Refracting telescopes have an objective lens at the front of the tube with the light emitting through the back of the tube to the eyepiece. Since many observations are made high in the sky, a right angle diagonal is used to avoid neck strain. This also provides an upright image making them suitable for terrestrial observations. A refractor has several advantages over other telescope designs. The tube is enclosed so that dust and moisture do not enter the tube, and they do not have a central obstruction which reduces the light entering the tube. A refractor typically will give higher quality images of planets than other telescopes of similar aperture.