Wndsn Telemeter Instructions
Telemeters setup, operation, and maintenance.
Many uses of the Telemeter are apparent after simple instructions, but some techniques require some in-depth practice and special attention to detail.
Initial Telemeter Setup
To install, knot the string to the card through the provided hole. Measure a length of 57.3 cm (22.44 inches) from eye to card. You can make a knot at the end or a loop; for measuring distances, hold onto the string with your teeth.
Why is the string length so important, and what's special about the 57.3?
It says "baked-in trigonometry"; also, on the tool itself, there is a hint:
For d = 57.3 cm, arctan(s/d) in deg = s => 1° = 1 cm
The explanation is the definition of radian, or rad: The length of an arc of a unit circle is numerically equal to the measurement in radians of the angle that it subtends; one radian is just under 57.3° (when the arc length is equal to the radius). The circumference subtends an angle of 2π radians.
Now, at a radius (hence the string length) of 57.3 cm, 1 cm in measured size subtends exactly 1° and makes for elegant calculations;
D = S * d / α where:
- D = distance from eye to observed object of known size
- S = known size of the object observed
- d = distance from tool to observer's eye
- α = the angular size of the object observed at the distance d