Aug 13

Restoring the US/C-3 Infrared Signalling Telescope

Thursday, August 13, 2015 2:15 PM


Bottom view

Bottom view of the US/C-3 Infrared Signalling Telescope. (Collection of LTCDR Thomas J. Cutler, USN [Ret.])

In Greek mythology, the prophet Tiresias was blinded by the gods as punishment for revealing their secrets. He begged the goddess Athena to restore his sight, but she could not. Instead, she gave him the gift of foresight, and Tiresias spent the remainder of his days spouting prophesy.

Tiresias had seen too much and had paid the price for it. Such, too, may be the case of a battered US/C-3 infrared signaling telescope that came into this writer’s care for restoration.

data plate

Data Plate of the US/C-3 telescope. The warning to not point the instrument towards the sun was to prevent damage to the image tube, not for the sake of the viewer’s eyes.

Before one can even begin to address the question of how to fix something, one has to know first just what one has, and how it is supposed to work. Thankfully, there were clues enough to figure out what the answers to all of these questions in a fairly short order.

The US/C-3 was developed during World War II as a response to the need for secure nighttime signaling. In conjunction with RCA and its laboratories, the Navy developed what was then a virtually (or so it was thought) unbreakable method: the use of infrared light. Aldis lamps, spotlights, and beacons could be filtered through glass to only emit infrared light. Such tools could be used by covert operations to help guide vessels to landing sites, or transmit signals covertly. Not being within the range of visible light, detection could be kept to a minimum. Both Germany and Japan had experimented with the use of IR light and had developed detectors, and had even deployed them operationally.

Side view of the 1P25 infrared image tube. From the original patent by Flory, et. al., 1950. Though the technology had been developed in secret during the war, the patent was not filed until 1946.

Side view of the 1P25 infrared image tube. From the original patent by Flory, et al., 1950. Though the technology had been developed in secret during the war, the patent (No. 2,506,018) was not filed until 1946.

But the key to good signalling is detection. A signal sent but not received is all but useless. Such was the need for the US/C-3 telescope. The key to the whole operation was the RCA 1P25 image vacuum tube, developed in secret and patented after the war. Tubes were high-technology then, and though largely supplanted by solid-state electronics, their continuing use in the future of electronic warfare is garnering renewed-interest in some circles.

At one end of the 1P25 tube, a thin film coating of metal oxides sensitive to infrared light would emit electrons. This stream of energy would then be focused with adjustable high-voltage electric fields onto a phosphorescent screen at the other end, producing a green, glowing image viewed through an eyepiece.

The image on the cathode itself was focused via a traditional telescope, manufactured by the familiar Polaroid Corporation. The focal length was set quite short, to better permit scanning for signals. The entire unit was built very ruggedly to combat corrosion and water at sea, with rubber seals around each joined part, a sturdy metal case, and several options for mounting, including hand grips and a plate for sliding into a shipboard mount.

The technology of the telescope is the same used in the much more famous “Sniperscopes” and “Snooperscopes” night-vision rifle scopes of the period, the key difference being these scopes featured a large IR-filtered spotlight that would illuminate targets in the dark.

But its ruggedness belies the rather fragile nature of its components. Time is often unkind to old electronics, and the challenge of restoring one of these pieces is finding out just how to fix parts that have not been manufactured in decades.


View of the US/C-3 Infrared Signaling Telescope’s battery compartment after restoration.

Like Tiresias, this telescope had seen a great deal but been “blinded,” in a sense, by the awesome power of time. It was in a rough condition.

The telescope was powered by two D-cell batteries, which drove electric vibrators to power the high-voltage rectifying circuits for the image tube. The alkali (not acid, is is commonly believed) within the batteries can leak and corrode metal, which is what happened here. Judicious use of a plumbing wrench managed to pry off the battery cover and expose the extent of the corrosion. It was in a bad state, but not impossible to restore. The only real damage was to the cover itself, which had been eaten through in spots was reconstructed with metal epoxy. All the rest of the damage from the leaking batteries was easily cleaned up. A new ball chain and rivet was required, however, to replace the original which had all but dissolved in places.

Another matter were the internal vacuum tubes. The image tube had shattered, scattering glass and pieces of tube into the telescope mount. Not exactly a desirable situation by any account. Replacement tubes are hard to come by, not having been manufactured for nearly 60 years, but one unused one was found—and worked.

The other matter was one of the dirty little secrets of old electronics: components break down with time. We like to think that the systems we design are static things, at least in terms of hardware. But this is far from the case. Over time, capacitors—back when this unit was made, encased in paper and wax—start to leak; resistors drift; and insulation on wires hardens and cracks. The time span is a long one, however, so most of the time maintenance is not an issue—until it becomes one. It such instances forethought given to maintenance in the design of a piece of equipment becomes obvious.

A critical piece to replace was a series of four 100-megohm resistors that ran between two pins of the tube and were crammed in between the socket and the eyepiece. Being part of the electrical focusing mechanism, these vital resistors had drifted to become unusable. The circuit needed to be as close to 400 megohms as possible. New resistors would not fit in place, but a single 400 megohm resistor could just be squeezed in without too much of a fuss.


A view through the eyepiece of the restored US/C-3 binoculars. Image shows a table and a coffee percolator. The “specs” in the image are from damage to the telescope’s focusing mirror.

Tubes were put in place, wires soldered, connections checked and rechecked, screws tightened, lens cleaned. Then came the moment of truth: Batteries were loaded in, and the unit switched on. A hum as the high-voltage vibrating circuit came to life, followed by an increasing green glow in the eyepiece. It worked! The damage of time is apparent when one looks through the tube. Just as one appreciates a patina on an old painting, so too must one respect the evidence of time when gazing through the eyepiece of such a rugged little device.

Restored scope

The restored US/C-3 Telescope. Note the filter on the front of the objective lens.

Today the little telescope adorns the office of its proud owner, a now-working testament to the ingenuity of wartime, and the design principles that can stand the test of time. займы на карту займ по паспорту онлайнзайм наличными онлайнонлайн займ на карту быстро займ денег в москведеньги займ ульяновсккиви займ онлайн займ переводом на картузайм на карту 50000 на годпетрозаводск займ оформить займ на карту без отказазайм на карту срочно онлайнваши деньги займ взять займ с открытыми просрочкамизайм под залог участкасамый быстрый займ на карту

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