Draw a labelled ray diagram of an astronomical refracting telescope in normal adjustment and derive its magnifying power. Give two reasons why large telescopes use mirrors.

1. 1 · Define magnifying power. m = β/α: the angle subtended at the eye by the final image ÷ the angle subtended by the object at the objective.
2. 2 · Draw the ray diagram. [Objective and eyepiece fₒ + fₑ apart; parallel rays from a distant object at angle α; real, inverted image at the common focus; rays leaving the eyepiece parallel at angle β; label α, β, fₒ and fₑ]
3. 3 · Write the angles.
   • α ≈ h/fₒ (image of height h at the objective's focus)
   • β ≈ h/fₑ (the same image at the eyepiece's focus)
4. 4 · Derive the result. m = β/α = fₒ/fₑ, and the tube length is fₒ + fₑ.
5. 5 · Say how to increase m. Use an objective of long focal length and an eyepiece of short focal length.
6. 6 · Give the advantages of mirrors.
   • No chromatic aberration
   • A paraboloidal mirror removes spherical aberration
   • A mirror is lighter and supported over its whole back, so large apertures are possible
7. 7 · Real life. The Vainu Bappu Telescope at Kavalur, Tamil Nadu, is a reflecting telescope.
8. 8 · Close in one line. A telescope magnifies the angle a distant object subtends, not its actual size.

Scoring tip: Draw the incoming rays parallel to each other. Make the objective clearly bigger than the eyepiece.