This diagonal is for astronomers who prefer the convenience of 90-degree viewing but want to view a fully corrected image, ie the right way up and not mirror-reversed left to right.
Superb finish - the nosepiece is chromed brass, fully threaded for standard 1.25" filters. Internally blackened to eliminate reflections for improved contrast. The eyepiece holder is black-anodised aluminium with a chrome cross screw.
Screw-assembled prism body in dense resin (ie not flimsy plastic), gives the whole unit a great quality feel.