<p>In addition to what was mentioned in the accepted answer, there's the issue of data transfer speeds, from high orbit satellites, it maxes out at several megabytes a second, so even if we had a radio telescope in Earth orbit, and we had solved the timing issue, it would've taken months to get the data, while the link is saturated, which is a significant waste of radio telescope resources.</p>

CC BY-SA 4.0
2022-11-06T21:30:18.347Z
2022-11-06T21:30:18.347Z
Didi Kohen
<p>No, it would not, because it operates in the visible spectrum and the EHT is an array of radio telescopes. For the &quot;very long baseline interferometry&quot; technique to work, all the telescopes have to be operating at the same wavelength, because combining the signals involves measuring exactly how well the peaks and troughs of the radio waves from the different telescopes line up.</p>
<p>You can do VLBI in the visible spectrum, but you have to match up the waves even more precisely since light has shorter wavelengths than radio waves. The EHT collected all the data first, and then spent a lot of time combining it by computer, but that required using very precise clocks to sync the data. We don't have clocks precise enough to do that for light, so a direct optical connection is required between the multiple telescopes. So there isn't a good way to do a planetwide VLBI optical telescope yet.</p>
<p>Addendum inspired by the comments, especially from TazAstroSpacial: To get the same resolution with optical light, you can use a much smaller telescope array. I didn't mention that fact because I was thinking about keeping the size of the array the same and getting better resolution. But in any case the problem is that the challenges of doing the interferometry at smaller wavelengths more than make up for the advantages of needing a smaller aperture. At least at the current state of the art.</p>

CC BY-SA 4.0
2019-05-01T21:37:33.060Z
Adding a space-based radio telescope to the array would certainly help, but there aren't any that are currently operational.
Mark
CC BY-SA 4.0
2019-05-01T23:52:50.250Z
@Mark currently unanswered: [How is Spektr-R doing these days?](https://space.stackexchange.com/q/35641/12102) and for some historical instruments: [Has VLBI been done using any space-based receivers besides Spektr-R?](https://space.stackexchange.com/q/35642/12102)
uhoh
CC BY-SA 4.0
2019-05-02T06:04:05.033Z
In the optical you don't need a planet wide optical interferometer to get the same resolution as the radio telescope array. Shorter wavelengths yeild better resolution. EHT used 1.3mm, red light 0.0007mm so only need optical array (0.0007/1.3 factor smaller) about 6.5 km across. This is still a lot larger than possible at optical wavelengths. The largest optical interferometer is by [ESO](https://www.eso.org/public/australia/teles-instr/technology/interferometry/)
TazAstroSpacial
CC BY-SA 4.0
2019-05-02T06:34:48.620Z
"This is still a lot larger than possible at optical wavelengths." Really? I'm no rocket scientist, but I expect it'd be theoretically possible to launch the mirrors and their assorted supporting systems up into space in multiple pieces, then assemble and polish them in orbit. It'd just be really, really, *really* expensive.
nick012000
CC BY-SA 4.0
2019-05-02T06:59:31.867Z
@nick012000: The issue is not the overall distance or physical construction of the devices. It is the timing and other precision requirements to be able to combine the observations. Both are engineering problems, just the one you picked up on is solvable using known/practiced techniques whilst the measurements are not.
Neil Slater
CC BY-SA 4.0
2019-05-02T07:02:25.480Z
@NeilSlater Why would you need to combine the observations if you just build a set of mirrors 6.5km across for a telescope that large? Like I said, probably theoretically possible, but really, really, *really* expensive. ;)
nick012000
CC BY-SA 4.0
2019-05-02T07:05:02.640Z
@nick012000: Oh, you literally mean a 6.5km aperture? No, that's not feasible either. We've built nothing close to that size in space, and don't have technology to do so. It is more that just "very expensive", there would be tons of hard, possibly unsolvable problems in doing this - it's a cool idea though, so perhaps ask a question about the challenges involved.
Neil Slater
CC BY-SA 4.0
2019-05-02T07:51:21.090Z
@nick012000 You would need to fix the distance between the mirrors accurately to a few wavelengths of light.
Aron
CC BY-SA 4.0
2019-05-02T11:01:30.450Z
@NeilSlater: doesn't the [LISA project](https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna) do this kind of "multiple satellite with very precise distances to each other"? I.e. this sounds to me (as a complete layman) like the problems involved with building a 6.5km aperture are about to be solved anyway?
oliver
CC BY-SA 4.0
2019-05-02T11:32:06.630Z
@oliver: It is a similar situation in some ways, and I don't know what else may need to be solved for a virtual optical wavelength aperture at a similar scale. I suspect this is not "solved" though. Might make a good follow-up question. You need someone who knows the maths and technical challenges of the optical VLBI, and who understands what parts of LISA are applicable,
Neil Slater
CC BY-SA 4.0
2019-05-02T15:53:19.580Z
There is also to much gas in the way for us to be able to observe the "light ring around the photon sphere effect" using optical wavelengths?
Agerhell
CC BY-SA 4.0
2019-05-01T17:40:26.070Z
2020-12-16T15:44:07.017Z
2020-12-16T15:44:07.017Z
Mark Foskey
<p>My question is related to black hole image released in April. As far as I understand idea of EHT, it joins observations from multiple locations to work like one telescope with radius that is equal to the distance between the farthest telescopes in array. Moreover Earth atmosphere affects observation, e.g. by introducing delays. Would it beneficial to add Hubble Space Telescope to EHT and if yes, why HST was not included in observation array?</p>

<black-hole><supermassive-black-hole><event-horizon-telescope>
Would Hubble Space Telescope improve black hole image observed by EHT if it joined array of telesopes?
Marcin
# Would Hubble Space Telescope improve black hole image observed by EHT if it joined array of telesopes?

