ATLAS: A High-cadence All-sky Survey System

Technology has advanced to the point that it is possible to image the entire sky every night and process the data in real time. The sky is hardly static: many interesting phenomena occur, including variable stationary objects such as stars or QSOs, transient stationary objects such as supernovae or M dwarf flares, and moving objects such as asteroidsand the stars themselves. Funded by NASA, we have designed and built a sky survey system for the purpose of finding dangerous near-Earth asteroids(NEAs). This system, the " AsteroidTerrestrial-impact Last Alert System" (ATLAS), has been optimized to produce the best survey capability per unit cost, and therefore ATLAS is an efficient and competitive system for finding potentially hazardous asteroids(PHAs) but also for tracking variables and finding transients. While carrying out its NASA mission, ATLAS now discovers the greatest number of bright ($m < 19$) supernovae candidates of any ground based survey, frequently detecting very young explosions due to its 2 day cadence. ATLAS discovered the afterglowof a gamma-ray burst independent of the high energy trigger and has released a variable starcatalogue of 5$\times10^{6}$ sources. This, the first of a series of articles describing ATLAS, is devoted to the design and performance of the ATLAS system. Subsequent articles will describe in more detail the software, the survey strategy, ATLAS-derived NEA population statistics, transient detections, and the first data release of variable starsand transient lightcurves.