# What is this optical device (USSR WWII) used to aim(?) an anti-aircraft gun? **URL:** \ **Category:** Factual Questions\ **Created:** [September 27, 2013, 11:56pm UTC](https://boards.straightdope.com/t/what-is-this-optical-device-ussr-wwii-used-to-aim-an-anti-aircraft-gun/669847 "2013-09-27T23:56:05Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [September 27, 2013, 11:56pm UTC](https://boards.straightdope.com/t/what-is-this-optical-device-ussr-wwii-used-to-aim-an-anti-aircraft-gun/669847/1 "2013-09-27T23:56:05Z")
Please take a look at [this picture](http://en.rian.ru/photolents/20130705/182073194_2/Battle-of-Kursk-Began-70-Years-ago.html). It shows a Soviet anti-aircraft battery team in action at Kursk in August 1943. I am curious to know more about the device that the soldier on the right is using. From the looks of it, I imagine that it may be some sort of optical equipment that gives a stereoscopic view of distant targets (although that is a total guess). But, if I _am_ right about that, does it work by the parallax principle? Given the relatively short distance between the two ends of the tube, is it even possible to exploit the parallax phenomenon? Bottom line is that I don’t really have a clue! Thanks! ---
**Author:** ![ExTank](https://avatars.discourse-cdn.com/v4/letter/e/35a633/32.png) [@ExTank](https://boards.straightdope.com/u/ExTank)\ **Post date:** [September 28, 2013, 12:11am UTC](https://boards.straightdope.com/t/what-is-this-optical-device-ussr-wwii-used-to-aim-an-anti-aircraft-gun/669847/2 "2013-09-28T00:11:34Z")
Best guess: an optical rangefinder. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [September 28, 2013, 12:14am UTC](https://boards.straightdope.com/t/what-is-this-optical-device-ussr-wwii-used-to-aim-an-anti-aircraft-gun/669847/3 "2013-09-28T00:14:30Z")
> [@ExTank](#): > > Best guess: an optical rangefinder. Yes, that’s what I was getting at - but, if it is a rangefinder, does it work by parallax? ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [September 28, 2013, 12:20am UTC](https://boards.straightdope.com/t/what-is-this-optical-device-ussr-wwii-used-to-aim-an-anti-aircraft-gun/669847/4 "2013-09-28T00:20:02Z")
It has to, why else have it shaped like that? Pretty slick idea really. ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [September 28, 2013, 12:26am UTC](https://boards.straightdope.com/t/what-is-this-optical-device-ussr-wwii-used-to-aim-an-anti-aircraft-gun/669847/5 "2013-09-28T00:26:00Z")
There are several very similar examples in the parallax rangefinder Wikipedia entry: > **[Coincidence rangefinder](https://en.wikipedia.org/wiki/Coincidence_rangefinder)** > > A coincidence rangefinder or coincidence telemeter is a type of rangefinder that uses mechanical and optical principles to allow an operator to determine the distance to a visible object. There are subtypes split-image telemeter, inverted image, or double-image telemeter with different principles how two images in a single ocular are compared. Coincidence rangefinders were important elements of fire control systems for long-range naval guns and land-based coastal artillery circa 1890–1960. They w... ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [September 28, 2013, 1:06am UTC](https://boards.straightdope.com/t/what-is-this-optical-device-ussr-wwii-used-to-aim-an-anti-aircraft-gun/669847/6 "2013-09-28T01:06:44Z")
> [@jnglmassiv](#): > > There are several very similar examples in the parallax rangefinder Wikipedia entry: > [Coincidence rangefinder - Wikipedia](http://en.wikipedia.org/wiki/Coincidence_rangefinder) Perfect! Thank you. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [September 28, 2013, 2:04am UTC](https://boards.straightdope.com/t/what-is-this-optical-device-ussr-wwii-used-to-aim-an-anti-aircraft-gun/669847/7 "2013-09-28T02:04:48Z")
Naval ships had [REALLY big ones](http://everywhereonce.files.wordpress.com/2010/11/uss-north-carolina.jpg). See that thin vertical rectangle to the right of that yellow square? That’s ONE side of the rangefinder for that turret. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [September 28, 2013, 3:04am UTC](https://boards.straightdope.com/t/what-is-this-optical-device-ussr-wwii-used-to-aim-an-anti-aircraft-gun/669847/8 "2013-09-28T03:04:29Z")
As the Wikipedia article notes, you do indeed get enough parallax from that distance to get a meaningful reading. The pentaprism they refer to bends the light path through a 90 degee angle, reardless of the angle of incidence (unlike , say, a right angle prism, in which the angle between the incoming and outgoing ray depends on the angle of incidence). Typically, there was a thick glass plate or another prism inside that you rotated to get the two images to coincide. The readout on the rotating knob gave you the range to target. ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [September 28, 2013, 3:08am UTC](https://boards.straightdope.com/t/what-is-this-optical-device-ussr-wwii-used-to-aim-an-anti-aircraft-gun/669847/9 "2013-09-28T03:08:56Z")
Have you ever heard of a rangefinder film camera? The two mirrors are about 2" apart. Pretty accurate out to 40’ or so. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 28, 2013, 3:40am UTC](https://boards.straightdope.com/t/what-is-this-optical-device-ussr-wwii-used-to-aim-an-anti-aircraft-gun/669847/10 "2013-09-28T03:40:13Z")
How far away parallax is good for depends both on the length of the baseline, and on the angular resolution you have available. These devices are presumably designed to maximize the angular resolution. ---
**Author:** ![OttoDaFe](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ottodafe/32/356_2.png) [@OttoDaFe](https://boards.straightdope.com/u/OttoDaFe)\ **Post date:** [September 28, 2013, 5:16am UTC](https://boards.straightdope.com/t/what-is-this-optical-device-ussr-wwii-used-to-aim-an-anti-aircraft-gun/669847/11 "2013-09-28T05:16:51Z")
> [@bump](#): > > Naval ships had [REALLY big ones](http://everywhereonce.files.wordpress.com/2010/11/uss-north-carolina.jpg). > > See that thin vertical rectangle to the right of that yellow square? That’s ONE side of the rangefinder for that turret. And if you want bigger still, look just under the radar antenna. That’s the main fire control rangefinder.