Earth Based Telescopes – Optical Telescopes

The stars and galaxies in our universe lie at vast distances from the Earth. These are way beyond our reach and technological capabilities to travel to in order to gain information about them. However, the electromagnetic radiations, including light from these galaxies and stars, travel the vast distance to Earth. By studying these electromagnetic radiations information and great insight can be can be gained about the stars, galaxies and universe.

Telescopes are instruments used to study the electromagnetic radiations from the universe. The word telescope is derived from the Greek words “tele” (from afar) and “skopos” (viewer). The underlying principle of all telescopes is to gather radiation from distant objects and focus it. This increases the intensity of the radiations thereby allowing the objects to be magnified.

There are different types of telescopes available depending on the wavelength or type of radiation to be studied.

Optical Telescopes

Optical telescopes gather visible light rays from the electromagnetic spectrum. There are three basic types of optical telescopes.

  1. Refracting telescopes
  2. Reflecting telescopes
  3. Cassegrain telescopes

 

Refracting telescopes

Refracting telescopes consist of a closed tube. At one end of the tube is an object glass also known as the objective lens and at the opposite end of the tube there is the eyepiece lens.

Light from a distant star or planet enters the objective lens and the rays are refracted and an image is formed at the point of focus at the lower end of the tube. This image acts as an object for the eyepiece which acts as a magnifying lens enlarging the bright image. The observer views the object through the eyepiece or can attach a camera to the telescope to record images.

The animation below shows how a refracting telescope works.

Light rays reflected from the moon enter the objective lens and an image is formed at the focus F. This image acts as the object for the eyepiece. The focus point a distance f from the eyepiece lens coincided with the position of F. The eyepiece acts as a magnifying lens and an enlarged imaged of the object in this case the moon is formed.

Reflecting Telescope

Reflecting telescopes also called Newtonian telescopes are named after their inventor Sir Isaac Newton. Reflecting telescopes focus light rays using a large concave mirror covered with a thin coating of aluminium. Light rays are collected by a primary curved mirror at the bottom of the tube and reflected forwarded to a flat secondary mirror mounted at a 45 degree angle. This deflects the converging light rays by 90 degrees to an eyepiece.

The animation below shows how a reflecting telescope works.

Light is collected by the curve mirror at the rear of the telescope and reflected back up the tube to a flat secondary mirror mounted at a 45 degree angle. The image is then deflected by a 90 degree angle to the eyepiece.



Cassegrain Telescopes

Cassegrain telescopes are also reflecting telescopes named after their French inventor. Cassegrain telescopes consist of a concave primary mirror which reflects collected light back up the tube to a secondary convex surface mirror. Light is then reflected straight back down the tube through a hole in the center of the primary mirror, where it is received in the eyepiece.

The combined action of the two mirrors in this arrangement significantly increases the effective focal length over the actual length of the telescope allowing for a more compact telescope.

The animation below shows how a reflecting telescope works.

Light is collected by the concave mirror at the rear of the telescope and reflected back up the tube to a secondary convex surface mirror mounted at a 45 degree angle. Light is then reflected straight back down the tube through a hole in the center of the primary mirror, where it is received in the eyepiece.

Disadvantages and limitations of Earth based optical telescopes

  • They can only be used at night.
  • They cannot be used in poor or cloudy weather.
  • They are limited by other atmospheric conditions; particularly convection currents and air turbulence which distort the wave-fronts from the reference object i.e. a star.  However this can be overcome with the use of powerful computers integrated with the telescope which can measure the distortions in the atmosphere and adjust the mirror shape accordingly.
  • They can be limited by minute mechanical stresses in the primary mirror shape. These can also be compensated for by a computer which makes alterations to the mirror shape.

Advantages of reflecting telescopes

  • The primary mirrors are suitable for compensating techniques controlled by computers.
  • Larger mirrors can be made lighter reducing mechanical stresses due to weight.
  • Large size mirrors can be more easily manufactured.