THE IMAGING SYSTEM

THE  COMPLEX TASK

    The primary purpose of the Lunar Orbiter missions was an unmanned close-up reconnaissance of potential landing sites for the Surveyor and Apollo missions, which were due to begin before the end of the sixties.  When John Kennedy first imposed this challenge on the U.S., very little of the infrastructure was in place to even consider such an enormous task, so everything required had to be invented or improvised as the various stages were implemented.  This included the camera system on the Lunar Orbiters.

    The challenge of the Lunar Orbiter program was to somehow capture high-resolution images of relatively wide areas of the Moon's surface.  These images must then somehow get back to the planners on Earth so safe landing sites could be found for the unmanned Surveyor and manned Apollo missions. 

    Some high-resolution images had been returned by the unmanned Ranger missions, but these were obtained by transmitting TV images back to Earth as the spacecraft simply plunged headlong into the Moon.  These were good early attempts at getting to the Moon, but this was not the way to get close-up images of the surface.  Another way had to be found.

    The technology for such high-res photography already existed, but it required photographic film that must be returned to Earth to be processed and examined. It would be difficult enough just to get the camera to the Moon, but getting the film safely back to Earth was too risky and complicated.  The TV cameras of the day did not have the required resolution and digital imaging did not yet exist.  A way had to be found to send high quality photographic film to the Moon, capture high-res images, process them  onboard and transmit them back to Earth.  All this using a small experimental unmanned craft.  The method that was invented was truly ingenious [1].

    The best analogy to the film processing technique of the Lunar Orbiter camera is the old Polaroid type film that must be peeled apart after extraction from the camera.  With the Polaroid film each print carried its own little pocket of developer fluid.  This fluid would not touch the print until it was pulled through a roller as it was extracted from the camera.  The developer was sandwiched between the print and a second layer as the roller spread it across the print.

    But this technique had to work automatically onboard the Lunar Orbiter, so it had to be simplified.  The components were  essentially the same, but they were arranged differently.  Instead of separate opaque prints, transparent film was used on one long roll of film while the developer was coated onto another long roll.  Once the film was exposed, it was pressed into contact with the developer as the two rolls were fed together for processing.  After the film had been processed the two rolls were pulled apart again.

    This process [2] is known as a "Bimat" process, and the pulling apart of the processed film and the developer caused unexpected problems that plagued each of the five Lunar Orbiter missions [3].  This is the source of the infamous "bimat defect" which plays an indirect role in the story of the double craters.

THE MANY PROBLEMS

    It should be remembered that each Lunar Orbiter mission was an experiment in itself, and the first mission experienced many problems that would resurface in varying degrees on later missions [4].  It had trouble orienting itself, it overheated, the batteries discharged faster than planned, but most importantly, the shutter of the 610 mm (high-resolution) lens was out of synchronization with the motion compensation sensor.  This last problem was important to the mission because the high-res images were the reason for the mission in the first place.

    At lower altitudes the surface of the Moon swept through the field of the high-res camera quickly enough to cause blurring unless the motion were compensated for.  This was accomplished with a "velocity-to-height" (V/H) sensor which basically compared the spacecraft velocity to it's altitude and caused the film to move with the image projected on it during exposure, thus preventing blurring due to the motion of the spacecraft .

    But on the first mission the V/H sensor was out of sync with the shutter and the result was severe smearing and some double exposures of many of the high-res images [5].  Very few were usable on the first mission.  This image smear, to a much lesser degree, occurred again during Lunar Orbiter 3, the mission that captured all of the "double crater" images.

    Another unexpected problem was that the Bimat was sticking to the processed film in some places.  This would cause some of the Bimat to be left as spots on the image, or cause the emulsion to be pulled off of the image.  In an attempt to avoid this, fresh bimat was rolled out more frequently, creating the necessity of taking of photographs more frequently.  This led to some unplanned photography [3].  This problem also figures into the story of the crater pairs.

The Case For Artificiality       The Case For Motion Blur      The Test

Footnotes

¹ Destination Moon. Chapter 3, paragraphs 1 and 2; Chapter 9, paragraph 15
² Destination Moon. Chapter 3, paragraphs 3 and 4
³ Destination Moon. Chapter 9, paragraph 21
⁴ Destination Moon. Chapter 9, paragraphs 4, 7, 17 and 22
⁵ Destination Moon. Chapter 9, paragraph 17

The above notes are from DESTINATION MOON: A History of the Lunar Orbiter Program.  This  work is highly recommended for anyone interested in learning more about the early preparations for Apollo in general and the Lunar Orbiter program in particular.

Author: MikeLomax@aol.com