A 90° diagonal prism for Schmidt-Cassegrain and refractor telescopes, the Meade #918A renders upright but laterally-reversed images which are brighter than erect image designs, which render images both upright and laterally correct at the expense of a bit of light. When at lower magnifications it is fine to use an erecting device, but when maximizing your magnification option given the scope, seeing conditions & object of interest it is important to employ a device which leaves the image reversed.