= Telescope aperture and resolved surface brightness
{title2=$A_{\rm tel}\Omega_{\rm res}\sim\lambda^2$}
For a diffraction-limited <telescope>, collecting area scales as $D^2$ while a native <angular resolution> element scales as $(\lambda/D)^2$. Counts from a smooth resolved source per native resolution element therefore do not increase with aperture, at fixed radiance, exposure and throughput. Counts in a fixed angular bin instead grow as $D^2$, and the photon-noise-limited <signal-to-noise ratio> grows as $D\sqrt{t}$. <Conservation of etendue> and <cosmological surface-brightness dimming> do not forbid this latter gain.
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