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I Telescope Design

  1. Jul 28, 2016 #1
    I am currently in the middle of constructing a Newtonian reflecting telescope and I had a question. The focal length for my primary mirror is 1500mm and 3mm for my eyepiece. My question relates to the path of the light inside the tube of a telescope. I understand that in a Newtonian reflector, the light travels to the primary mirror at the end of the tube and reflects onto the secondary mirror into the eyepiece. My question is, is the total distance between the primary mirror and the eyepiece 1500mm or 1500mm plus the 3mm focal length on the eyepiece? With a primary mirror of 1500mm Focal length and an eyepiece of 3mm length, what is the total distance from the primary mirror to the eyepiece?
    Thanks in advance for the answers. I do appreciate your time and knowledge.
  2. jcsd
  3. Jul 29, 2016 #2


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    hi there

    Every telescope has a stated focal length, which is effectively the distance from the primary lens or mirror to the point at which it forms an image of a very distant object. This is a figure that you'll usually see printed or engraved near the eyepiece focuser.

    That is the focal length of the scope. The eyepiece sits just past that focal point


    Focal length of objective = fo
    Focal length of eyepiece = fe

    does that help ?

    EDIT: ... the principle is the same regardless of it it is a reflector or a refractor

  4. Jul 30, 2016 #3
    I have used this http://www.dalekeller.net/ATM/newtonians/newtsoft/newtsoft.htm [Broken] to design Newtonians to good effect.

    Regards Andrew
    Last edited by a moderator: May 8, 2017
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