Refractor telescopes and spotting scopes are the most common varieties of optical instruments and consist of a tube with an objective lens on the end. Light passes through that lens, through the tube, then into a prism that flips the image before it enters the eyepiece at the opposite end. Catadioptric scopes, by contrast, use an array of lenses and mirrors to “fold” the light path and generally have larger lenses suited for astronomy. On the whole, refractors are typically more compact, durable, and portable, which is why you see this design on spotting scopes often.