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Recently, we got the first picture of a black hole. From what I heard, it took multiple telescopes to capture the image and combine the pictures captured by each telescope. Why did we need to use multiple telescopes instead of one? (e.g what about the nature of the black hole that made it challenging to capture the image of black hole using one telescope?)

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    $\begingroup$ Most VLBI images take at least 3 radio telescopes - it makes data analysis easier. The more telescopes, the better the resolution and the easier the data analysis. $\endgroup$ – Jon Custer Apr 17 at 23:40
  • $\begingroup$ en.wikipedia.org/wiki/Astronomical_interferometer $\endgroup$ – BowlOfRed Apr 18 at 0:01
  • $\begingroup$ @JonCuster, even a brief answer is an answer rather than a comment. $\endgroup$ – Alfred Centauri Apr 18 at 2:23
  • $\begingroup$ @AlfredCentauri - yeah, but I’m not really an expert, having been interested in VLBI roughly 40 years ago through Scientific American. I had hoped we might have some more knowledgeable people here to answer. $\endgroup$ – Jon Custer Apr 18 at 2:30
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The angular diameter of the photon ring that has been detected is about 40 microarcseconds.

To adequately resolve this structure required observations with an angular resolution better than this.

The angular resolution of a single telescope (in radians) is approximately the wavelength of observation divided by the telescope diameter.

Observing at a wavelength of 1.3 mm, then $<40$ microarcsecond resolution requires a telescope with a diameter of $\sim 6700$ km.

Clearly this is impractical for a single instrument. A solution is to link multiple telescopes that have separations between them greater than this distance. Providing the location of these telescopes and the timing of the signals they receive is precisely known, then their results can be combined to reconstruct images of the source brightness that have an angular resolution similar to that of a single telescope with a diameter equal to the largest pairwise separation.

The largest baselines possible are about the diameter of the Earth, yielding angular resolutions of $\sim 20$ microarcseconds at 1.3 mm wavelengths.

Adding more telescopes, with different separations, improves sensitivity to features at different angular scales and ultimately the final reconstructed images.

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  • $\begingroup$ The use of 3 stations allows the phase differences to be worked out uniquely, adding more is even better. Using just two requires absolute timing, 3 just means the relative timing has to be good. $\endgroup$ – Jon Custer Apr 18 at 2:32
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Resolution ... Wavelength/ Diameter.... You can gather alot more light with a bigger receiver when you got a faint signal comming in. This galactic center of a blackhole is massive but its also very far away.

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