<blockquote>
<p>What part of the EM spectrum was used in the black hole image?... Is it an image in visible light, infra-red or longer?</p>
</blockquote>
<h2>Microwaves, (<a href="https://en.wikipedia.org/wiki/Extremely_high_frequency" rel="nofollow noreferrer">millimeter waves</a> actually), and the hairy edge of far-infrared</h2>
<p>at a nominal frequency of 230 GHz or 1.30 mm wavelength, the bandwidth is roughly 2 to 6 GHz wide depending on how many channels of data were used to produce the published image.</p>
<hr />
<p>From <a href="https://iopscience.iop.org/article/10.3847/2041-8213/ab0ec7" rel="nofollow noreferrer">First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole</a> the first of four articles published together:</p>
<blockquote>
<p><strong>4. Observations, Correlation, and Calibration</strong></p>
<p>We observed M87* on 2017 April 5, 6, 10, and 11 with the EHT. Weather was uniformly good to excellent with nightly median zenith atmospheric opacities at <strong>230 GHz</strong> ranging from 0.03 to 0.28 over the different locations. The observations were scheduled as a series of scans of three to seven minutes in duration, with M87* scans interleaved with those on the quasar 3C 279. The number of scans obtained on M87* per night ranged from 7 (April 10) to 25 (April 6) as a result of different observing schedules. A description of the M87* observations, their correlation, calibration, and validated final data products is presented in Paper III and briefly summarized here.</p>
<p>At each station, the astronomical signal in both polarizations <strong>and two adjacent 2 GHz wide frequency bands centered at 227.1 and 229.1 GHz</strong> were converted to baseband using standard heterodyne techniques, then digitized and recorded at a total rate of 32 Gbps.[...]</p>
</blockquote>
<p>So if we use 230 GHz, the wavelength is given by <span class="math-container">$c/f$</span> or 1.30 millimeters. It's hard for me to say right now if the image comes from only one 2GHz wide channel, or all three, which means that the bandwidth is either about 0.9% or  2.1%, but that's still pretty narrow compared to images taken at optical frequencies. That's (ultimately) because interferometry is done digitally these days and the computational size and time scales fairly fast with the size of the baseband.</p>
<p>I should note that these days it's more and more common for astronomers to refer to all kinds of different wavelengths as &quot;light&quot; in a loose way. Circa 1.3 millimeter wavelength certainly could be thought of as far-infrared, though <a href="https://en.wikipedia.org/wiki/Far_infrared" rel="nofollow noreferrer">Wikipedia</a> puts the cutoff at 1 mmm (300 GHz).</p>

CC BY-SA 4.0
2019-04-12T23:10:02.453Z
@user71659 you are right, I've made and edit. with your link thanks! While we don't call CMB the "[Cosmic Millimeter wave Background](https://en.wikipedia.org/wiki/Cosmic_microwave_background)", we do call ALMA the "[Atacama Large Millimeterr/submillimeter Array](https://en.wikipedia.org/wiki/Atacama_Large_Millimeter_Array)". I don't know what "Large Millimeters" are (humor!!) but I do at least know that [there are no large hadrons!](https://physics.stackexchange.com/q/454554/83380)!
uhoh
CC BY-SA 4.0
2019-04-11T10:10:25.397Z
2019-04-12T23:03:23.887Z
2020-06-17T09:47:40.653Z
uhoh
<p>The EHT has published the first image of a black hole.  It is the event horizon of the singularity in M87 against its accretion disk.  I've been through the reports in the popular press but none mention the light wavelengths this was taken at.  Is it an image in visible light, infra-red or longer?</p>

<p><a href="https://i.sstatic.net/j0EGm.jpg" rel="noreferrer"><img src="https://i.sstatic.net/j0EGm.jpg" alt="enter image description here"></a></p>

CC BY-SA 4.0
2019-04-11T10:45:21.123Z
@Hobbes I wonder then if it is time to see if we can better codify where the line is for the future? Maybe the existence these two tags unfairly invites people to ask questions that will then be closed for example. Maybe we can think of a "rule of thumb" that will help people know where better to ask. The problem with closing a good question as off-topic is that it either stays on hold, then closed (and is therefore rendered unanswerable) or eventually gets moved, which is extra work for the mods.
uhoh
CC BY-SA 4.0
2019-04-11T10:49:25.377Z
@DaveGremlin if you click the word "edited" below your question, to the left of your icon, you can see the [revision history of your question](https://space.stackexchange.com/posts/35446/revisions), and that I added it. I did primarily because the question turns out to be about radiotelescopes, and adding an appropriate tag makes it easier for future readers to find it. It also helps in the migration process I believe. There needs to be some overlap in tagging between the old site (here) and the new site (Astronomy) if it is to be migrated. This probably is a good candidate for that.
uhoh
CC BY-SA 4.0
2019-04-11T11:34:27.353Z
I'm not sure  "It is the event horizon of the singularity in M87 against its accretion disk." is a very accurate statement. The image arises from  the paths that microwaves from the accretion disk follow in this highly warped space-time. The end result is that fewer come to us along a straight path from the event horizon and more come from the sides, especially one side, but it would be a mistake to think of this as too direct a representation of what is there.
Steve Linton
CC BY-SA 4.0
2019-04-11T12:08:47.140Z
When the close votes are "this question belongs on another site", the 5th close vote will alert a moderator: https://meta.stackexchange.com/questions/85017/how-do-i-move-my-own-question-to-another-stack-exchange-site
Hobbes
CC BY-SA 4.0
2019-04-11T13:37:21.687Z
@Hobbes This has never happened for me.
called2voyage
CC BY-SA 4.0
2019-04-11T13:42:33.290Z
Interesting. A bug, perhaps?
Hobbes
CC BY-SA 4.0
2019-04-11T14:10:49.807Z
@Hobbes Maybe, I don't know. My recommendation is flag a question for moderator attention and mention migration, if you think it needs to be migrated and has some close votes. By the way, where at that link are you getting that information from? I don't see anything there that says the 5th close vote will alert a mod.
called2voyage
<radio-telescope><black-hole>
What part of the EM spectrum was used in the black hole image?
Dave Gremlin
# What part of the EM spectrum was used in the black hole image?

