"A star diagonal "consumes" back-focus distance, which most reflectors have precious little of to begin with. With the star diagonal in place in the reflector's focuser, you will be unlikely to have enough inward focuser travel to make up for the back-focus distance the diagonal consumes in the optical path to bring most, if not all eyepieces into focus." http://www.astronomyforum.net/te ... tors-diagonals.html