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  1. #1
    Jon Isaacs's Avatar
    Jon Isaacs Guest

    Default Measuring eyepiece focal length

    I am interested in measuring the focal lengths of various eyepieces with
    reasonable accuracy, maybe 5% or less. I am aware of that one can just measure
    the exit pupil but measuring the exit pupil accurate, especially for a short
    focal length piece seems difficult.

    Thoughts, suggestions, links...


  2. #2
    Jim Easterbrook's Avatar
    Jim Easterbrook Guest

    Default Measuring eyepiece focal length

    In article <>, Jon Isaacs

    I've had a go at using Don Taylor's method which he gave details of in
    February 2000:
    I've written a web page describing a variation of Don's method:
    My accuracy is not very good, particularly at the shorter focal lengths.
    I hope you find a better method.
    Jim Easterbrook <>
    N51.36 E0.25

  3. #3
    Don's Avatar
    Don Guest

    Default Measuring eyepiece focal length

    Jon Isaacs wrote:

    Here is a method that worked very well for me when I wanted to check on the focal
    lengths of my eyepieces. It gave results accurate to about 0.1 mm in the focal
    even with short focal length eyepieces.
    It basically works in reverse of the classic method of measuring exit pupils.
    Instead of measuring the exit pupil we place an illuminated grid of closely spaced
    lines lines where the exit pupil would normally be and use the eyepiece lens to
    project the grid onto a distant screen. For the grid I used a ronchi grating. Now
    instead of measuring a tiny exit pupil we measure the vastly larger projected grid
    spacing on the screen.

    The setup consists of a flashlight shining through a Ronchi grating (120
    lines/mm in my case) which is placed about where the eye
    would go if it were looking through the eyepiece. Be sure to shield any stray
    light from the flashlight that doesn't go through the gratiing so that it doesn't
    fall on the screen.

    The eyepiece is then
    carefully moved toward or away from the Ronchi grating (I used an old
    rack and pinion focuser to aid this but it wasn't essential) until the
    lines are sharply in focus on a screen (I used a white wall of the room)
    a couple of meters away. One then measures the center to center spacing
    of the projected Ronchi lines. For the longer focal length eyepieces I
    measured the distance covered by 5 lines and then divided that by 5.

    To find the focal length one first calculates the magnification, M = D/d
    where d is the spacing between lines on the grating itself (1/120 mm in
    my case) and D is the spacing as magnified by the projection. I then
    calculate the focal length of the eyepiece as f = L/(M+2) where L is
    the distance from the Ronchi to the projection screen. This is an
    approximation (but a good one) in that it assumes that the separation
    between principal planes of the eyepiece is small compared to the
    distance to the screen. Thats why you want the screen to be on the order
    of at least 100 times the eyepiece focal length from the Ronchi grating.
    I also tried this method using a transparent millimeter scale instead
    of the Ronchi. That works OK for the longer focal length eyepieces. If one doesn't
    have either of these a single wire of known thickness might also serve.

    The technique can also be used to measure the focal length of
    Barlow + eyepiece combinations. In that case it gives the effective
    focal length of the eyepiece as used with the Barlow. By dividing that
    result into the focal length measured for the eyepiece by itself, one
    can get the Barlow's amplification factor as used with that eyepiece.

    A more exact formula for finding the focal length is according to my
    f = (L-h)/(M + 2 + 1/M)
    where h is the separation of the principal planes.

    I have been trying to think of something that would work as a substitute for the
    Ronchi grating. A single wire of known thickness ought to work, but I would want
    to check the thickness with a micrometer.

    Have fun if you try it,
    Don Taylor

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