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Optical Making a Dobsonian Telescope DIY

  1. Mar 30, 2015 #1
    I am going to make a Dobsonian telescope. Primarily a 6" f/8. I have no idea what the secondary mirror size would be an its distance from the centre of the primary. I have been told that if the secondary is too small then the effective aperture would be reduced. And i do not understancd these terms: central obstruction and secondary offset.
    Thank you in advance
  2. jcsd
  3. Mar 31, 2015 #2


    Staff: Mentor

  4. Mar 31, 2015 #3


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    For a 6" f8 primary, the secondary needs a minor axis of about 1-1/4" for a fully illuminated [captures the entire light cone of the primary] field of view. Frankly, I would just buy the secondary. Figuring optical flats is more tedious than figuring a parabolic primary and the flat should be corrected to double that of the primary - i.e., a 1/8 wave primary needs a 1/16 wave secondary. I mentioned 1/8 wave because that is about as good as can be achieved on most home brew primary mirrors. You can get a secondary mirror like this for about $50 from Antares.
  5. Apr 7, 2015 #4
    We had a chance to make a 6" telescope in class run by Dobson himself. Lot of work, but so worth it. Not sure we saved any money, but really learned a lot.
  6. Apr 7, 2015 #5


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    Well done ! ..... And that is the sole reason for building your own scope :smile:

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