The Ionospheric Mapping and Geocoronal Experiment (IMAGER) is a space-based, multispectral, imaging payload, designed at the U.S. Naval Research Laboratory. The IMAGER's primary science mission is to find, track, and measure ionospheric irregularities as they move across the surface of the Earth and vary with time. IMAGER will observe the ionosphere of the Earth in narrow extreme- and far-ultraviolet passbands centered at 83.4, 130.4, 135.6, and 143.0 nm. These emissions are produced by naturally occurring airglow emission from the nighttime and daytime ionosphere and thermosphere. The IMAGER consists of an imaging telescope with a filter wheel assembly and a pair of microchannel plate-based imaging detectors with cross delay line readouts. The telescope of the instrument consists of a 160 mm diameter, F/4.0 off-axis very fast aplanatic Gregorian telescope. The focal length is 640 mm and the field of view is 1.6° × 1.6° which will cover approximately 1000 × 1000 km2 on the Earth's surface. The modulation transfer function is above 0.90 at 2.8 line pairs-millimeter-1 over the field, which corresponds to a line pair separated by 20 km on the Earth. The spatial resolution is approximately 10 × 10 km2 and is oversampled by a factor of 9 (3 × 3 pixels per resolution element). A system of reflective filters is used to select different wavelengths of interest. The telescope will be gimbaled to provide a field-of-regard encompassing the entire disk and limb of the Earth. The gimbal will also allow the telescope to track the ionospheric irregularities as they move. This paper describes the design of the optical and mechanical systems and their intended performance and includes an overview of the mission and science requirements that defined the aforementioned systems.