The Mars Orbiter Camera (MOC) narrow angle system is a 3.5 m focal length camera that has operated for the last five years as part of the Mars Global Surveyor (MGS) Mission. Folded into a total package (including electronics) of less than 1 m length and weighing just over 20 kg, MOC's Ritchey-Chretien optical design is extremely sensitive to primary-to-secondary despace. Because of this, providing proper focus over the range of operational conditions was the primary challenge of the MOC development effort. As initially proposed, the instrument used a graphite-epoxy metering structure to provide a completely athermal system. Given of the sensitivity of the design and large operational temperature range, this turned out not to be realizable. The first fallback from a completely athermal design was to model the response of the system over temperature, and set the detector so the system would be focus over the range of operational conditions. Prototype testing revealed this was also not a workable solution. Late in the development flow, the system was retrofitted with a set of heaters to control focus in flight by application of radial thermal gradients across the primary mirror. Despite the loss of the first copy of the MOC on Mars Observer in 1993, the MOC on MGS has been an outstanding success, returning over 140,000 images of Mars to date and making a number of new discoveries about the planet.