ETX-90AT and ETX-125AT Astro Telescopes include an internal optical-flat mirror to reflect light to the telescope's 90 astronomical observing position. In this position, the telescope's image is upright, but reversed left-to-right. This is of little consequence for astronomical viewing, but for terrestrial observing it is highly desirable to have a correctly oriented image. The #932 Erecting Prism delivers a fully corrected image and provides a convenient 45 observing angle for optimal terrestrial viewing.