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Thread: AFOV

  1. #1
    Mike Thomas's Avatar
    Mike Thomas Guest

    Default AFOV



    How does one calculate AFOV knowing the stop diameter of an EP?



  2. #2
    Lawrence Sayre's Avatar
    Lawrence Sayre Guest

    Default AFOV

    Mike Thomas wrote:

    AFOV = FS/FL_eyepiece * 180/Pi

    Plus add up to ~5% extra AFOV for good measure if the design is a
    ultrawide type. This accounts for the extra real AFOV that barrel or
    pincushion distortion will contribute.

    Lawrence Sayre

    --
    My philosophy, in essence, is the concept of man as
    a moral being, with his own happiness as the moral
    purpose of his life, with productive achievement as
    his noblest activity, and reason as his only absolute.

    Ayn Rand (in the appendix to 'Atlas Shrugged')


  3. #3
    Martin R. Howell's Avatar
    Martin R. Howell Guest

    Default AFOV

    Hi Mick



  4. #4
    David Knisely's Avatar
    David Knisely Guest

    Default AFOV

    Mike Thomas wrote:


    Well, there is only an approximate method (the best way is to just measure the
    apparent field in a simple bench test). The old TF = AFOV/Mag formula for
    true field is the one most often quoted, although it isn't quite as accurate
    as the Field Stop formula. The Field Stop formula for true field is:
    TF = (180/Pi)*EFSD/Fl, where EFSD is the eyepiece field stop diameter and Fl
    is the focal length of the telescope. You can set the two formulae equal to
    each other and then solve for AFOV:

    AFOV = (180/Pi)*(EFSD/Efl), where Efl is the eyepiece focal length.

    For two examples, my 27mm Kellner has a field stop diameter of 25.38mm.
    Using the above formula, its apparent field calculates out to be 53.86
    degrees. Its measured true field is 52.5 degrees, so its just a bit off the
    mark. My Meade 14mm Ultrawide has a field stop diameter of 20.2mm. Using the
    above formula, the apparent field should be 82.7 degrees. The actual
    *measured* field of view of my 14mm Ultrawide is 83.1 degrees, which is
    closer, but not quite dead-on. Thus the formula for calculating apparent
    field of view of an eyepiece is only approximate. Clear skies to you.

    --
    David W. Knisely KA0CZC@navix.net
    Prairie Astronomy Club: http://www.prairieastronomyclub.org
    Hyde Memorial Observatory: http://www.hydeobservatory.info/

    **********************************************
    * Attend the 11th Annual NEBRASKA STAR PARTY *
    * July 18-23, 2004, Merritt Reservoir *
    * http://www.NebraskaStarParty.org *
    **********************************************




  5. #5
    Mike Thomas's Avatar
    Mike Thomas Guest

    Default AFOV


    "Lawrence Sayre" <lsayre@neo.rr.com> wrote in message
    news:r0GEc.8938$2T2.6105@fe2.columbus.rr.com...


    Ok but 57.3*FS / FL(ep) is the same thing and is the CFOV (calculatedFOV).
    Also, you forgot to divide that value by the magnification of the EP in
    question. so CFOVorAFOV / Mag = TFOV







  6. #6
    Lawrence Sayre's Avatar
    Lawrence Sayre Guest

    Default AFOV

    Mike Thomas wrote:
    There is clearly no mistake here. Please check your math again. Here's
    an example:

    TeleVue 13mm Nagler Type 6, FS = 17.6mm

    AFOV = 17.6/13 * 57.296

    AFOV = 77.57 degrees (if distortion free, which it is not)

    Add up to 5%:

    77.57 * 1.05 = 81.45 degrees AFOV (with pincushion distortion added)

    Lawrence Sayre

    --
    My philosophy, in essence, is the concept of man as
    a moral being, with his own happiness as the moral
    purpose of his life, with productive achievement as
    his noblest activity, and reason as his only absolute.

    Ayn Rand (in the appendix to 'Atlas Shrugged')


  7. #7
    skypilot's Avatar
    skypilot Guest

    Default AFOV




    How on earth do you "measure" the measured FOV?

    Happy trails to you

  8. #8
    Etok's Avatar
    Etok Guest

    Default AFOV

    Martin R. Howell wrote:
    That would be my guess.
    Heh.

    Regards,
    Etok

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  9. #9
    Mike Thomas's Avatar
    Mike Thomas Guest

    Default AFOV


    "> >


    Read again. I didn't say anything about a mistake.



  10. #10
    Jon Isaacs's Avatar
    Jon Isaacs Guest

    Default AFOV

    >Read again. I didn't say anything about a mistake.

    This is the exchange in question:

    This is what you asked:


    This is what Lawrence replied:


    This is the correct formula for estimating the AFOV from the FS (Field Stop)
    and the focal length of the eyepiece.

    However you then wrote:


    You did ask how to calculate the FOV from the FS and the FL of the eyepiece....
    This is the answer.

    question. so CFOVorAFOV / Mag = TFOV

    Your question was not about TFOV so Lawrence forgot nothing.

    jon





 

 
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