EP3: Aperture and Resolution

In this episode, we examine what aperture actually controls in a telescope system. Aperture defines two independent physical properties, photon collection and angular resolution. The first is governed by geometry, where collecting area scales with the square of diameter. The second is governed by diffraction, where a circular aperture produces an Airy pattern that limits how closely two point sources can be separated. We introduce the Rayleigh criterion as the standard definition of angular resolution and distinguish it from the full width at half maximum (FWHM) of the point spread function. These two quantities describe different aspects of the same diffraction process, separation of two sources versus the width of a single stellar image. Using real instrument examples, we show that under typical observing conditions in Melbourne, atmospheric seeing dominates the final image. While larger apertures provide finer theoretical resolution, this improvement is often masked by the atmosphere. In practice, increased aperture primarily improves photon collection and signal-to-noise ratio rather than resolving finer detail. This episode establishes the physical foundation required to evaluate telescope performance without relying on product specifications or marketing claims. Full derivations, worked examples, and extended analysis are available in the weekly journal: https://github.com/AUSTRONOMIC/CONTENT