A small telescope is generally considered by professional astronomers to be any reflector-type telescope with a primary mirror of less than 2 metres (80 in) diameter. By amateur standards a small telescope can have a primary mirror/aperture less than 6–10 inches (150–250 mm) in diameter. Little if any professional-level research is performed with the refracting type of telescope in the modern era of astronomy.
Small telescopes dominate astronomical research in the fields of asteroid and comet discovery and observation, variable star photometry, and supernova and nova discovery, and colorimetry and polarimetry of the solar system's planets.
Because of their limited light-gathering capability, small telescopes are usually not well-suited to spectroscopy, although some useful spectroscopic work can be performed with reflecting-type telescopes with a primary mirror as small as 14 inches (35 cm) when equipped with the increasingly sophisticated modern ccd imaging and spectroscopic instrumentation recently becoming available to amateur astronomers.
Most telescopes within the field of amateur astronomy are considered to be small, ranging in general from 2-inch (50 mm) achromatic refracting types, to reflecting type telescopes featuring primary mirrors up to (and sometime exceeding) 36 inches (90 cm) in diameter. Most small telescopes are dedicated to visual observing, although many are applied to such uses as gathering scientific data, or astrophotography.
The range of amateur astronomer's telescopes is wide, with numerous types and designs, such as achromatic and apochromatic refractors, Newtonian reflectors, Schmitt-Cassegrain, Maksutov–Cassegrain and Maksutov–Newtonian compound reflectors. However in more recent years manufacturers of telescopes for the amateur market have begun offering more sophisticated designs, such as the Ritchey–Chrétien and (corrected) Dall-Kirkham, which have traditionally been the preserve of large professional-grade instruments.