SATI (Spectral Airglow Temperature Imager), a passive optical instrument, is a two-channel spatial scanning Fabry-Perot spectrometer with a back illuminated CCD detector providing spectral information in the radial direction of the image and spatial information in the azimuthal direction. It allows monitoring of the dynamics and temperature of the mesosphere by alternate observations of O2 Atmospheric (0-1) nightglow emission layer at 94 km and the OH Meinel (6-2) layer at 87 km. The instrument consists of an optical module, a controller and a data acquisition computer. The temperature retrieval is based on the intensity dependence of the selected vibrational-rotational lines on temperature yielding a temperature uncertainty of 1.7 K. The spatial resolution in the azimuthal direction and an integration time set at 2 minutes allows detection of gravity waves. SATI is automated for a standalone operation and is maintenance-free under normal operation conditions. The instrument can be accessed remotely to download data or to change operation modes. The optical module requires installation under a half sphere transparent dome and operates in a temperature range between 10C and 25C. For more details, please see http://stpl.cress.yorku.ca/SATI/.