An attachment for a small telescope, containing a small plane mirror or prism, used to turn through a right angle the beam of light in the telescope and the direction of the draw-tube into which the eyepiece is inserted. Use of a diagonal may be helpful when the visual user of a small amateur telescope would have difficulty accessing the eyepiece in its normal tube. However, there is some loss of light when a further optical element is introduced, and there may also be loss of image quality. The image in a diagonal is reversed right-to-left.