I wanted a 1.25 inch diagonal with a compression ring rather than one with the standard set screw that marks eyepiece barrels, and originally decided on the William Optics Erecting prism with the helical fine focusser. However reading the about the issues with diffraction spikes made me think again and I decided instead to go for this Sky-Watcher di-electric mirror diagonal, foregoing the bonus of the fine focusser.

Its of all metal construction and looks and feels nicely put together, a big step up in quality to the standard issue Sky-Watcher diagonals supplied with my Skymax 127 Mak and Startravel 102 Refractor.

My ancient eyes can't see any difference in light transmission between this and the standard diagonal, however i'm more than happy with it.