Binoviewer

From AstroPedia

Binoviewers allow an observer to in effect turn their telescope into a pair of binoculars so he/she can use both eyes to observe.

Typically a binoviewer has a 1.25" barrel, a split prism, and two 1.25" eyepiece holders. The barrel end slides into the focuser, and two 1.25" eyepieces of equal f/l go in each holder.

Binoviewers allow the luxury of enabling you to view with both eyes, but half the amount of light coming to each eye and mean you need to purchase twice as many eyepieces.