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A spotting scope is a compact high-power telescope optimized for detailed observation of distant objects. They are used as portable optical enhancement devices for various outdoor activities such as birdwatching, skygazing and other naturalist activities, for hunting and target shooting to verify a marksman's shot placements, for tactical ranging and surveillance, and for any other application that requires higher magnification than ordinary binoculars (typically 20× to 60×).
The light-gathering power and resolution of a spotting scope is determined by the diameter of the objective lens, typically between 50 and 80 mm (2.0 and 3.1 in). The larger the objective, the more massive and expensive the telescope.
The optical assembly has a small refracting objective lens, an internal image-erecting system, and an eyepiece that is usually removable. The image-erecting system may use relay lenses, prisms such as Porro or roof prisms, or a catadioptric system of the Schmidt or Maksutov design. Spotting scopes may have a ruggedized design, a mounting interface for attaching to a tripod, and an ergonomically designed and located control knob for focus adjustment. Some spotting scopes also have in-built reticles for stadiametric rangefinding.
Spotting scope eyepieces are usually interchangeable to adapt for different magnifications, or may have variable zoom to give a range of magnifications. Magnifications less than 20× are unusual, as are magnifications more than 60× since the latter can lead to poorer image brightness, a narrow field of view and too much image shaking, even on a tripod. The eyepiece mount layout can be "straight-through" (the eyepiece is on the same axis as the scope body), or "angled" (the eyepiece is at an angle to the scope body—usually 45 degrees).
The high magnification of spotting scopes makes them prone to image disturbance from vibrations, so they are often stabilized with tripods or (less commonly) monopods, which provide a stationary and steady platform. Tripod heads can be used to control any required movements of the scope.