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  1. #1
    canopus56's Avatar
    canopus56 Guest

    Default Right angle and prime focus distance



    Measuring the back of a refractor housing to prime focus, using only
    straight eyepiece lenses, I consistently get about 130mm to prime
    focus. When I put a right angle in optical path, and measure down the
    optical axis of the eyepiece from prime focus to to the mirror surface,
    and then down the telescope axis, I consistently get 124mm. I don't
    think the difference is a measuring error. Is there something about
    the optics of right angle mirrors that explains the missing 6mm?

    - Optically perplexed on a cloudy night - Canopus56


  2. #2
    Dan Mckenna's Avatar
    Dan Mckenna Guest

    Default Right angle and prime focus distance

    Canopus,

    If it is a front surface mirror then only thing can change the focus
    is a non flat flat. Most of the non-flats are made from float glass and
    show astigmatism and not a pure defocus as you seem to indicate.
    An astigmatic surface at 45 degrees may produce a optic that has little
    astigmatism and some power producing a focus shift. On second thought
    if it is a float glass mirror then it might be installed
    upside down and you are going through some glass.

    Yes optical perplextion (is that a word ?) keeps me off the streets on
    cloudy nights.
    why, we are having one right now..what are the chances ?

    Have a nice night

    dan



    canopus56 wrote:


  3. #3
    canopus56's Avatar
    canopus56 Guest

    Default Right angle and prime focus distance

    It is a low-end crappy right angle. Let me remeasure with my
    higher-end right-angle and see if I get the same result.


    The light goes through a minor additional refraction index as it
    travels twice through the the thin glass surface and this might change
    the focus? Sounds right.

    - Canopus56


  4. #4
    Bob May's Avatar
    Bob May Guest

    Default Right angle and prime focus distance

    Make sure that you are indeed measuring the center of the flat mirror. You
    don't have to go far to get a 6mm error in distance.

    --
    Why isn't there an Ozone Hole at the NORTH Pole?



 

 

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