Survey cadence

How often a telescope revisits the same patch, which decides what it can possibly discover.

A survey that images the whole sky once a decade will find galaxies and miss everything transient. One that returns every three nights finds supernovae, asteroids and anything that flares. Cadence is therefore a scientific decision disguised as a scheduling one, and it is the central design parameter of Rubin Observatory, which will revisit the whole southern sky every few nights for ten years and generate about ten million alerts a night. Cadence also decides what a survey can never see. A three-day return finds a supernova but samples a gamma-ray burst afterglow once, by which time it has faded, and it will miss anything that varies on the timescale it happens to sample at — an aliasing problem borrowed straight from signal processing, and one the schedulers argue about for years before first light.