Question

(i) Draw a neat labelled ray diagram of an astronomical telescope in normal adjustment. Explain briefly its working.

 (ii) An astronomical telescope uses two lenses of powers 10D and 1D. What is its magnifying power in normal adjustment?

Answer 1

(i) Working: Suppose AB is the point object whose end A is on the axis of telescope. The objective lies (L₁) forms image A'B' of object AB at the second principal focus F₀ . The image A'B' is real, inverted and diminished. For normal adjustment position this image also lies at first focus (F_e ) of eye lens (L₂). This image acts as an object for eye lens and final image is formed at infinity. The final image A''B'' (say), is magnified and inverted.