Binoviewers allow you to observe through your telescope using both eyes. They do this by means of an optical beam splitter that directs half of the available light to each of two eyepieces. These eyepieces need to be identical in order for you to make sense of what your individual eyes are seeing. Some objects, such as the planets, do tend to take on more of a three dimensional appearance when observed through a binoviewer. Some experienced observers also claim to be able to see more detail in faint objects when using two eyes.