Papa Flash’s Rapatronic Camera

For background music in the lab, I’ve been listening to 1940’s-50’s atomic energy videos on Youtube. I was curious about the origins of the blast pictures. A quick search revealed Harold ‘Doc’ Edgerton’s (AKA Papa Flash) rapatronic (“rapid action electronic”) shutter.

The birth of a blast.

After the war, the US wanted to acquire single gated pictures of large explosions. They wanted to know the diameter of the expanding fireball at certain times in addition to a host of other properties. The diameter as a function of time gave them a measure of yield.

At a given test tower at some distance from ground zero, they would mount an array of special cameras triggered at appropriate times after t=0. Eastman Kodak of Rochester, NY ( developed an ingenious camera system called the multiple aperture focal plane scanner but it needed an appropriate shutter technology. Each shutter needs to open and close at specific times with low jitter. Doc advanced an idea based on a Faraday cell with no moving parts and did the early work at EG&G (Edgerton, Germeshausen, and Grier). Charles Wyckoff, a student of Edgerton’s from the late 30’s at MIT, was working on an alternative shutter based on ammonium dihydrogen phosphate (ADP), but it was not working out and the decision was made for him to re-join with Edgerton and focus on the Faraday shutter.

The schematic above shows the “subject”, which illuminates a phototube. (The phototube found widespread use in “talking movies,” but we’ll leave that for another post.) In this application, the phototube produces a photocurrent and that signal fires the gap and actuates the magneto-optical shutter. The shutter then remained open for some predetermined microseconds and the photographic film was exposed to the blast.

Rapatronic shutter assembly, 1952.

Kodak had to develop a new type of film for the complex camera and it had to endure the conditions of the test. Below is a picture of a rapatronic camera subassembly.

Rapatronic camera subassembly. National Atomic Testing Museum
The shutter is inside the gray tube to the right of the hump, which is where the coil resides. The film pack would be attached at the left end of the gray tube. The black box is the programmable time delay.
A good view of the film plane to the left of the black time delay box.

A microsecond is an eternity today. But in the immediate post-war period, all they had were phototubes, thyratrons, vacuum-tube timing circuits, and mostly passive optical elements. Studying equipment from that period gives one a great appreciation for the work involved and helps place our work in perspective.

A Nevada Tumbler-Snapper blast illustrating the rope trick effect. NB the background.

Speaking of perspective, while Doc’s rapatronic cameras were imaging megaton fireballs, he was generating beautiful images for the public.

Edgerton Moving Skip Rope (1952)
Edgerton Baton (1953)

The contrast of the times is very interesting. Below is a picture from The Oscars, 1952, followed by a view over the pacific ocean taken around the same time. Technology was advancing at an amazing pace. It must have been an exciting time to be an engineer.

Bogart wins Best Actor, 1952
Eniwetok Atoll (1951)

I’d like to know more about the team at Kodak that developed the camera system.