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Stargazing Magnification of telescope

  1. Oct 23, 2009 #1
    hi, i don't understand why the 2 converging lens telescope's magnification is given by M = f_o / f_e

    where f_o = focal length of objective, and f_e = focal length of eyepiece...

    issn't M = image distance / object distance...

    in the case of a infinity object, the image is also infinity for the eye to view it without straining.

    so why is there still a magnification ? infinity over infinity?

    need help thanks a lot!
  2. jcsd
  3. Oct 24, 2009 #2


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    Staff: Mentor

    This is angular magnification.

    This is linear magnification.

    They're different things.
  4. Oct 24, 2009 #3
    oh. i see.

    i look up the angular magnification and the formula was

    M = tan E / tan E_0 (wikipedia)

    where E_0 = the angle subtended by the object at the front focal point of the objective and E =the angle subtended by the image at the rear focal point of the eyepiece.

    so how does this become the formula

    M = f_o / f_e ?

    even with small angle approximation the formula M = tan E / tan E_0 is still not M = f_o / f_e ?
  5. Nov 15, 2009 #4
    The two formulas for angular magnificaion are equivalent by their very nature and you dont need to prove equality by making arithmetic substitutions
    They are both ratios (the focal length equation is simply a ratio of spatial dimensions, where as the angle equations is a ratio of the angles subtended by the magnitude of these dimensions)
    Both ratios are in direct proportion.
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