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  1. #1
    Kevin's Avatar
    Kevin Guest

    Default Apparent field of view of an eyepiece

    What is a good way for measuring the apparent field of view of an eyepiece?

  2. #2
    Larry G.'s Avatar
    Larry G. Guest

    Default Apparent field of view of an eyepiece

    On Tue, 09 Jan 2007 20:06:40 -0600, Kevin <> wrote:

    The empirical method is the find a star near Declination 0 degrees.
    Set your telescope so that the star is on the eastern edge of the
    field. Then time it as it traverses the field, until it disappears
    off of the western edge.

    Then, you calculate your true field thusly:

    True Field (deg) = Transit Time (minutes) * 1 deg / 4 minutes

    This result will give the true field in arc degree(s). Transit
    Time should be measured in minutes.

    Next, calculate the Magnification of your telescope:

    Magnification (x) = Focal Length Objective (mm) / Focal Length Eyepiece

    Finally, to get the Apparent Field

    Apparent Field (deg) = True Field (deg) * Magnification (x)

    Actually, these are all approximations, but should be good enough
    for non-precision work.

    Larry G.

    Your mind is a terrible thing to waste - TURN OFF YOUR TV!

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  3. #3
    David Knisely's Avatar
    David Knisely Guest

    Default Apparent field of view of an eyepiece

    Kevin posted:

    OK, here is how you do it:


    MATERIALS: 1. A Meterstick, Yardstick, or other linear device whose
    length is accurately known, which can be hung vertically on a wall, and
    whose exact middle or center is accurately marked. This could also be a
    narrow strip of paper of known length with its exact middle and ends
    marked clearly. This object will be known as the observing "target".
    2. A method of holding and properly supporting an eyepiece rigidly in a
    horzontal position (like a bracket attached to a camera tripod), but
    which can be manually moved towards or away from a measuring target.
    3. A tape measure.

    STEP #1: Mount the vertical "target" (ie: the Yardstick or its
    substitute) on the wall so that its exact middle is will be about same
    height above the floor as the center of the eyepiece. For a meter
    stick, the midpoint will be the 50cm mark, and for a yardstick, it will
    be the 18 inch mark. Mark this midpoint with a visible marking like a
    small piece of tape or a black felt tip marker, so the middle can be
    easily seen from a distance.

    STEP #2: Mount the eyepiece at a height above the floor which is exactly
    the same as the mid-point of the target, so that the observer can look
    into the eye lens with the eyepiece optic axis or barrel horizontal and
    parallel to the floor. Make certain the eyepiece is as horizontal as
    possible, and that it can be easily moved towards or away from a nearby
    wall from as little as two feet from the wall to as much as six feet away.

    STEP #3: place the eyepiece straight out from the wall from where the
    observing "target" is located. Look into the eyepiece with *both* eyes
    open and merge the images of the eyepiece field of view and the target.
    Make the center of the superimposed eyepiece field centered on the
    mid-point mark of the observing target as closely as possible, and keep
    your head level with the floor (ie: keep your eyes at the same height
    above the floor).

    STEP #4: Look at the top and bottom of the target, again with both eyes
    open. Try to make the top and bottom edges of the eyepiece field match
    the top and bottom edges of the target on the wall by carefully moving
    the eyepiece towards or away from the wall. Make certain when moving
    the eyepiece that it remains pointed exactly towards the center of the
    observing target, and that its height above the floor does not change.
    Once the edges of the eyepiece field match the top and bottom of the
    target, take the tape measure and measure the distance from the back of
    the eyepiece just beyond the eye lens (ie: where your eye was sitting
    when you were looking through the eyepiece) to the middle of the target
    on the wall. If the target has a length of "2Y" and the distance to the
    wall you measured is "D", then the apparent field of view of the
    eyepiece is then:

    AFOV = 2*ATAN (Y/D), where Y is *half* the total length of the target
    and ATAN the arc-tangent (or inverse tangent) function. For example, if
    you were using a yardstick (36 inches in length, or Y = 18.0 inches) and
    your eyepiece field matched its length at a distance of 37.0 inches from
    the center of the target, the apparent field of view of the eyepiece
    would be about 51.8 degrees. Do the measurements several times and
    average the numbers to reduce the variations induced my random
    measurement errors.

    Clear skies to you.

    David W. Knisely
    Prairie Astronomy Club:
    Hyde Memorial Observatory:

    * Attend the 14th Annual NEBRASKA STAR PARTY *
    * July 15th-20th, 2007, Merritt Reservoir *
    * *

  4. #4
    Brian Tung's Avatar
    Brian Tung Guest

    Default Apparent field of view of an eyepiece

    Kevin wrote:

    From MOPFAQ (see link below):

    Q. How can I measure the apparent field of view of my eyepiece?

    A. Here's a method that's worked for me. You need a tape measure and
    the ability to use your two eyes for slightly different purposes.

    As mentioned in the answer to the preceding question, if you look
    through an eyepiece just on its own, without a telescope, you'll usually
    see a sharp edge to the field of view. That's because you're seeing the
    field stop, which lies at the focal plane of the eyepiece. If the
    barrel itself is being used as the field stop, the edge might not be as
    clean, but you can still use this method.

    Set up in front of an object of known height (or width) h. I use a
    doorway that's 80 inches tall. Holding both eyes open, look through
    the eyepiece with one eye. Be sure to keep the eyepiece level with
    the midpoint of the object. You should see an indistinct field with
    the eyepiece eye, and the object with your other eye.

    Now, move back and forth until the object is just as tall (or wide)
    as the field of view. Measure the distance d between your *eye* (not
    eyepiece) and the object. The apparent field of view is then measured
    directly as

    aFOV = 2*atan ---

    Here's a rough ASCII diagram:

    / ^
    / |
    / |
    / |
    / |
    / |
    eye<EP h
    \ |
    \ |
    \ |
    \ |
    \ |
    \ v

    |<-- d -->|

    For example, my 6 mm Radian yielded a distance d of 69 inches, so the
    aFOV was 2*atan(80/138) = about 60 degrees. I estimate the error on
    this method to be on the order of 1.5 degrees, plus or minus.

    Brian Tung <>
    The Astronomy Corner at
    Unofficial C5+ Home Page at
    The PleiadAtlas Home Page at
    My Own Personal FAQ (SAA) at

  5. #5
    Kevin's Avatar
    Kevin Guest

    Default Apparent field of view of an eyepiece

    Thank you very much for the excellent suggestions.

    What is interesting is that, if I hold the eyepiece at arms length so I
    actually see a small image of a tape measure or an object with a measured
    width in the eyepiece, and I adjust the distance of the eyepiece so the
    known width matches the apparent field of view, and I calculate the apparent
    field of view angle, this seems to correspond well with the method of using
    both eyes with superimposed images.

    "Kevin" <> wrote in message



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