Types of Telescope: Optical Telescope

An optical telescope is a device used to observe distant objects by gathering and focusing light from the visible part of the electromagnetic spectrum. It helps create magnified images for direct viewing, photography, or data collection through electronic sensors.

Working:

• Optical telescopes primarily use two or more lenses.
• The main lens, called the objective lens, is designed to collect as much light as possible from a distant object.
• A larger objective lens ensures more light is gathered, enabling a clearer and brighter image.
• The light collected by the objective lens is directed to a smaller lens, known as the eyepiece.
• The eyepiece magnifies the image, making it larger and easier to observe.
• When light passes through a lens, it changes direction due to a process called refraction. This occurs as light enters and exits the lens.
• Telescopes that rely on refraction for image formation are called refracting telescopes.

Refracting telescope

Refracting telescopes are useful for observing space, but they have limitations

• The objective lens must be very large to collect more light, which makes it heavy and difficult to manufacture without distortion.
• Larger lenses increase the length of the telescope, making it cumbersome to handle.
• Lenses cause chromatic aberration, where colours in the image appear distorted.

To address these challenges, reflecting telescopes are used. These telescopes use concave mirrors instead of lenses.

• Large mirrors are easier to build and can be made by combining smaller pieces.
• Mirrors are lighter than lenses of the same size.
• Images formed by mirrors are free from chromatic aberration.

Reflecting telescopes enable the observation of distant stars and galaxies that are invisible to the human eye.

1. Newtonian Telescope

• Light from space is collected by a concave mirror.
• It is then deflected by a small flat (plane) mirror to the side.
• The light passes through an eyepiece, producing a magnified image.

The Newtonian telescope

2. Cassegrain Telescope

• Light is collected by a concave mirror and reflected back toward a smaller convex mirror.
• The light passes through a hole in the concave mirror and then through an eyepiece located at the back, creating a magnified image.

The Cassegrain telescope

In India, several reflecting telescopes with concave mirrors of 2-meter diameter have been in use for years. The largest optical telescope in India, and also in Asia, has a 3.6-meter-diameter mirror. It is located at the Aryabhatt Research Institute of Experimental Sciences in Nainital.