The mapping camera

The SIM bay's mapping camera was really two cameras in one package, with a third instrument included to aid interpretation of the imagery. It was based on a wide-field camera designed in the 1960s as part of the then secret Corona reconnaissance satellite programme to provide context images of target sites. The main instrument was the metric camera, a conventional photographic imager with a 76-millimetre lens that took wide-angle, often spectacular, images of the Moon's surface with a maximum resolution of about 20 metres onto 127-millimetre wide film to create large square negatives 114.3 millimetres to a side. A glass plate in front of the film had tiny

The crater Alphonsus as photographed by Apollo 16's mapping camera.

crosses inscribed that allowed researchers to compensate for changes in the film's geometry over time. A large cassette carried over 450 metres of film which was sufficient for over 2,500 images.

When using such imagery for mapping purposes, it was vital to know the direction in which the camera was pointing. This information was supplied by the associated stellar camera. At the same time as a frame was being exposed with the metric camera, another was taken of the stars looking out to the side. Since researchers knew the precise angle between the axes of the two cameras' optical systems, they could deduce exactly where in space the metric camera was aimed. To accommodate the sideways view of the stellar camera, the entire mapping camera system was mounted on a track that allowed it to extend out of the service module bay.

The third part of the mapping camera system, the laser altimeter, determined the distance between the camera and the lunar surface to an accuracy of about 1 metre. This worked by sending extremely brief pulses of laser light to the surface along an axis parallel to the metric camera's axis. A detector then received the pulse after it had reflected off the surface and timed its return with high accuracy, yielding the distance. Altitude information was sent to Earth by telemetry and, when carried out simultaneously with a metric camera frame, was photographically coded onto the film. Both the stellar camera and the laser altimeter could continue operating while over the Moon's darkened hemisphere - that is, the stellar camera served to locate the terrain sampled by the laser pulses in the darkness.

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