ETX telescopes feature an internal flat mirror in order to reflect light to the telescope's 90° astronomical observing position. In this position the telescope's image is reversed yet upright. For terrestrial observing the 45 degree correcting prism results in a correctly oriented image and a convenient 45 degree observing angle. For use with the ETX-90, ETX-105, and ETX-125 telescopes (Use the Meade #933 for ETX-70AT).