ALIS The ALIS (Auroral Large Imaging System) project ( Steen and Brändström, 1993) is a ground-based optical system for measuring the aurora, high-altitude clouds, and other atmospheric optical signatures at high latitude. It is composed of a grid of stations with approximately 50 km separation and consists currently (summer 2000) of six stations in regular unmanned operation ( Brändström and Steen, 1994). In joint campaigns ( Aso et al., 1998b) the system has been set up and operated with six manned and unmanned stations. Each station is equipped with a sensitive high-resolution CCD camera operated and controlled remotely. At the time of writing, the ALIS system consists of four CCD cameras with telecentric optics and $57$ fields-of-view (side to side) and two CCD cameras with approximately $90$ fields-of-view. The cameras are equiped with narrow ( $d\lambda\approx 40$ Å) interference filters for imaging of the auroral green line $(\lambda=5577$   Å$)$, the auroral red line $(\lambda=6300$   Å$)$, a $N_2^+$ 1NG emission $(\lambda=4278$   Å$)$, and a near IR emission $(\lambda=8446$   Å$)$. The approximate sensitivity is 24 Rayleigh per count. The instrument noise is between 5 and 10 counts, which should be compared with the photon count statistics that give random fluctuations that are larger; the ALIS cameras actually measure the random fluctuations of the photon emission. With the current cameras and software it is possible to take images with exposure times down to 50 ms, with a frame rate of one image every 10 s at full resolution, and up to one image every 3-5 s with reduced resolution (Brändström, private communication). One of the major scientific objectives of ALIS is to retrieve three-dimensional (3-D) distributions of auroral emissions with the aid of tomographic inversion techniques. This has influenced the design of the entire experimental configuration, i.e., geographical location of the stations, the optics, and other relevant experimental parameters. The geometry of the stations is optimised for reconstruction of the spatial distribution of aurora in the Kiruna region. In order to obtain a three-dimensional reconstruction of the aurora which is accurate in as large a volume as possible, a number of orientations of the cameras are designed to make as large a volume as possible fall within the field-of-view of all cameras. In Figure 1.1 the approximate fields of view at 100 km altitude are plotted with all cameras in the ``core mode''; that is, the cameras are directed towards a point above Kiruna. Figure 1.1: field-of-view at 100 km altitude for the cameras oriented towards a point above the Kiruna station, the ``core mode''.