Reflecting telescope in the context of "40-foot telescope"

Diffraction spikes are lines radiating from bright light sources, causing what is known as the starburst effect or sunstars in photographs and in vision. They are artifacts caused by light diffracting around the support vanes of the secondary mirror in reflecting telescopes, or edges of non-circular camera apertures, and around eyelashes and eyelids in the eye.

While similar in appearance, this is a different effect to "vertical smear" or "blooming" that appears when bright light sources are captured by a charge-coupled device (CCD) image sensor.

The history of astronomy focuses on the efforts of civilizations to understand the universe beyond earth's atmosphere.Astronomy is one of the oldest natural sciences, achieving a high level of success in the second half of the first millennium. Astronomy has origins in the religious, mythological, cosmological, calendrical, and astrological beliefs and practices of prehistory. Early astronomical records date back to the Babylonians around 1000 BC. There is also astronomical evidence of interest from early Chinese, Central American and North European cultures.

Astronomy was used by early cultures for timekeeping, navigation, spiritual and religious practices, and agricultural planning. Ancient astronomers observed and charted the skies in an effort to learn about the workings of the universe. During the Renaissance Period, revolutionary ideas emerged about astronomy. One such idea was contributed in 1543 by Polish astronomer Nicolaus Copernicus, who developed a heliocentric model that depicted the planets orbiting the sun. This was the start of the Copernican Revolution, with the invention of the telescope in 1608 playing a key part. Later developments included the reflecting telescope, astronomical photography, astronomical spectroscopy, radio telescopes, cosmic ray astronomy, infrared telescopes, space telescopes,ultraviolet astronomy, X-ray astronomy, gamma-ray astronomy, space probes, neutrino astronomy, and gravitational-wave astronomy.

A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally, it was an optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects – an optical telescope. Nowadays, the word "telescope" is defined as a wide range of instruments capable of detecting different regions of the electromagnetic spectrum, and in some cases other types of detectors.

The first known practical telescopes were refracting telescopes with glass lenses and were invented in the Netherlands at the beginning of the 17th century. They were used for both terrestrial applications and astronomy.

The VISTA (Visible and Infrared Survey Telescope for Astronomy) is a wide-field reflecting telescope with a 4.1 metre mirror, located at the Paranal Observatory in Chile. It is operated by the European Southern Observatory and started science operations in December 2009. VISTA was conceived and developed by a consortium of universities in the United Kingdom led by Queen Mary University of London and became an in-kind contribution to ESO as part of the UK's accession agreement, with the subscription paid by the UK Science and Technology Facilities Council (STFC).

VISTA is a survey telescope working at infrared wavelengths, and is by far the largest telescope in the world dedicated to surveying the sky at near-infrared wavelengths. The telescope has only one instrument: VIRCAM, the Vista InfraRed CAMera. This is a 3-tonne camera containing 16 special detectors sensitive to infrared light, with a combined total of 67 million pixels.

Active optics is a technology used with reflecting telescopes developed in the 1980s, which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, and mechanical stress. Without active optics, the construction of 8 metre class telescopes is not possible, nor would telescopes with segmented mirrors be feasible.

This method is used by, among others, the Nordic Optical Telescope, the New Technology Telescope, the Telescopio Nazionale Galileo and the Keck telescopes, as well as all of the largest telescopes built since the mid-1990s.

A curved mirror is a mirror with a curved reflecting surface. The surface may be either convex (bulging outward) or concave (recessed inward). Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are sometimes used in optical devices. The most common non-spherical type are parabolic reflectors, found in optical devices such as reflecting telescopes that need to image distant objects, since spherical mirror systems, like spherical lenses, suffer from spherical aberration. Distorting mirrors are used for entertainment. They have convex and concave regions that produce deliberately distorted images. They also provide highly magnified or highly diminished (smaller) images when the object is placed at certain distances. Convex mirrors are often used for security and safety in shops and parking lots.

A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and astronomical telescopes but is also used for long-focus camera lenses. Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by the reflecting telescope, which allows larger apertures. A refractor's magnification is calculated by dividing the focal length of the objective lens by that of the eyepiece.

Refracting telescopes typically have a lens at the front, then a long tube, then an eyepiece or instrumentation at the rear, where the telescope view comes to focus. Originally, telescopes had an objective of one element, but a century later, two and even three element lenses were made.