Defects In Glass Type Telescopes

Description:

Spherical aberration

All the incident rays parallel to the principle axis of the lens should pass through focus F after refraction but, the rays closer to the principle axis pass through a point F′ slightly ahead of the focus as shown in figure.

To solve this defect we can make adjustment in the lens i.e. we will only use the middle portion of the lens and rest of the lens will be made black so no rays can be refracted from the black portion but, while doing so we have to compromise with the intensity of image which is un-acceptable.

Chromatic aberration

We know that when a ray is incident on a prism the phenomenon of dispersion of light takes place.

If we observe the lens carefully, we can see that each portion of the lens behaves like a prism and each portion has dispersion of light hence the violet light bends more and red light bends the least i.e. focus for different colors of light is different. This generates a blurred image.

Handling is difficult due to weight

To gain better results through a telescope, the aperture of the objective lens should be as large as possible but large lens comes with a large weight and to hold this large lens in place a metal frame is required that also has a large weight. This makes handling to the telescope very difficult.