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?)
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.
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.