3. Line of Sight

Many, but not all, instruments have a telescope for sighting on a target. The telescope, Figure E-7, is a tube with an objective lens at one end and operator’s eyepiece at the other. Just forward of the eyepiece are the crosshairs (aka, crosswires) which are mounted to the reticle ring. The telescope has a main focusing knob for the objective lens and a focusing ring for the crosshairs.

Figure E-7

The Line of Sight (LoS) is defined by the optical center of the objective lens and the crosshairs intersection, Figure E-8.

Figure E-8
Line of Sight

Depending on the instrument, the telescope may rotate in only a horizontal plane or in both horizontal and vertical planes. This will be discussed in the appropriate instrument topics later.