"For a 4m sail, for example, the diffraction limit spot size on Earth would be on order of 1000m." That must be a misprint; the diffraction-limited spot si

"For a 4m sail, for example, the diffraction limit spot size on Earth would be on order of 1000m." That must be a misprint; the diffraction-limited spot size would be on the order of 10 million kilometers. It's correct when it says a 1 km^2 collector would intercept ~10^{-14} of the transmitted signal. https://www.google.com/search?q=4+light+years+*+1+micron+%2F+4+meters https://www.google.com/search?q=4+light+years+*+1+micron+%2F... https://en.wikipedia.org/wiki/Airy_disk#Size https://en.wikipedia.org/wiki/Airy_disk#Size