A Star Diagonal brings the eyepiece out at 90 degrees from the telescope tube, allowing comfortable viewing when the telescope is pointed at, or near, the zenith (i.e. directly overhead). The resulting image is the right way up, but is reversed from left to right.
The Bresser Messier 2" Mirror Diagonal - 93% Transmission is very good diagonal mirror for refracting and Cassegrain telescopes with 2" focusers (and for the Explore Scientific and Bresser Messier telescopes with 2.5"- HEX focusers).
High quality 2" star diagonal. Makes observing at high angles much more comfortable when using Refractor and SCT telescopes. Comes complete with the 2" - 1.25" adaptor so you can use both eyepiece sizes with this diagonal.