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The passage around this formula

This reach is the approach’s defining strength and its defining limitation is spatial resolution, which is fixed by the physics of diffraction rather than by mission budget. For a telescope with aperture diameter D observing light of wavelength $\lambda$ , the smallest angular separation it can resolve is approximately $\theta \approx 1.22\,\frac{\lambda}{D}$. This relation is why an exoplanet studied by transmission spectroscopy is a single point of light with no visible surface at all — the target subtends an angle far smaller than $\theta$ for any existing telescope, so every observation is necessarily a disk-integrated average over the whole illuminated hemisphere. Even for solar-system bodies, where $\theta$ is…

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