Terrestrial telescopes are optical viewing devices primarily designed to look at objects on Earth, rather than in the night sky. There are several key differences that distinguish these telescopes from the more-common astronomic models, though some combination versions do exist. Terrestrial telescopes, also known as spyglasses, are frequently associated with the days of seafaring and piracy, but are still in use today for a variety of purposes.
Looking at objects on Earth requires different specifications that examining stars and planets. One of the primary considerations is the effect of air on a terrestrial image, since at high magnification levels, air tends to shimmer and distort the image. For this reason, the diameter of the lens, or aperture, is often a more important consideration when choosing terrestrial telescopes than the magnification capabilities. The larger the aperture, the less magnification is necessary at ground level, increasing the image quality.
Another important feature of terrestrial telescopes is the production of a non-inverted image. Many telescopes meant for astronomic viewing present an inverted image, which is not suitable for land viewing. Telescopes meant for terrestrial observation are designed to present an erect image, or one that is right-side up and left to right in presentation. In dual-use telescopes, a special adapter can be attached to the eye piece to create an erect image when viewing Earth objects.
Since terrestrial telescopes are often used for mobile observation, such as birdwatching, design and weight are important considerations. Most terrestrial scopes are prismatic, meaning that the image is viewed from the top or back of the scope. Newtonian telescopes, which are popular for star gazing, are usually unsuitable for a spyglass, since the eyepiece is on the side of the scope. Most terrestrial models are also compact and light, and can be easily carried around for day use. Larger models can be mounted on tripods for stability and to bear their far heavier weight.
The history of the terrestrial telescope is inextricably linked to the great days of maritime exploration. Ships would use terrestrial scopes to identify other ships as friends or foes, but could also use the instruments to gauge possible reef locations or search for land on the far distant horizon. In modern times, terrestrial telescopes are frequently used by photographers, scientists, and bird watchers to help identify subjects in large landscapes. Hunters may also use spyglasses to help identify camouflaged or distant prey.