What is an astronomical telescope? Describe its construction and working. What is its magnifying power? Under what conditions does an astronomical telescope act in normal adjustment?

Astronomical Telescope:

An astronomical telescope is an optical instrument designed to observe distant celestial objects like stars, planets, and galaxies. It uses lenses or mirrors to gather and magnify light, allowing astronomers to view objects that are too far away to be seen with the naked eye.

Construction of an Astronomical Telescope:

An astronomical telescope consists of the following main parts:

  1. Objective Lens (or Mirror):

    • The objective is the primary component responsible for gathering light from a distant object and forming an image.
    • In refracting telescopes, the objective is a large convex lens.
    • In reflecting telescopes, the objective is a large concave mirror.

  2. Eyepiece (Ocular Lens):

    • The eyepiece is a small lens used to magnify the image formed by the objective.
    • It is placed at the focal point of the objective lens or mirror, where the image is formed, allowing the viewer to observe the magnified image.

  3. Tube:

    • The tube of the telescope holds the objective lens and the eyepiece in alignment and ensures that the light entering the telescope follows a straight path.

  4. Mount and Tripod:

    • The mount supports the telescope and allows it to be aimed at different parts of the sky. The tripod ensures stability and keeps the telescope steady during observation.

  5. Focuser:

    • The focuser allows fine adjustment of the eyepiece position to bring the image into sharp focus.

  6. Diagonal Mirror (in some telescopes):

    • In some telescopes, especially refractors, a diagonal mirror is used to deflect the light path so the observer can view the image more comfortably.

Working of an Astronomical Telescope:

The basic working principle of an astronomical telescope involves two main steps:

  1. Light Gathering:

    • The objective lens (or mirror) gathers light from a distant celestial object. The larger the diameter of the objective, the more light the telescope can gather, and the brighter the image will be. This allows astronomers to see faint objects in the night sky.

  2. Magnification:

    • The light gathered by the objective lens or mirror is focused to form an image at its focal point. The eyepiece lens then magnifies this image, making distant objects appear larger and more detailed.

In refracting telescopes, light passes through the objective lens, is bent (refracted), and forms an image at the focal point. In reflecting telescopes, light is reflected off the concave mirror, converging at the focal point.

Magnifying Power of an Astronomical Telescope:

The magnifying power (or magnification) of a telescope depends on the focal lengths of the objective lens and the eyepiece.

The formula for magnifying power is:

M= Focal Length of Objective (fo)/Focal Length of Eyepiece (fe)

Where:

  • fo is the focal length of the objective lens (or mirror),
  • fe is the focal length of the eyepiece.

For example, if the focal length of the objective is 1000 mm and the focal length of the eyepiece is 25 mm, the magnifying power would be:

M=1000/25=40M 

Thus, the telescope would magnify the image 40 times.

Normal Adjustment of an Astronomical Telescope:

An astronomical telescope is said to be in normal adjustment when the following conditions are met:

  1. Parallel Light from Distant Objects:

    • The telescope is set up to observe distant objects (such as stars), where the light rays entering the telescope are effectively parallel. This ensures that the image formed by the objective is at the focal point of the objective lens or mirror.

  2. Eyepiece at Focal Point of Objective:

    • In normal adjustment, the image formed by the objective lens is at the focal point, and the eyepiece is positioned so that the observer can see the image clearly at infinity (i.e., the eye can focus without straining). This is a comfortable viewing condition, where the observer looks through the eyepiece without needing to focus on objects at different distances.

  3. Clear Focus at Infinite Distance:

    • The image is formed at the focal plane of the eyepiece in such a way that when viewed, the image appears sharp at a near-infinite distance (no focusing required from the observer’s eye).

Related Questions:

  1. What is reflection of light? Define the following terms: Incident ray, reflected ray, normal, angle of incidence, and angle of reflection. Describe the laws of reflection.
  2. What are different spherical mirrors? Define different terms involved in spherical mirrors.
  3. Why is a concave mirror called a converging mirror? Why is a convex mirror called a diverging mirror?
  4. Define the principal focus of convex and concave mirrors. Define focal length. Which mirror has a positive focal length? Which has a negative focal length and why?
  5. What is the spherical mirror formula? Also, write down the sign conventions for the spherical mirror formula.
  6. Differentiate between real and virtual images. What are three basic rays by which we can draw a ray diagram to form an image by a spherical mirror?
  7. What is refraction of light? Show by ray diagram how light bends when it travels from rare to denser medium and from denser to rare medium. Define the refractive index of a medium. What are its different formulae?
  8. What are the laws of refraction? Explain with the help of ray diagrams
  9. Explain the phenomenon of total internal reflection. What are necessary conditions to perform total internal reflection? Derive the formula for the refractive index of a medium in terms of the critical angle.
  10. What is optical fibre? How does light travel through it? Explain.
  11. What is a lens? What are its types? Define the basic terminologies of lenses.
  12. Define and explain the terms: resolving power and magnifying power.
  13. What is a magnifying lens or simple microscope? How can a convex lens behave as a magnifying glass? What is its magnifying power? Why must a convex lens of short focal length be used as a magnifying glass?
  14. What is a compound microscope? Describe its construction and working. What is its magnifying power?
  15. When you look at the front side of a polished spoon, your image is inverted, and from the back of the spoon, your image is erect. Explain why?
  16. Which mirror is used by girls for makeup and why?
  17. Why are large convex mirrors fixed at blind turns of mountains?
  18. Which mirrors are used for the rear view of vehicles and why?
  19. If a person is walking in a pool, why do his legs appear shorter in water?
  20. When white light passes through a prism, it disperses into its seven colors. Why does dispersion take place in a prism?