Auroral arcs (curtains) are extremely thin. A calculation of the minimum-possible arc thickness is presented; this minimum thickness is found to be about 9.5 m. Four requirements for designing an optical system that can image the thinnest curtains are discussed: (1) angular resolution, (2) temporal resolution, (3) light-gathering power, and (4) data-recording convenience. An optical system meeting these four requirements was constructed. With this system, an observing campaign in Fort Smith, Northwest Territories, has begun and images of the small-scale structure of auroral arcs are presented. Arc thicknesses of approximately 40 m were observed. These measurements of arc thicknesses may provide a critical test for the many theories about the origins of auroral arcs.Key words: aurora, geomagnetism, ionosphere, Northwest Territories, photography, polarMots clés: aurore boréale, géomagnétisme, ionosphère, Territoires du Nord-Ouest, photographie, polair
