Telescope

A telescope is a device that observes distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally, it was an optical instrument that used lenses, curved mirrors, or a combination of both to observe distant objects. It was called an optical telescope.

Nowadays, the word “telescope” is used for a wide range of instruments capable of detecting different regions of the electromagnetic spectrum and, in some cases, other types of detectors (for example, particle and gravity detectors). The first telescopes were refracting telescopes with glass lenses, which were invented in the Netherlands at the beginning of the 17th century. They were used for both terrestrial applications and astronomy. The reflecting telescope, which uses mirrors to focus light, was invented a few decades after the first refracting telescope.

In the 20th century, many new types of telescopes were developed, including radio telescopes in the 1930s, infrared telescopes in the 1960s, and satellite-based observatories from the 1970s onwards.

Telescope Types

A refracting telescope uses lenses to create an image, whereas a reflecting telescope uses mirrors to create an image.

A Galilean telescope is a refracting telescope that consists of a convergent lens as objective (i.e., the lens that forms the image); and an eyepiece (or ocular), placed in front of the focus, which is a divergent lens. An upright image is produced. This simple refracting telescope is still used in modern opera glasses, which are low-powered binoculars.

A Keplerian telescope is a refracting telescope usually used in astronomical observations, including a positive objective lens and a positive eyepiece, giving an inverted image and a relatively wide field-of-view.

The Newtonian telescope, also called the Newtonian reflector, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton, using a concave primary mirror and a flat diagonal secondary mirror.

Catadioptric Telscopes

A catadioptric telescope is an optical telescope that uses both lenses and mirrors to create an image. The word “catadioptric” comes from the words “catoptric” (mirrors) and “dioptric” (lenses). One example is a Schmidt telescope.

Principles

Catadioptric telescopes use a folded optical path to reduce their length/mass.
They often have a corrector plate that corrects optical aberrations (spherical aberration) of a spherical mirror, and a flat or curved secondary mirror that re-directs the light to a convenient focal position.
They are designed to correct field aberrations and have a wide field of view.

Advantages

They are smaller and more portable than other types of telescopes.
They are easier to manufacture.
They have a wider aberration-free field of view.

Disadvantages

Tube currents can cause poor images before they cool.
They can be expensive per inch of aperture.
They can have a lot of light loss.
They can have a large central obstruction (entrance pupil is obscured).

Astronomical Telescopes

An astronomical satellite observatory is a spacecraft that orbits Earth and studies celestial objects and radiation from above the atmosphere. These observatories can study cosmic phenomena in many regions of the spectrum, including gamma-ray, X-ray, ultraviolet, visible, and infrared.

Advantages

They can place instruments in space, allowing observation of all regions of the spectrum.
They avoid atmospheric turbulence and airglow.
They can observe dim objects during the day.
They avoid light pollution.

Astronomical satellite observatories

Orbiting Astronomical Observatory (OAO): A series of four American space observatories launched between 1966 and 1972
Infrared Astronomical Satellite (IRAS): The first telescope to study the sky in infrared light
Hubble Space Telescope: One of the Great Observatories
James Webb Space Telescope: A new observatory launched in 2021 has dramatically improved our knowledge of the distant parts of the universe.