The Event Horizon Telescope (EHT) does interferometry from an array of telescopes spread all across Earth. The data is locally stored on a hard drive and shipped to a central location, so the telescopes in the array don't need to be connected to each other in real time. Would it be possible to make an EHT-like interferometer with telescopes on the Earth and on the Moon? It seems like such a telescope would have a much higher resolution than the EHT, and would make it possible to observe in wavelengths shorter than radio.

Of course, money is likely an issue here, but is there any technical reason why this wouldn't be feasible?

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    $\begingroup$ Related: astronomy.stackexchange.com/q/30383/2153 $\endgroup$
    – HDE 226868
    Mar 16 '20 at 14:17
  • $\begingroup$ That question is more about the performance of this kind of telescopes. This question is rather about technical issues that might prevent us from building it. $\endgroup$
    – usernumber
    Mar 16 '20 at 14:21
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    $\begingroup$ That question specifically explains the key technical issue preventing this usefully being done. There's no point building something we know won't work properly, and money certainly is a major issue. $\endgroup$
    – StephenG
    Mar 16 '20 at 16:48
  • $\begingroup$ @StephenG It can certainly work properly if done properly and there's nothing there that supports anything to the contrary. You just need a source that doesn't change shape significantly over one month, and ideally it should be far from the Moon's orbital plane. A single pair of telescopes can generate enough uv coverage in a night to allow for some image solutions. The related question is simply related, nothing more. $\endgroup$
    – uhoh
    Mar 17 '20 at 2:02

Put simply: controlling the phasing would be impossible (not just difficult). Not only do you have to deal with a constantly changing physical distance but also with the dynamics of the atmosphere. It's difficult enough to maintain transverse phasing across an earth-based receiver (think adaptive optics); I can't imagine a reliable way to track the radial variation. Without that, unless you are sticking with multi-meter wavelength radio systems, you cannot hope to phase up the receivers.

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    $\begingroup$ You have a third receiver in space and use phase closure techniques I would have thought. Just like the EHT. $\endgroup$
    – ProfRob
    Mar 16 '20 at 20:53
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    $\begingroup$ The EHT does not "control phasing" either, so that point is moot. We already know Earth-Moon distances to centimeter precision from orbital mechanics and fifty years of laser ranging. Other answers here explain how the EHT is calibrated by recording data from other sources simultaneously, and if one is still worried after all of that one can also use a direct optical or microwave beam for additional ranging. It's not like the Earth-Moon system moves randomly. I'll add supporting links when I get to a real meyboard. $\endgroup$
    – uhoh
    Mar 17 '20 at 1:59
  • $\begingroup$ This is information is certainly outdated. The russian Spekt-R satellite project successfully managed to tackle Earth-Satellite interferometry on an elliptic orbit reaching nearly the lunar semi-major axis. It took them nearly the entire last decade to work all details out, but they recently have started publishing AGN observations of hitherto unknown spatial resolution. $\endgroup$ Mar 19 '20 at 3:43

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