Diopter

Diopter

Diopter ( dr. Greek δι-όπτρα ) - the simplest device for setting the direction to the object. Represents two plates, mounted on the ends of the alidade . One plate, called the eye diopter , has a small opening or narrow gap. In the opposite plate, called the subject diopter , a fly or a thin hair is fixed in a much larger hole. The use of a diopter is to rotate the alidade so that the small hole, hair and object are on the same line.

The measurement error using a diopter (with a scale) can be no more than 2 angular minutes. A greater distance between the eye and subject diopters corresponds to a smaller measurement error.

The first mention of the diopter is dated back to the 1st century. ( Heron of Alexandria ). In the past, the diopter was widely used in geodetic instruments, now it is preserved only in some types of bussolas .