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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?

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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.

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    $\begingroup$ Adding a space-based radio telescope to the array would certainly help, but there aren't any that are currently operational. $\endgroup$ – Mark May 1 at 21:37
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    $\begingroup$ @Mark currently unanswered: How is Spektr-R doing these days? and for some historical instruments: Has VLBI been done using any space-based receivers besides Spektr-R? $\endgroup$ – uhoh May 1 at 23:52
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    $\begingroup$ 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 $\endgroup$ – TazAstroSpacial May 2 at 6:04
  • $\begingroup$ "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. $\endgroup$ – nick012000 May 2 at 6:34
  • $\begingroup$ @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. $\endgroup$ – Neil Slater May 2 at 6:59

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