I bought the WO Erecting Prism diagonal to use with my ZenithStar 61 scope when I want to do some birdwatching. The 45 degree angle puts the eyepiece in a more comfortable position for terrestrial use than a typical 90 degree diagonal, and when combined with a good quality zoom eyepiece such as Baader's Hyperion Zoom Mk IV turns a small refractor into a superb spotting scope. The only limitation of the design is that Amici prisms generally aren't ideal for high magnification use so while the image is razor sharp at 45x magnification, it looks soft if I add the Baader 2.25x Hyperion Barlow. Switching to a good quality mirror or prism diagonal will enable higher magnifications if you don't mind the left-right reversal and don't fancy paying 500 quid for Baader's excellent Astro Amici Prism.