Astronomy - Telescopes

Telescopes

Lenses and their properties were known well before the invention of the optical telescope; simple lenses made from rock crystal have been known from before recorded history.

1611--The term "telescope" is coined by Prince Frederick Sesi at a reception where Galileo was demonstrating his instruments.

The earliest known working telescopes appeared in 1608 in the Netherlands and are credited to Hans Lippershey and others.

The design of these early refracting telescopes consisted of a convex objective lens and a concave eyepiece. They are constructed this way to not invert the image.

Lippershey's original design had only 3x magnification.

In 1611, Johannes Kepler described how a telescope could be made with a convex objective lens and a convex eyepiece lens and by 1655 astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces.

Telescopes seem to have been made in the Netherlands in considerable numbers soon after the date of their invention, and rapidly found their way all over Europe.

Keplerian telescope

Isaac Newton is credited with building the first "practical" reflector in 1668 with a design that incorporated a small flat diagonal mirror to reflect the light to an eyepiece mounted on the side of the telescope.

Laurent Cassegrain in 1672 described the design of a reflector with a small convex secondary mirror to reflect light through a central hole in the main mirror.

Newton Reflector

 

Gassegrain Reflector

About the year 1774 William Herschel (then a teacher of music in Bath, England) began to occupy his leisure hours with the construction of reflector telescope mirrors, finally devoted himself entirely to their construction and use in astronomical research.

In 1783, Herschel completed a reflector of approximately 18 inches (46 cm) in diameter and 20 ft. (6.1 m) focal length. He observed the heavens with this telescope for some twenty years, replacing the mirror several times.

In 1789 Herschel finished building his largest reflecting telescope with a mirror of 49 inches (120 cm) and a focal length of 40 ft. (12 m), (commonly known as his 40-foot telescope) at his new home, at Observatory House in Slough.

This telescope was world's largest telescope for over 50 years. However, this large scope was difficult to handle and thus less used than his favourite 18.7-inch reflector.

Essentially all major optical research telescopes since 1900 have been reflectors. A number of 4-metre class (160 inch) telescopes were built on superior higher altitude sites including Hawaii and the Chilean desert in the 1975–1985 era.

The development of the computer-controlled alt-azimuth mount in the 1970s and active optics in the 1980s enabled a new generation of even larger telescopes.

An altazimuth or alt-azimuth mount is a simple two-axis mount for supporting and rotating an instrument about two perpendicular axes – one vertical and the other horizontal.

Rotation about the vertical axis varies the azimuth (compass bearing) of the pointing direction of the instrument. Rotation about the horizontal axis varies the altitude (angle of elevation) of the pointing direction.

John Dobson popularized a simplified altazimuth mount design for Newtonian reflectors because of its ease of construction; Dobson's innovation was to use non-machined parts for the mount that could be found in any hardware store such as plywood, Formica, and plastic plumbing parts combined with modern materials such as nylon or Teflon.

A Newtonian Telescope on a Simple Dobsonian Mount

An equatorial mount is a mount for instruments that compensate the rotation of earth by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras.

The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any object in the sky by driving one axis at a constant speed. Such an arrangement is called a sidereal drive.

A computerised telescope is a telescope with the so-called "GoTofunction". A GoTo-system prevents you from having to locate an object in space yourself. You can always be sure that you focus on the right star or planet.

There is no need to adjust the telescope yourself either, this is done automatically! All in all, this is a great invention, especially for the beginning or amateur astronomer. The GoTofunction enables you to find your way in space, with just a push of a button.

All celestial objects with a temperature above absolute zero emit some form of electromagnetic radiation. In order to study the universe, scientists use several different types of telescopes to detect these different types of emitted radiation in the electromagnetic spectrum. Some of these are gamma ray, x-ray, ultra-violet, regular visible light (optical), as well as infrared telescopes.

An infrared telescope is a telescope that uses infrared light to detect celestial bodies. Infrared light is one of several types of radiation present in the electromagnetic spectrum.

In 1800, William Herschel discovered infrared radiation.

A radio telescope is a specialized antenna and radio receiver used to receive radio waves from astronomical radio sources in the sky.

Radio astronomy studies the radio frequency portion of the electromagnetic spectrum emitted by astronomical objects,

The era of radio telescopes (along with radio astronomy) was born with Karl GutheJansky'sdiscovery of an astronomical radio source in 1931. Many types of telescopes were developed in the 20th century for a wide range of wavelengths from radio to gamma-rays.

The development of space observatories after 1960 allowed access to several bands impossible to observe from the ground, including X-rays and longer wavelength infrared bands.

The 76 metre Lovell, Jodrell Bank

IACT stands for Imaging Atmospheric (or Air) Cherenkov Telescope. It is a device or method to detect very-high-energy gamma-ray photons. There are currently three operating IACT systems: H.E.S.S., MAGIC and VERITAS.

Set to be the world's largest telescope at the highest altitude, the Major Atmospheric Cerenkov Experiment Telescope (MACE) is currently being established at Hanle, Ladakh, India. Also, currently under design is the Cherenkov Telescope Array (CTA).

The SubmillimeterTelescope (SMT), formerly known as the Heinrich Hertz SubmillimeterTelescope, is a submillimeterwavelength radio telescope located on Mount Graham, Arizona. It is a 10-meter-wide parabolic dish inside a building to protect it from bad weather. The telescope's construction was finished in 1993. Along with the 12 Meter Telescope on KittPeak, this telescope is maintained by the Arizona Radio Observatory, a division of Steward Observatory at the University of Arizona.

Ultraviolet astronomy is the observation of electromagnetic radiation at ultraviolet wavelengths between approximately 10 and 320 nanometres; shorter wavelengths—higher energy photons—are studied by X-ray astronomy and gamma ray astronomy.

Light at these wavelengths is absorbed by the Earth's atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space.

An X-ray telescope (XRT) is a telescope that is designed to observe remote objects in the X-ray spectrum. In order to get above the Earth's atmosphere, which is opaque to X-rays, X-ray telescopes must be mounted on high altitude rockets, balloons or artificial satellites.

The basic elements of the telescope are the optics, that collects the radiation entering the telescope, and the detector, on which the radiation is collected and measured. A variety of different designs and technologies have been used for these elements.

A solar telescope is a special purpose telescope used to observe the Sun. Solar telescopes usually detect light with wavelengths in, or not far outside, the visible spectrum.

Professional solar observatories may have main optical elements with very long focal lengths and light paths operating in a vacuum or helium to eliminate air motion due to convection inside the telescope.