MAKE YOUR TOY ASTRONOMICAL TYPE TELESCOPE

 

MAKE YOUR TOY ASTRONOMICAL TYPE TELESCOPE

Nikunja Bihari Sahu

 

  Telescope (Refracting astronomical type):

      A telescope is a device that shows a distant object in magnified form. The telescope was first invented by a Dutch lens-maker Hans Lippershey in 1608. He believed that the telescope can be potentially used as a spyglass to detect pirates in the ships going along the seas. However, it was the Italian astronomer Galileo who first pointed his telescope skyward for astronomical investigations in 1609.

    A telescope primarily consists of two lenses; i.e. an Objective Lens and an Eyepiece Lens. The Angular Magnification of a Telescope is the ratio of the focal length of the Objective lens to the focal lens of the Eyepiece lens. The Angular Magnification of a Telescope is defined as the ratio of the angle subtended at the eye by the final image formed by the optical instrument to the angle subtended at the eye by the object when not viewed through the instrument. This is given by the following formula:

M = f_o/f_e

Where

M= Angular Magnification  of the Telescope

 f_o = Focal length of the Objective Lens, and

f_e = Focal length of the Eyepiece Lens

        For a telescope, the focal length of the Objective lens is fixed and, hence, cannot be changed. However, a telescope’s Magnification can be changed by using eyepieces with different focal lengths: the shorter the focal length, higher is the Magnification.

The Tube length (L) of the Telescope (when the Object distance for the Eyepiece matches with its focal length) is given by the formula:

L= f_o + f_e

We will make a small Refractive telescope having 3x Magnification. An astronomical telescope always produces inverted image, as there is no harm in viewing a celestial object like a star or a planet or the Moon upside down.

   Materials Required:

1. Double Convex Lens ( f= 30 cm, Dia= 50 cm) to serve as the Objective

2. Double Convex Lens ( f= 10 cm, Dia= 50 cm) to serve as the Eyepiece

3. Black Cardboard Paper

4. Cellotape

 

Construction:

     Roll the Cardboard sheet to form a cylinder of 50 mm in diameter and glue the edges with the help of cellotape so that the tube remains sturdy in shape.  Fit the Objective lens at one end of this cylinder with the help of cellotape. Prepare a second cylinder using the same cardboard sheet by rolling it over the first cylinder so that the first cylinder can be tightly pushed into it and slide freely to and fro. Attach the Eyepieces lens at one end of this second cylinder again with the help of cellotape. Now, push the Eyepiece cylinder into the Objective cylinder so that the Eyepiece lens remains outside for the observer to place his/her eyes to view. Our kit is now ready for operation. Care should be taken to keep the lens surface free from any dust, oil or dirt and, hence, direct contact of the lens surface with hand should be avoided.

How does it Work

   The Objective lens produces a real, inverted and diminished image of a distant object in its focal plane. The position of the Eyepiece lens is adjusted so that the image is formed within the focal length of the Eyepiece. The image produced by the Objective lens acts as an object for the Eyepiece lens which magnifies the image to produce a final image. The final image produced after the light rays refracted from the Eyepiece lens is Inverted, Virtual and Magnified.

How Do You Go

      Point the telescope to a distant object so that the Objective lens remains towards the object and look through the Eyepiece. Slide the Eyepiece cylinder over the Objective cylinder to and fro so as to vary the distance between the two lenses till the image looks sharper. The final image will be virtual, inverted and magnified.

Caution:

Never look to the Sun with telescope. It might permanently impair your vision by damaging the eyes!

 

 

 

 

Further Scope

       You will soon realize that this telescope is not free from various optical aberrations like Spherical aberration and Chromatic aberration. Spherical aberration arise due to the fact that the Paraxial rays and the Marginal rays, after refraction through the lens,  don't come to the focus at the same point on the Principal axis of the lens. This aberration can be minimized by using an Aperture Stop for the Objective Lens. In this arrangement, a black coloured Paper ring can be put over the Objective lens reducing its aperture to only one-third. This results in the fact that most of the marginal rays are blocked allowing only paraxial rays to pass through the Objective lens. In this case, although the image will be fainter, it will be much clearer.

   Similarly, the telescope also suffers from Chromatic aberration and the final image is bordered by coloured fringes. Chromatic aberration arises because of the failure of all the wavelengths of the visible light to come to a common focus, as the Refractive Index of  lights of different colours are different for a given refracting medium like glass. In professional telescope, an Objective lens incorporating an Achromatic doublet is generally used to minimise the effects due to Chromatic aberration which is a complicated technology beyond the reach of common people. Hence, in our telescope, the Chromatic aberration cannot be eliminated in part or full and, hence, the final image will look surrounded by coloured spectrum.

 

 

 

 

PHOTOGRAPHS

 

 A girl testing her telescope by adjusting the distance between the two lenses  in a Telescope Making Workshop

 

Ray diagram for an Astronomical telescope

 

 

A 4" astronomical telescope is used to view the four Galilean moons of Jupiter using an Objective lens of focal length 660 mm and an Eyepiece having focal length 25 mm, adjusted to yield a Magnification of 26.4

 

Education Officer

Regional Science Centre

Bhopal