Astro-Observer

Reflector Telescopes

Published by Astro-Observer · Last reviewed November 8, 2026

A Comprehensive Overview

Reflector telescopes, also known as reflecting telescopes, are one of the most widely used types of telescopes in both amateur and professional astronomy. They utilize mirrors to gather and focus light, which provides clear and detailed images of celestial objects. This article delves into how reflector telescopes work, their history, notable examples, and consumer options available today.

How Reflector Telescopes Work

Reflector telescopes use a combination of mirrors to reflect light and form an image. The primary components include:

  1. Primary Mirror: This is the main mirror, typically concave (parabolic in a Newtonian), which collects incoming light and reflects it to a focal point.
  2. Secondary Mirror: Positioned in the path of the light reflected by the primary mirror, this smaller mirror redirects the light to the eyepiece or a camera. In a Newtonian it is a flat mirror tilted at 45 degrees that sends the light out the side of the tube.
  3. Eyepiece: This component magnifies the image formed by the primary and secondary mirrors, allowing the observer to view it.

The primary mirror gathers light from distant objects and brings it to a focus. The secondary mirror then reflects this focused light to the eyepiece, where the image is magnified for observation. This design avoids chromatic aberration, a common issue with refracting telescopes, where different colors of light are focused at different points.

For the sharpest views, the mirrors must be lined up precisely with each other and with the focuser. This adjustment is called collimation. Most amateur reflectors have adjustment screws on both mirrors, and checking collimation with a collimation cap, Cheshire eyepiece or laser collimator is a routine part of owning one, especially for fast (low f-ratio) mirrors.

History of Reflector Telescopes

The idea of using a curved mirror instead of a lens was discussed in the early 17th century, soon after the refractor appeared. Key steps include:

  1. James Gregory (1663): Gregory published a design with a concave primary and a concave secondary mirror, though he could not get a working telescope built at the time.
  2. Isaac Newton (1668): Isaac Newton is credited with building the first working reflector telescope, known as the Newtonian reflector. His design used a concave primary mirror and a flat diagonal secondary mirror to reflect light to an eyepiece mounted on the side of the telescope tube.
  3. Laurent Cassegrain (1672): A design attributed to Cassegrain featured a concave primary mirror and a convex secondary mirror, with the light focused through a hole in the primary mirror. This design, known as the Cassegrain reflector, and its variants are widely used in modern telescopes due to their compact size and long focal length.
  4. John Hadley (1721): Hadley built the first Newtonian with a successfully parabolic primary mirror, which greatly reduced spherical aberration and improved image quality.

Notable Examples of Reflector Telescopes

Reflector telescopes have evolved significantly over the centuries, leading to the construction of some of the most powerful and famous telescopes in history:

  1. Hubble Space Telescope: Launched in 1990, the Hubble Space Telescope is a Ritchey-Chrétien reflector (a variant of the Cassegrain design) that orbits Earth. Its 2.4-meter primary mirror has provided some of the most detailed images of distant galaxies, nebulae, and other celestial objects, significantly advancing our understanding of the universe.
  2. Keck Observatory: Located on Maunakea in Hawaii, the Keck Observatory consists of two 10-meter telescopes, each with a primary mirror made up of 36 hexagonal segments. These telescopes have been instrumental in numerous astronomical discoveries, including the study of exoplanets and the structure of galaxies.
  3. James Webb Space Telescope (JWST): Launched on December 25, 2021, and observing since 2022, JWST is the largest space telescope ever flown. It features a 6.5-meter primary mirror composed of 18 hexagonal segments and works in infrared light, allowing it to peer through dust clouds and observe some of the earliest galaxies in the universe.
  4. Gran Telescopio Canarias (GTC): The GTC, located on La Palma in the Canary Islands, is currently the world's largest single-aperture optical telescope, with a 10.4-meter primary mirror made of 36 segments. It has been used for a wide range of astronomical research, from studying black holes to exploring the properties of distant stars.

Consumer Reflector Telescopes

For amateur astronomers and hobbyists, several consumer telescopes built around mirrors offer excellent performance and value. Here are some well-known examples:

  1. Celestron NexStar 8SE: Strictly a compound (catadioptric) telescope rather than a pure reflector, this Schmidt-Cassegrain has an 8-inch aperture, a computerized GoTo mount, and SkyAlign technology for easy setup. See Compound Telescopes.
  2. Orion SkyQuest XT8: A classic Dobsonian reflector telescope with an 8-inch aperture, offering excellent light-gathering capabilities and ease of use. Orion closed its US operations in 2024, so it is now mainly found used; similar 8-inch Dobsonians are made by Sky-Watcher, Apertura and others.
  3. Sky-Watcher 6-Inch Dobsonian: This Dobsonian reflector features a 6-inch aperture, providing bright and clear images of planets, stars, and deep-sky objects. It's known for its affordability and simplicity, making it a great choice for beginners.
  4. Meade Polaris 130 EQ: This Newtonian reflector telescope has a 130mm aperture and comes with a manual equatorial mount, which makes following objects across the sky easier. Meade closed its US operations in 2024, so this model is now mainly found used.
  5. Zhumell Z12 Deluxe Dobsonian: With a 12-inch aperture, this Dobsonian reflector offers impressive views of deep-sky objects. Its large mirror and smooth altazimuth motion make it a powerful tool for serious amateur astronomers.

Advantages of Reflector Telescopes

Reflector telescopes offer several advantages over other types of telescopes:

  1. No Chromatic Aberration: A mirror reflects every color of light at the same angle, so all colors come to the same focus and there is none of the color fringing seen in simple refracting telescopes.
  2. Larger Apertures: Reflectors can be built with larger apertures than refractors, allowing them to gather more light and resolve finer details.
  3. Cost-Effective: Mirrors are generally cheaper to produce than large lenses, making reflectors more cost-effective for large telescopes.

Conclusion

Reflector telescopes have revolutionized the field of astronomy since their invention in the 17th century. By using mirrors to gather and focus light, they offer clear and detailed views of the cosmos without the chromatic aberration that affects refracting telescopes. From Isaac Newton's early designs to modern giants like the Hubble Space Telescope and the James Webb Space Telescope, reflector telescopes continue to play a crucial role in advancing our understanding of the universe. For amateur astronomers, a wide range of consumer reflector telescopes provide accessible and powerful tools to explore the night sky.