation. What is preventing you from obtaining a more detailed and clearer image over a longer period of time?

**Tags:** observational-astronomy, supermassive-black-hole, m87

> **Comments:**
> **username_1**: would be nice, if you could quote the source of your knowledge  
> **username_2**: Generally speaking, time on the scopes are very valuable, and observations are limited to what is strictly necessary to collect only the data they're after.  
> **username_0**: @username_1 "First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole"  

---

## 1 answers

### Answer by username_3 on 2022-07-09. Score: 4 | Accepted answer

This is a really good question in that there are good answers to it.

Alas I'm no expert, but I will get the ball rolling.

About the "only four days", this is almost certainly only the period for the data that ended up in the final image. There is probably a much larger body of observing time spent in technology development. The EHT really pushed VLBI radio Astronomy to new limits, one of which is processing the recorded data offline to discover the exact distances and orientations of each telescope dish with respect to the other to sub-millimeter *accuracy*. For more on that see Physics SE's [Precise Event Horizon Telescope locations?](https://physics.stackexchange.com/q/473320/83380)

### The "light" (radio waves) in the image comes from swirling, changing hot gas in the accretion disk

Now for the data contributing to the image, I can't state with certainty that this is the exact and complete answer, but it is certainly a limiting factor for exposure time.

The goal is to produce a static image, not a movie. To use interferometry to build up a clear "snapshot" requires the exposure time to be fast enough that the object doesn't blur.

The algorithms they used at the time to generate that image from that pretty sparsely populated array of telescopes, for which at any given moment a significant fraction can't see the target, assumed that the the target was static, that it didn't change. See

* [How can the Event Horizon Telescope image Sgr A\* when it's not visible from all sites at one time?](https://astronomy.stackexchange.com/q/30311/7982)
* [How does the Event Horizon Telescope implement the interferometry?](https://astronomy.stackexchange.com/q/20082/7982)

[![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](https://i.sstatic.net/Ndfbsm.png)](https://i.sstatic.net/Ndfbs.png)

**above:** (click for larger) "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).

It's the same way we assume, or at least hope that our targets don't move much during our photograph exposure times of a fraction of a second, but for the EHT's image of M87\* that shutter time was four days.

The problem of the motion of the material in the accretion disk is one reason that M87\* was chosen instead of our own black hole SgrA\* in the center of our own Milky Way galaxy. M87's black hole is much much larger than ours and the accretion disk is much brighter and more active at radio wavelengths, offsetting to some extent its larger distance, but because its event horizon and accretion disk are so much larger than ours, it changes much more slowly.

To my understanding that's why it was chosen as the EHT's first target, rather than SrgA\*. For more on that see answers to:

* [How will they know when to start taking the picture of the black hole at the center of the Milky Way?](https://astronomy.stackexchange.com/q/39354/7982)
* [Will the first Event Horizon Telescope image of the Milky Way's black hole Sgr A\* just be another lumpy orange donut?](https://astronomy.stackexchange.com/q/39352/7982)
* [Why didn't the Event Horizon Telescope team mention Sagittarius A\*?](https://astronomy.stackexchange.com/q/30313/7982)
* [Why not take a picture of a closer black hole?](https://astronomy.stackexchange.com/q/30339/7982)
* [Why doesn't the black hole in the center of the Milky Way glow similarly to the famous M87 image?](https://astronomy.stackexchange.com/q/39340/7982)
* [Why does the Event Horizon Telescope (EHT) not include telescopes from Africa, Asia or Australia?](https://astronomy.stackexchange.com/q/30441/7982)

---

Illustration of what photographing a spinning object might look like where the exposure time (shutter speed) is too long and the motion too fast:

[![clothes blurred in photograph of a spinning washing machine with the top open from https://jerrysappliancerepair.com/washer-repair/why-wont-my-washing-machine-stop-spinning/](https://i.sstatic.net/OHuc0m.jpg)](https://i.sstatic.net/OHuc0.jpg)

[Source](https://jerrysappliancerepair.com/washer-repair/why-wont-my-washing-machine-stop-spinning/)

> **Comments:**
> **username_4**: Another factor is the fact that the telescopes making up the EHT are used for many different thing in addition to EHT observations, so the EHT team did not have (potentially) unlimited time for their observations.  
> **username_3**: @username_4 Presumably the whole EHT collaboration and effort is based on some serious long-term commitment by each of them to make it happen and and it's quite possible EHT publications provide a level of visibility and funding leveraging that might not have been possible for each instrument on its own. So I think that's not at all a limitation - every group in the collaboration will do whatever it will take to make the whole project a success. Everybody wins. The whole is larger than the sum of its parts, etc.  
> **username_5**: The EHT collaboration are not the people in charge of the telescopes (there may be overlap individuals). While the collaboration may get some special privileges in the scheduling (as it needs all telescopes to be scheduled to be online at the same time). Requests for telescope time get reviewed by observation panels. While the science outcome is high impact, that does not mean they would be given unlimited telescope time to make it happen. They would have had to put in their request how many hours of observations they want.  
> **username_3**: @username_5 I'm not seeing "unlimited" mentioned anywhere, and I'm still having a difficult time believing that if X days of observing time are necessary or the the project fails that the review committee at one of the observatories will say "Tough noodles, you only get X/2." I have a hunch there was some substantial commitments made at administrative levels of each observatory from the beginning. But instead of folks leaving comments to me, perhaps someone should add an additional answer for the benefit fo the OP and future readers to the effect of "four days is all they could get"?  

---

