20.Telescope (a little bit of history)

 

In 1608 Hans Lippershey, an eyeglass maker, submitted the first patent of telescope. But who made the first telescope, it is not clear. Later on, Galileo improved the design and started applying in Astronomy. This telescope had convex objective lens and concave eyepiece lens. In 1611 Kepler developed the design using convex lens in both objective and eyepiece and this design was more useful than the previous one developed by Galileo.

Diagram of Galilean and Keplerian Telescope

In 1655 Huygen was making powerful and enhanced Keplarian telescopes using compound eyepiece but those telescopes were proved inaccurate. In 1688 Isaac Newton built the first reflector telescope which consisted of a tiny flat diagonal mirror to reflect the incoming light on an eyepiece. Using achromatic lens, in 1733, Chester Moore Hall built a telescope.

First Reflector Telescope built by Newton

Diagram of Achromatic Lens

When light of white color passes through a medium it splits into several colors. For example, when light of white color passes through glass medium it splits into several colors because different colors have different refracting indexes in the glass medium which causes the velocity of light to vary. As a result, several images of different colors of an object gets created. To solve this problem an achromatic lens can be used which helps to create a single image of an object.

 

An achromatic lens helps to reduce colour aberrations of objective lens. Telescopes with achromatic lens were more functional and shorter. In 1721 reflecting telescopes were further developed after the production of larger paraboloidal mirrors by John Hadley. In 1932 when the adoption of long-lasting aluminized coatings on reflector mirror started and when the process of silvering glass mirrors was introduced by Leon Foucault more development on reflecting telescopes had taken place. Around 1910 the Cassegrain Reflector variant named Ritchey-Chretien was invented but after 1950 it was widely accepted and used in many modern telescopes including Hubble Space Telescope.

Diagram of Cassegrain Telescope

Hubble Space Telescope

During 1850 to 1900, because of using metal mirrors, reflector telescopes faced some problems and in the mean time refractor telescopes were developed a lot and telescopes with 60 c.m. to 1 meter aperture were built. In this period of development the largest refractor telescope at Yerkes Observatory was built in 1897. However, from early 1900s larger reflectors with glass mirrors were started building. In 1917 the Hooker Telescope and in 1948 the Hale Telescope were built, both of them are reflector telescopes.

Refractor Telescope of Yerkes Observatory

Hooker Telescoope

Hale Telescope

In short, after 1900 all major telescopes that are built for research purposes are mainly reflector telescopes. The development of computer controlled alt-azimuth mount in the 1970s and active optics in the 1980s paved the way for building a new generation of telescopes that are even larger than the previous ones such as, the 10 meter (400 inch) Keck telescopes from 1993 to 96 and also a number of 8 meter telescopes including the Subaru Telescope, the ESO Very Large Telescope and Gemini Observatory.

Diagram of Alt-Azimuth Mount

Subaru Telescope

Gemini Observatory

ESO Very Large Telescope

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

The technology, which is used in reflecting telescopes, to actively shape a telescope’s mirror to prevent deformation due to external influences such as wind, temperature and mechanical stress is known as active optics.

 

In 1931 the era of radio telescopes were born after Karl Guthe Jansky’s accidental discovery of an astronomical radio source. In the 20^th century various types of telescope were developed for a wide range of wavelengths from gamma rays to radio waves. In 1960, after the development of space observatories it became possible to access the wavelengths that are not accessible from ground such as, x-rays and longer wavelength infrared bands.

Radio Telescope

Classification of telescopes:

1.    Optical Telescope Types.

2.    Telescopes classified by the task performed.

3.    Telescopes that work outside the optical spectrum.

4.    Astronomical Interferometer.

5.    Types of telescopes according to mount types.