1. in astronomical telescope; the rays from the object after refraction at the objective are brought to a focus before entering the eyepiece to produce a real inverted image in front of the eye piece
2.in Galileo's telescope; the rays from the object get refracted at the objective and are intercepted by the eyepiece before a real image is formed
3.a line passing through the optical centre of the objective traversing through the eyepiece; is called line of sight
4.all the above.