Cassegrain Telescope | Reflecting Type Telescope

 

Cassegrain Telescope | Reflecting Type Telescope :- An optical instrument designed for observing distant objects that uses mirrors to collect light from objects is called a reflecting type telescope.

Here we will read about Cassegrain telescope. It is a type of reflecting type telescope that uses two mirrors to collect and focus light. It is named after its French inventor, Laurent Cassegrain, who developed the design in the 17th century.

Structure of reflecting type telescope (Cassegrain telescope):-

The objective in a Cassegrain telescope is a parabolic concave mirror (primary mirror) of large focal length and large aperture with a small hole at the center pole. It is attached at the end of a wide tube and the other open end of this tube is directed towards the object. A smaller convex mirror (secondary mirror) is placed before the focus of the objective (concave mirror). This convex mirror reflects the rays reflected inside the tube by the objective before they are focused at its focus and sends them towards the eyepiece.

Reason for installing convex mirror (secondary mirror)

If the observer sits at the focus of a concave mirror to see the image in a Cassegrain telescope, the following problems arise :-

(1) Inaccessibility: In a Cassegrain telescope the focus of the concave mirror is located deep inside the telescope tube. It is neither practical nor convenient for an observer to sit inside the telescope tube.

(2) Practicality: Following celestial objects would also be challenging due to the Earth’s rotation, as the observer would need to move with the orientation of the telescope.

(3) Obstruction in the path of incident rays: If the observer sits at the focus of the concave mirror, then due to obstruction in the path of incident rays, some light gets reduced due to which the image does not become clear.

For these reasons, astronomers and observers do not use the primary focus of a Cassegrain telescope for visual observations.

 

Ray Diagram of Cassegrain telescope

Working of reflecting type telescope – Cassegrain Telescope

In a Cassegrain telescope, light rays coming parallel to the principal axis are reflected by the concave mirror (primary objective mirror) and focused at focus point in front of the mirror. A secondary mirror (convex mirror) stops the focused light rays and reflects them towards a hole located at the center of the primary mirror, where it converges to a point behind the primary mirror, known as the secondary focus. The final image is viewed through an eyepiece (a convex lens).

Magnifying Power Of Reflecting type Telescope

 

m = angle subtended at the eye by the final image (β)/ angle subtended at the eye by the object (α)

It can be proved that when the final image is formed at least distance of distinct vision (D), then

  …..(1)

And when the final image is formed at infinity (∞), then

  …..(2)

In equations (1) and (2) f_o and f_E are the focal lengths of the objective concave mirror and eyepiece lens respectively.