An optical instrument that gathers and focuses light into a camera, CCD, spectrograph , or an astronomer's eye. A primary mirror gathers and focuses light to the focal point. These telescopes are most common in astronomical observatories.
Aperture ranges from a few centimeters to several meters.
Mirror diameter is limited by the flexible nature of glass, mirror weight, and design of the telescope. Some telescopes overcome these problems with a segmented primary mirror. Aperture size is limited by the purity and weight of the glass lens. Several of the most magnificent telescopes are refractors. Yerkes Observatory's 40-inch refractor is the largest.