Astronomical Telescope

An astronomical telescope is an optical instrument used to observe distant celestial objects such as stars, planets, and galaxies.It produces a magnified image of a distant object using two lenses: the objective and the eyepiece.

Components:

• Objective lens: Large convex lens; forms a real, inverted image of a distant object.
• Eyepiece lens: Convex lens; acts as a magnifier to produce a virtual image.
• Tube: Holds lenses in position and prevents external light interference.

Working Principle:

• The objective lens collects light from a distant object and forms a real, inverted image at its focal plane.
• The eyepiece lens magnifies this image to produce a virtual, magnified image that can be viewed by the eye.
• Used for astronomical observation, so the final image is usually inverted (not a problem for stars and planets).

Ray Diagram of an Astronomical Telescope (Normal Adjustment):

• In normal adjustment, the final image is at infinity, making it easier for the relaxed human eye to view.
• Diagram explanation:
  
  • Parallel rays from a distant object pass through the objective lens and converge at its focal point F₀, forming a real image.
  • The eyepiece lens is positioned such that its focal point Fₑ coincides with F₀ of the objective.
  • Rays emerging from the eyepiece are parallel, producing a virtual image at infinity.

Magnification of an Astronomical Telescope:

Angular magnification \( M \) is given by:

\( M = \dfrac{\text{Focal length of objective}}{\text{Focal length of eyepiece}} = \dfrac{f_o}{f_e} \)

• It shows how much larger the telescope makes the observed object appear compared to the naked eye.

Key Points:

• Used for observing distant stars, planets, and moons.
• The image is inverted, but for astronomy this usually does not matter.
• Can be adjusted to normal vision (final image at infinity) or for near vision (final image at finite distance).