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.