Range finder: German

The German 1.5 m range finder(unfortunately, I missed to note the exact model !) (German:Enfernungsmesser)

The coastal batteries never operated standalone. Based on the curvature of the earth and other factors, suitable vantage points were set where lookout stations were built. These lookout stations use to have such range finders, often longer ones(4, 5, 6, 7 and 10 meters). Longer the device, more the range and accuracy. These lookout stations directed the fire from the battery guns.

Range finders operated on different principles, the one in the picture used optical trigonometry(I hope such examples are explained to the kids learning trigonometry in school or colleges, lest they end up hating that  topic which deserves reverence!). Put shortly – the two ends of the range finder had prisms which formed an image on the optical piece. Note that one end was fixed while the other moved. The images produced by both the ends were superimposed to give a uniform image, the angles made by both the ends were known and also the distance between two ends was known which helped to calculate the distance to the actual object (mostly ships but also aircrafts at times). Of course, weather, moving targets, curvature of earth and so on posed challenges but within 20-25 km, the above shown range finders were quite accurate.