Physlet Illustration: Keplerian Telescope

 

 

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Focal Length of Eyepiece = mm

In this simulation, a Keplerian telescope is constructed from two converging lenses.  The objective has a focal length of 9.0 mm. The user may drag the source and the eyepiece as well as change its focal length (1 to 7 mm).  What determines the angular magnification of such a telescope?   

Hints:

  1. Drag the eyepiece until the image is in focus.  (Hint: The object is at infinity so the rays arrive parallel to each other. The rays that form the final, virtual image must also be parallel.)
  2. Change the focal length of the eyepiece.  Does the focus change?  What else changes?
  3. What can one say about the distance between the lenses in this type of telescope?
  4. Angular magnification may be calculated from the ratio of the the outgoing and incoming angles of the rays.  
  5. Another way to calculate the magnification involves the ratio of the focal lengths of the two lenses.  Does this calculated value agree with the measured value?

Reference

See The New Cosmic Universe, Sections 31.7

Illustration written by Chuck Niederriter and Steve Mellema