Rangefinder #2

photo by Gayle Laird, Exploratorium

Rangefinder #2 is based off an interwar British model by Barr and Stroud. The main difference between this model and the first one is that it does not use mirrors only prism wedges and penta prisms to manipulate the lines of sight. I was lucky enough to find and download a training manual for this particular device and reverse engineer it. Unfortunately, in the interim I seem to have lost the particular web address it came from. Searching for "Barr and Stroud rangefinder" images will find several diagrams outlining its function, but I will try to post a link to the original documents I used once I find them. Generally speaking you look into the center eyepiece via an integrator prism (which I got from a old 3 lcd projection television) you see out through 2 objective lenses and then 2 penta prisms at the ends of the rangefinder, which direct your vision forward. There are 2 prism wedges between the objective lenses and the integrator in the prism. The one on the right of the photo is for calibration and the one on the left is for range finding. Other than the above mentioned optical components the range finder consists of 8020 aluminum stock, machined delrin and laser cut acrylic.

photo by Gayle Laird, Exploratorium

Here you can see the range rule on the top of the sliding prism. When used the two channels of the Rangefinder appear red (the adjustable side) and blue (the fixed side) which makes its somewhat easier to orient yourself when looking through the Rangefinder. This is because the integrator prism had filters for red, green and blue for the corresponding lcds. I only used the 2 side channels, the green channel faces forward and is covered by cardboard. In general this set up was way more accurate than the first version. It was able to get the same reading over and over on objects under 1000 yards, but I found that it is very sensitive to vibration so it might not give the same readings after being moved. I continue to try and improve the design.

A detail of the eye piece and range rule

What you see looking through the eyepiece with coincidence not yet been achieved

Rangefinder #1

This is a Coincidence Range Finder prototype I built for the Exploratorium late last year. Its essentially an optical trigonometry tool which allows you to measure the length to a distant object based on the distance between the pods and the angle of deflection needed to see the same thing from both pods. When the range finder is dialed in you will see 2 perfectly superimposed images of the same object, meaning you set the angle of the mirror in the control pod so it sees what is directly in front of the telescope pod. For a general overview of the theory behind Coincidence Range Rinders work take a look at this wikipedia article and the diagrams at the bottom of this page.

This is the inside of the telescope beam splitter pod. I mounted an inexpensive 10x monocular in some pvc pipe fittings. In front of the monocular I mounted a half height adjustable first surface mirror to receive light from the control pod window and the window directly in front of the monocular simultaneously.

This is the inside of the control pod, a larger first surface mirror collects light from the closed window and sends it out the removed one to the telescope pod. The mirror is adjustable via a knob driven screw which also drives a gear train to the range gauge.

Here is the back of the control pod. Note that the range gauge window is made to open as the ranges were written on by hand using a laser range finder for this first prototype.

Looking into the control pod mirror from the front you can see the light path back to the beam splitter and into the telescope. I used speed rail to connect the 2 pods for easy adjustment and disassembly.

While you could achieve coincidence (see 2 super imposed images) this range finder only proved vaguely accurate, mainly because the range gauge was driven by the control knob and not directly on the position of mirror. The gear train tended to slip disconnecting the mirror position thus the angle of coincidence from the read out on the gauge. In response to the problem of inconsistent accuracy I decided to look at some military range finder designs, which lead to the creation of Rangefinder #2.