Why should the design keep the eyepieces horizontal?
The eyepieces should both be aligned horizontally with the ground. Even though collimation doesn’t require it, the viewer tends to hold his or her head level so the eyes are parallel to the ground. Unfortunately, the binocular telescope is not like a pair of hand-held binoculars that the user easily aligns with his or her eyes. If the telescope isn’t aligned this way, the viewer must tip his or her head to one side or another in order to align with the eyepieces. Inexperienced telescope users are bothered by this. Rotating upper cages simplify this adjustment, since the viewer will naturally keep the eyepieces in a horizontal line.
