Depending on the telescope’s position, it might be very uncomfortable for the viewer to look straight through the eyepiece! For that reason, a simple piece of equipment was invented: the diagonal. The star diagonal is easily recognizable. It contains an angled mirror or prism that allows for comfortable viewing, especially if the telescope is pointed at or near the zenith. At the same time, it can erect the image, making it appear “right side up”, but it also reverses left to right.