(Asked by: username_0 on 2019-04-11)

The EHT has published the first image of a black hole. It is the event horizon of the singularity in M87 against its accretion disk. I've been through the reports in the popular press but none mention the light wavelengths this was taken at. Is it an image in visible light, infra-red or longer?

[![enter image description here](https://i.sstatic.net/j0EGm.jpg)](https://i.sstatic.net/j0EGm.jpg)

**Tags:** radio-telescope, black-hole

> **Comments:**
> **username_1**: @username_3 I wonder then if it is time to see if we can better codify where the line is for the future? Maybe the existence these two tags unfairly invites people to ask questions that will then be closed for example. Maybe we can think of a "rule of thumb" that will help people know where better to ask. The problem with closing a good question as off-topic is that it either stays on hold, then closed (and is therefore rendered unanswerable) or eventually gets moved, which is extra work for the mods.  
> **username_1**: @username_0 if you click the word "edited" below your question, to the left of your icon, you can see the [revision history of your question](https://space.stackexchange.com/posts/35446/revisions), and that I added it. I did primarily because the question turns out to be about radiotelescopes, and adding an appropriate tag makes it easier for future readers to find it. It also helps in the migration process I believe. There needs to be some overlap in tagging between the old site (here) and the new site (Astronomy) if it is to be migrated. This probably is a good candidate for that.  
> **username_2**: I'm not sure  "It is the event horizon of the singularity in M87 against its accretion disk." is a very accurate statement. The image arises from  the paths that microwaves from the accretion disk follow in this highly warped space-time. The end result is that fewer come to us along a straight path from the event horizon and more come from the sides, especially one side, but it would be a mistake to think of this as too direct a representation of what is there.  
> **username_3**: When the close votes are "this question belongs on another site", the 5th close vote will alert a moderator: https://meta.stackexchange.com/questions/85017/how-do-i-move-my-own-question-to-another-stack-exchange-site  
> **username_4**: @username_3 This has never happened for me.  
> **username_3**: Interesting. A bug, perhaps?  
> **username_4**: @username_3 Maybe, I don't know. My recommendation is flag a question for moderator attention and mention migration, if you think it needs to be migrated and has some close votes. By the way, where at that link are you getting that information from? I don't see anything there that says the 5th close vote will alert a mod.  

---

## 1 answers

### Answer by username_1 on 2019-04-11. Score: 5 | Accepted answer

> What part of the EM spectrum was used in the black hole image?... Is it an image in visible light, infra-red or longer?

## Microwaves, ([millimeter waves](https://en.wikipedia.org/wiki/Extremely_high_frequency) actually), and the hairy edge of far-infrared

at a nominal frequency of 230 GHz or 1.30 mm wavelength, the bandwidth is roughly 2 to 6 GHz wide depending on how many channels of data were used to produce the published image.

---

From [First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole](https://iopscience.iop.org/article/10.3847/2041-8213/ab0ec7) the first of four articles published together:

> **4. Observations, Correlation, and Calibration**
>
> We observed M87\* on 2017 April 5, 6, 10, and 11 with the EHT. Weather was uniformly good to excellent with nightly median zenith atmospheric opacities at **230 GHz** ranging from 0.03 to 0.28 over the different locations. The observations were scheduled as a series of scans of three to seven minutes in duration, with M87\* scans interleaved with those on the quasar 3C 279. The number of scans obtained on M87\* per night ranged from 7 (April 10) to 25 (April 6) as a result of different observing schedules. A description of the M87\* observations, their correlation, calibration, and validated final data products is presented in Paper III and briefly summarized here.
>
> At each station, the astronomical signal in both polarizations **and two adjacent 2 GHz wide frequency bands centered at 227.1 and 229.1 GHz** were converted to baseband using standard heterodyne techniques, then digitized and recorded at a total rate of 32 Gbps.[...]

So if we use 230 GHz, the wavelength is given by $c/f$ or 1.30 millimeters. It's hard for me to say right now if the image comes from only one 2GHz wide channel, or all three, which means that the bandwidth is either about 0.9% or 2.1%, but that's still pretty narrow compared to images taken at optical frequencies. That's (ultimately) because interferometry is done digitally these days and the computational size and time scales fairly fast with the size of the baseband.

I should note that these days it's more and more common for astronomers to refer to all kinds of different wavelengths as "light" in a loose way. Circa 1.3 millimeter wavelength certainly could be thought of as far-infrared, though [Wikipedia](https://en.wikipedia.org/wiki/Far_infrared) puts the cutoff at 1 mmm (300 GHz).

> **Comments:**
> **username_1**: @user71659 you are right, I've made and edit. with your link thanks! While we don't call CMB the "[Cosmic Millimeter wave Background](https://en.wikipedia.org/wiki/Cosmic_microwave_background)", we do call ALMA the "[Atacama Large Millimeterr/submillimeter Array](https://en.wikipedia.org/wiki/Atacama_Large_Millimeter_Array)". I don't know what "Large Millimeters" are (humor!!) but I do at least know that [there are no large hadrons!](https://physics.stackexchange.com/q/454554/83380)!  

---

