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Optical DIY Telescope theory and questions

  1. Sep 7, 2016 #1
    I'm confused about the theory of the telescope I am making.
    My questions are:
    1. Magnification - according to my image we should see magnification because the image we see is the image formed by the continuation of the dotted lines. Is that true?
    2. Why do we need lenses with different size? If you look at the picture I drew, we still need to be able to reach magnification, because we see the imaginary image as the continuation of the dotted lines.
    3. I understand that you have more light collected with bigger objective lens. More light from the object is coming in. However, why does it really matter?
    4. I have 2 plano-convex lenses. Is the configuration in the image correct to see magnification?


    Attached Files:

  2. jcsd
  3. Sep 7, 2016 #2


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    Hi C,

    Your picture isn't very helpful. google keplerian telescope angular magnification to find better ones. (e.g. here or here )

    1. Basically you look at an image formed by the objective lens. And you look with a magnifying glass, the ocular. (just like with a microscope!). You could have a piece of paper at the focal point of the objective: it would show the image. The longer the focal length of the objective, the bigger the picture. And the bigger the objective, the brighter the picture. (3) That's nice to see faraway stars that aren't all that bright.
    2. The ocular lens is an eyepiece. The shorter the focal length, the more magnification you get. It doesn't have to be so big because you have your eye close to the thing.
    4. yes (although the picture doesn't help in this).

    [edit] I really like this thorough site http://www.handprint.com/ASTRO/ae2.html (and the ae1 and ae3 there)
    Last edited: Sep 7, 2016
  4. Sep 7, 2016 #3
    In the first link you sent me there they have the eyepiece reverted unlike in my image. Does it matter? I think the refraction should be the same.
    I'll definitely check that last web out, although it may take some time.
  5. Sep 7, 2016 #4


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    Staff: Mentor

    When you magnify an image, the light is spread out into a larger area. Without a larger objective the image would be unacceptably dim. A larger objective collects more light and counteracts this spreading out of the light. In most cases it actually increases the brightness of the magnified image beyond that of the original image. This is why I can look through my telescope and see nearby galaxies and stars that would never be visible otherwise.
  6. Sep 8, 2016 #5
    Can you maybe draw a ray diagram for better understanding of this?
  7. Sep 8, 2016 #6


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    Staff: Mentor

    I'm afraid not. The only thing uglier than my face are my drawings.
  8. Sep 8, 2016 #7
    I'm sure you can do it, mspaint for the win!
    This is maybe the only way I can get what you are saying.
    And I'm sure you are pretty.
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