I want to use my binoviewer with a Celestron scope. How much back focus does it need?

Binoviewers have a string of prisms inside that bend entering light through a path of several inches as the light is split and then brought to a pair of eyepieces. A typical binoviewer for 1.25” eyepieces (like the Celestron #93691) uses about 3.5 inches of back focus. Other brands use less and some use more than 4 inches back focus.

This is not a problem for a Schmidt-Cassegrain telescope and its approximately 5 inches of focal accommodation. Many Maksutov-Cassegrains also have large back focus distances. It is a problem for spotting scopes that take 1.25” eyepieces and for Newtonians. They simply don’t have enough back focus to use a binoviewer. You won’t be able to focus on infinity using a binoviewer with these scopes. 

Updated 12/27/13