Like, if you flattened out Ceres to a 1 mm iron foil telescope mirror with 20x the surface area of the Sun, could you resolve details on the surface of an exoplanet? Could you make it arbitrarily bigger and continue to get better resolution?
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The resolving power of a telescope with a circular apertre is given by $$\theta = 1.22\frac{\lambda}{D}$$ Where $\lambda$ is the wavelength, $\theta$ is the smallest resolvable angular size and $D$ is the diameter of the aperture (lens/mirror) of the instrument. The larger $D$ gets the larger the fourier component you get to investigate, this represents the finer angular detail resolvable. I guess at some point you might run into some qm type issues but I'm not qualified to talk about that! To resolve details on exoplanets would actually require a modest few 100's of km's of diameter. |
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The resolving power of a device is limited by the so called Rayleigh criterion (commonly known as the diffraction limit). This is applicable for "classical" states of light (i.e light that has a coherent state representation). For non-classical light (squeezed light or entangled light), you can beat the diffraction limit. A few interesting articles What Diffraction Limit?, Resonant Lithography,Quantum Limits on Optical Resolution,Quantum Imaging--pdf file. An interesting experimental overview provided by Boyd's group at Rochester. The point is, for non-classical light the diffraction limit is $\lambda(N)=\frac{\lambda(1)}{N}$ where N is the number of photons in an entangled state (say a two mode state such as: $|N,0>-|0,N>)$ and $\lambda(1)$ is the wavelength dependence you would expect for classical light. |
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Some theoretical answers were provided, but here's a practical answer from an astronomer's point of view. (First off, the resolving power is given by diameter, not surface area. So we will talk about diameter here.) For visible light, the practical resolving power of a 100mm diameter mirror is 1 arcsec. A 200mm mirror: 0.5 arcsec. And so on. This is the rule used by astronomers. A mirror "with 20x the surface area of the Sun" (apparent area, I assume) would have approx 4.5x the diameter of the Sun, which is 6.3 x 10^6 km. Such a mirror would have a resolving power of 1.6 x 10^-11 arcsec. At 10 light years distance, such a mirror would resolve details as small as 7 meters. Please note this discussion is entirely theoretical, as there are no known technologies to manufacture such a big mirror with the required precision for visible light astronomy optics - which means the surface error cannot be bigger than 100 nanometers - in fact, for a good mirror, the acceptable error is 4x ... 5x smaller. There's no way to maintain such precision across millions of kilometers of reflective surface. Currently, the biggest monolithic mirror is 6m in diameter and it never performed very well. The biggest well-performing monolith mirror is 5m. The biggest segmented mirrors are 10m in diameter, with a 40m project having had its initial funding approved very recently. |
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