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As the James Webb telescope is getting ready for launch after years, I wonder in a purely hypothetical scenario, a telescope which is as huge as the diameter of Earth will make what kind of interesting observations possible. I am not asking about plain resolution improvements or the background math etc, but some examples which are actually the result of the known laws of physics to conceive what can be limits of what can be seen by telescopes, or if there is any limit at all if one aims to see farthest and faintest objects in the Universe.

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  • $\begingroup$ Have you made any research (internet search) effort? $\endgroup$ – Yashas Feb 16 '17 at 8:44
  • $\begingroup$ Diffraction limit of a telescope is the phrase you're looking for. $\endgroup$ – Kyle Kanos Feb 16 '17 at 12:12
  • $\begingroup$ @YashasSamaga finally I found something $\endgroup$ – Sourena Feb 20 '17 at 1:42
  • $\begingroup$ @KyleKanos good point. Helped me find relevant links. $\endgroup$ – Sourena Feb 20 '17 at 1:43
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The answer lies in the resolvable angle formula:

Angular resolution = 1.22 × Wavelength / D

D is the diameter of the apparatus. It also follows that by cleverly using an array of telescopes rather than one large telescope with the size of Earth, D can be effectively increased. In this regard, launching many telescopes in a solar orbit will be more practical.

Citing from a previous essay, seeing an alien creature with a comparable size to humans at 20 light years (the nearest candidate planet) will need an array with the diameter of 100 million kilometers with a cumulative mirror surface area of almost 1000x Earth surface. This way the alien will only map to a single pixel. Therefore, a direct observation of alien life outside the milky will be even more impractical using the available technology.

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