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This tweet called my attention to The Smithsonian National Air and Space Museum story Photographing Apollo 8's Orbit Toward the Moon. The story is quite an interesting bit of space history and worth a read. It includes a sequence of telescopic images of Apollo-8 from the Smithsonian archive. The first two show a periodic modulation in both brightness and in plume size.

What causes the oscillations/modulations shown in these two images?

Apollo-8 TLI burn Smithsonian

Source

Apollo-8 TLI burn Smithsonian

Source

Apollo-8 TLI burn Smithsonian

Source "David Le Conte with a Baker-Nunn camera in Florida, circa 1966. Credit: David Le Conte"

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Looks like Russell's right:

Baker Nunn Satellite Tracking Cameras

For a satellite photo run with the Baker Nunn camera the following operations occur: The Satellite Tracking Cameras consists of a continuous 35-mm length of film coming from a supply canister. The film is stopped for an exposure at the center of the film bridge and is then stretched over the optical shaped surface on a backup plate. The stretching of the film will place the emulsion surface of the film in focus over the total width and length of the films exposure. The film is then transported out to a take-up canister stopping with its trailing length ready for the next exposure.

Continuously, a pair of rotating shutter blades is passing around the film bridge, which will create clear stripes in the exposed image trail of the traveling satellite photo.

Continuously, another complete shutter is opening and closing, blocking unwanted exposure during the film transport.

A selectable speed worm and gear drive will drive the camera assembly tracking a satellite passing overhead.

Selectable transporting speeds can be implemented for tracking different satellite transit speeds.

Simultaneously an International time signal clock from the Norman Clock Co, Green Bay, Wisconsin is located at each Baker Nunn Satellite Tracking station. A time signal will be projected on the end of each exposure of the film.

A dozen f/0.75 Baker-Nunn cameras with 20-inch apertures – each weighing 3.5 tons including a multiple axis mount allowing it to follow satellites in the sky – were used by the Smithsonian Astrophysical Observatory to track artificial satellites from the late 1950s to mid-1970s.

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  • $\begingroup$ Great reference! $\endgroup$ – Organic Marble Jan 10 at 23:01
  • $\begingroup$ slam-dunk, thanks! $\endgroup$ – uhoh Jan 11 at 1:20

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