Courtesy of NASA's National Space Science Data Center
Five Lunar Orbiter missions were launched in 1966 through 1967 with the purpose of mapping the lunar surface before the Apollo landings. All five missions were successful, and 99% of the Moon was photographed with a resolution of 60 meters (197 feet) or better. The first three missions were dedicated to imaging 20 potential lunar landing sites, selected based on Earth-based observations. These were flown at low-inclination orbits. The fourth and fifth missions were devoted to broader scientific objectives and were flown in high-altitude polar orbits. Lunar Orbiter 4 photographed the entire nearside and 95 percent of the farside, and Lunar Orbiter 5 completed the farside coverage and acquired medium (20-meter or 660-foot) and high (2-meter or 6.6-foot) resolution images of 36 pre-selected areas.
The Lunar Orbiters had an ingenious imaging system, which consisted of a dual-lens camera, a film processing unit, a readout scanner, and a film handling apparatus. Both lenses, a 610-millimeter narrow angle high-resolution (HR) lens and an 80-millimeter wide-angle medium resolution (MR) lens, placed their frame exposures on a single roll of 70-millimeter film.
The axes of the two cameras were coincident so the area imaged in the HR frames were centered within the MR frame areas. The film was moved during exposure to compensate for the spacecraft velocity, which was estimated by an electric-optical sensor. The film was then processed, scanned, and the images transmitted back to Earth.