Reflecting Telescopes (AQA A-level Physics)

Overview

• Objective: Understand how to draw ray diagrams for Cassegrain reflecting telescopes, describe spherical aberration, and calculate resolving and collecting power.
• Specification Reference: AQA A-level Physics 3.9.1.2 Reflecting Telescopes.

Types of Optical Telescopes

• Refracting Telescopes
  • Use glass to refract radiation.
  • Issues: Expensive, easily distorted, difficult to manufacture.
  • Produce images with chromatic aberration.
• Reflecting Telescopes
  • Use mirrors to reflect radiation.
  • Advantages: Cheaper, easier support and manufacture.
  • Produce images with spherical aberration.

Cassegrain Reflecting Telescope

• Components:
  • Primary Mirror: Parabolic concave mirror, first to receive radiation.
  • Secondary Mirror: Convex mirror, placed in front of primary focus.
  • Eyepiece Lens: Focuses radiation into an observable image.
• Methodology:
  1. Radiation reflects from concave mirror to a principal focus.
  2. Secondary mirror reflects light through a hole in the concave mirror.
  3. Eyepiece lens magnifies the image.

Spherical Aberration

• Occurs when the primary mirror is not perfectly parabolic, leading to multiple foci and blurred images.
• Example: Hubble Space Telescope initially suffered from spherical aberration due to a manufacturing error.

Telescope Properties

1. Collecting Power

• Definition: Amount of radiation a telescope receives per second.
• Relation: Diameter squared of the telescope is directly proportional to collecting power.
• Importance: Larger diameter allows observation of fainter objects.

2. Resolving Power

• Definition: Ability to see separate objects clearly (resolution).
• Minimum Angular Resolution: Smallest angular separation for distinguishing two points.
• Limitation: Affected by diffraction and detector quality.
• Rayleigh Criterion: Two sources can be distinguished if their Airy discs meet specific conditions.
  • Formula: ( \theta = \frac{\lambda}{d} )
  • Lower ( \theta ) implies higher resolution.

Summary

• Cassegrain Arrangement: Involves parabolic and convex mirrors.

• Ray Diagrams: Illustrate light path through telescope.

• Merits of Reflectors vs Refractors: Reflectors are generally more practical due to fewer aberrations and support simplicity.

• With today's lesson, you should be able to:

  • Draw ray diagrams for Cassegrain reflecting telescopes.
  • Describe spherical aberration in reflecting telescopes.
  • Calculate resolving and collecting power of a Cassegrain reflecting telescope.

End of Lesson

Note: It's beneficial to refer to diagrams and examples of ray paths and aberration effects for a thorough understanding.