(Asked by: username_0 on 2019-05-01)

My question is related to black hole image released in April. As far as I understand idea of EHT, it joins observations from multiple locations to work like one telescope with radius that is equal to the distance between the farthest telescopes in array. Moreover Earth atmosphere affects observation, e.g. by introducing delays. Would it beneficial to add Hubble Space Telescope to EHT and if yes, why HST was not included in observation array?

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


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## 2 answers

### Answer by username_2 on 2019-05-01. Score: 19 | Accepted answer

No, it would not, because it operates in the visible spectrum and the EHT is an array of radio telescopes. For the "very long baseline interferometry" technique to work, all the telescopes have to be operating at the same wavelength, because combining the signals involves measuring exactly how well the peaks and troughs of the radio waves from the different telescopes line up.

You can do VLBI in the visible spectrum, but you have to match up the waves even more precisely since light has shorter wavelengths than radio waves. The EHT collected all the data first, and then spent a lot of time combining it by computer, but that required using very precise clocks to sync the data. We don't have clocks precise enough to do that for light, so a direct optical connection is required between the multiple telescopes. So there isn't a good way to do a planetwide VLBI optical telescope yet.

Addendum inspired by the comments, especially from username_5: To get the same resolution with optical light, you can use a much smaller telescope array. I didn't mention that fact because I was thinking about keeping the size of the array the same and getting better resolution. But in any case the problem is that the challenges of doing the interferometry at smaller wavelengths more than make up for the advantages of needing a smaller aperture. At least at the current state of the art.

> **Comments:**
> **username_3**: Adding a space-based radio telescope to the array would certainly help, but there aren't any that are currently operational.  
> **username_4**: @username_3 currently unanswered: [How is Spektr-R doing these days?](https://space.stackexchange.com/q/35641/12102) and for some historical instruments: [Has VLBI been done using any space-based receivers besides Spektr-R?](https://space.stackexchange.com/q/35642/12102)  
> **username_5**: In the optical you don't need a planet wide optical interferometer to get the same resolution as the radio telescope array. Shorter wavelengths yeild better resolution. EHT used 1.3mm, red light 0.0007mm so only need optical array (0.0007/1.3 factor smaller) about 6.5 km across. This is still a lot larger than possible at optical wavelengths. The largest optical interferometer is by [ESO](https://www.eso.org/public/australia/teles-instr/technology/interferometry/)  
> **username_6**: "This is still a lot larger than possible at optical wavelengths." Really? I'm no rocket scientist, but I expect it'd be theoretically possible to launch the mirrors and their assorted supporting systems up into space in multiple pieces, then assemble and polish them in orbit. It'd just be really, really, *really* expensive.  
> **username_7**: @username_6: The issue is not the overall distance or physical construction of the devices. It is the timing and other precision requirements to be able to combine the observations. Both are engineering problems, just the one you picked up on is solvable using known/practiced techniques whilst the measurements are not.  
> **username_6**: @username_7 Why would you need to combine the observations if you just build a set of mirrors 6.5km across for a telescope that large? Like I said, probably theoretically possible, but really, really, *really* expensive. ;)  
> **username_7**: @username_6: Oh, you literally mean a 6.5km aperture? No, that's not feasible either. We've built nothing close to that size in space, and don't have technology to do so. It is more that just "very expensive", there would be tons of hard, possibly unsolvable problems in doing this - it's a cool idea though, so perhaps ask a question about the challenges involved.  
> **username_8**: @username_6 You would need to fix the distance between the mirrors accurately to a few wavelengths of light.  
> **username_9**: @username_7: doesn't the [LISA project](https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna) do this kind of "multiple satellite with very precise distances to each other"? I.e. this sounds to me (as a complete layman) like the problems involved with building a 6.5km aperture are about to be solved anyway?  
> **username_7**: @username_9: It is a similar situation in some ways, and I don't know what else may need to be solved for a virtual optical wavelength aperture at a similar scale. I suspect this is not "solved" though. Might make a good follow-up question. You need someone who knows the maths and technical challenges of the optical VLBI, and who understands what parts of LISA are applicable,  
> **username_10**: There is also to much gas in the way for us to be able to observe the "light ring around the photon sphere effect" using optical wavelengths?  

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### Answer by username_1 on 2022-11-06. Score: 1

In addition to what was mentioned in the accepted answer, there's the issue of data transfer speeds, from high orbit satellites, it maxes out at several megabytes a second, so even if we had a radio telescope in Earth orbit, and we had solved the timing issue, it would've taken months to get the data, while the link is saturated, which is a significant waste of radio telescope resources.


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