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Optical Optics- DIY Telescope

  1. Oct 16, 2012 #1
    Okay so I want to make a telescope at home...

    Im not a noob. Even if I am please just help me.

    I've got a couple of magnifying glasses ..pretty big ..
    I don't know the exact focal length...
    But one of them is twice the size of other...

    As soon as the sun comes up I'll find their focal length s...

    What I want to know is how far apart the objective lens and eyepiece should be kept for clear and maximum magnification...
    I just need a formula or some way to calculate the dist between them.

    Also , how to fix the inverted image that will be formed by using the two lenses I have...I know I'll have to use another lens but again how far should it be placed and of what focal length should it be.
    I want an erect image..
    Assume I have two lenses of focal length F , one of 2F...if you need...

    Please correct me if i'm wrong...anywhere..

    I'm willing to get any other materials I might need ..
    Last edited by a moderator: Oct 16, 2012
  2. jcsd
  3. Oct 16, 2012 #2


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    As long as the focal point of the object lens is inside the focal distance of the eyepiece (distance between lenses is less than the sum of the two focal lengths: distance < 3F) you will get magnification. The magnification itself will depend upon how much smaller.
  4. Oct 17, 2012 #3


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    The magnification of a simple (astronomical) telescope, using two lenses is equal to the ratio of their focal lengths. The lens with the longer focal length is used as the 'objective' and the shorter is used for the 'eyepiece'. Most random 'magnifying glasses' will tent to have similar focal lengths so the magnification won't be all that special but you will be able to prove the point easily as long as the focal lengths are at all different.
    Btw, you don't need to wait for the sun. A distant light bulb will be quite sufficient to find the approximate focal length.
  5. Oct 17, 2012 #4


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    Remember that the distance will change if you are looking at terrestrial objects because of the shift in the focus.

    You may be better off using a mirror set at a 45 degree angle, positioned between the objective and the eyepiece if you have one.
  6. Oct 17, 2012 #5


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    You need the lenses to be separated by the sum of their focal lengths (adjust for focus) but you can do nothing to change the magnification using just two elements.

    A single mirror will not restore the image to what you want - it will be laterally inverted. Binoculars use a pair of prisms to invert the image twice so that writing appears correct.

    Have you Googled "Simple Telescope" Images yet?
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