The Lunar Orbiter program was a series of five unmanned Lunar orbiter missions launched by the United States 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 m 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 % of the farside, and Lunar Orbiter 5 completed the farside coverage and acquired medium (20 m) and high (2 m) 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 mm narrow angle high resolution (HR) lens and an 80 mm wide angle medium resolution (MR) lens, placed their frame exposures on a single roll of 70 mm 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 electro-optical sensor. The film was then processed, scanned, and the images transmitted back to Earth.
The main bus of the Lunar Orbiter had the general shape of a truncated cone, 1.65 metres tall and 1.5 m in diameter at the base. The spacecraft was composed of three decks supported by trusses and an arch. The equipment deck at the base of the craft held the battery, transponder, flight progammer, inertial reference unit (IRU), Canopus star tracker, command decoder, multiplex encoder, traveling wave tube amplifier (TWTA), and the photographic system. Four solar panels were mounted to extend out from this deck with a total span across of 3.72 m. Also extending out from the base of the spacecraft were a high gain antenna on a 1.32 m boom and a low gain antenna on a 2.08 m boom. Above the equipment deck, the middle deck held the velocity control engine, propellant, oxidizer and pressurization tanks, Sun sensors, and micrometeoroid detectors. The third deck consisted of a heat shield to protect the spacecraft from the firing of the velocity control engine. The nozzle of the engine protruded through the center of the shield. Mounted on the perimeter of the top deck were four attitude control thrusters.
Power of 375 W was provided by the four solar arrays containing 10,856 n/p solar cells which would directly run the spacecraft and also charge the 12 A·h nickel-cadmium battery. The batteries were used during brief periods of occultation when no solar power was available. Propulsion for major maneuvers was provided by the gimballed velocity control engine, a hypergolic 100 pound-force (445 N) thrust Marquardt rocket motor. Three axis stabilization and attitude control were provided by four one lbf (4 N) nitrogen gas jets. Navigational knowledge was provided by five Sun sensors, Canopus star sensor, and the IRU equipped with internal gyros. Communications were via a 10 W transmitter and the directional 1 m diameter high gain antenna for transmission of photographs and a 0.5 W transmitter and omnidirectional low gain antenna for other communications. Both antennas operated in S-band at 2295 MHz. Thermal control was maintained by a multilayer aluminized Mylar and Dacron thermal blanket which enshrouded the main bus, special paint, insulation, and small heaters.
Both links lead to a whole book on the program. For the HTML one, scroll down to see the table of contents link.