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Problem in making a small refracting Telescope

  1. May 1, 2015 #1
    It sounds silly to ask such question or stumble in such problem, indeed!

    But I was willing to make some small telescopes for young friends of mine, to encourage them to delve in the skies.

    So, I bought:
    1. Double concave, spherical glass lens, with a focal length of approx. -75mm and diameter of 38mm.
    2. Double convex, spherical glass lens, with a focal length of approx. 1000mm.

    I believed that by a tube of about a meter length, I should have an excellent refracting telescope!
    But unfortunately after collecting the lenses and the tube, the telescope did not work! Sth is hugely wrong in my estimations!

    Would anyone hint me where might the mistake be? Your illustrations are much appreciated.
  2. jcsd
  3. May 1, 2015 #2


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    This should give you what is called a Galilean telescope, since it is the type Galileo first used to explore the skies. What do you mean that "the telescope did not work"? You were probably just not in focus. You need to place the double concave lens before the focus of the double convex lens. Did you locate the focal point of the double convex lens? This is best done with a bright object like the moon or a street light. Then you can see the image point and accurately measure the focal length of the double convex lens. Then the double concave lens needs to be placed about 75 mm in front of the focal point. You will need to be able to move it back and forth to find the point of best focus. The top picture in attached image shows how the lenses should be placed.

  4. May 2, 2015 #3
    Yeah, I am planning to make a Galileo Type telescope.
    And I will try a tube with 92.5 cm according to your suggestions, and planning to do the detailed optics calculations for it soon. I tried 107.5 cm before, and it did not work.

    Thanks for your help. Will post here as soon as I assemble the pieces.
  5. May 4, 2015 #4

    Andy Resnick

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    Something to remember with the Galilean design, your eye needs to be as close to the negative lens as possible- the exit pupil is located slightly prior to the negative lens.
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