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Thread: How to find out the focal length of an eyepiece?

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    Default How to find out the focal length of an eyepiece?



    I bought a second hand telescope from a gentleman who owned it for 10 years, it has just one eyepiece. Everything that was printed on the eyepiece has been pretty much washed away. He told me that it's a 12 mm eyepiece but I want to confirm it. How can I do that? Thanks for reading the post.

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    Default Re: How to find out the focal length of an eyepiece?

    i would only be able to ballpark it from experience - would think you'd need an instrument

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    Default Re: How to find out the focal length of an eyepiece?


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    Default Re: How to find out the focal length of an eyepiece?

    Hi,

    This method may be a bit simpler.

    We take 2 objects with the same size (for example 2 pieces of paper). One piece of paper was put at a great distance from the telescope (about 40m). With one of his eyes he focused the object through the telescope. He wrote the distance between his eye and the piece of paper

    A distance: dA=………………..m


    Our partner takes the other piece of paper and stands with the paper in front of the telescope (slightly offset to not obscure the distant sheet of paper). We observe that sheet with our unaided eye. So with one eye looking into the EP we see the far sheet of paper and with the other eye we see the second sheet of paper not through the telescope. Our partner moves till the second sheet seems to have the same size as the far sheet.

    We write the distance between the second sheet and our eyes.

    B distance: dB=………………..m
    We use this formula to find the magnification:

    With EP we used, we found: dA=44m ; dB= 0.95m; Magnification=44/0.95=46

    Remember, this will only give you an approximate value for magnification, but if it is close to what 12mm EP would give you, I'd believe the guy who sold the scope to you.


    Hope this helps,
    JT
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    Default Re: How to find out the focal length of an eyepiece?

    If you know the focal lengthor focal ratio of your scope, and you have a set of calipers you can measure the size of the exit pupil and use this to determine the FL of the EP.

    EP = aperture/magnification
    magnification = FL_scope/FL_EP
    so
    EP = aperture*FL_EP/FL_scope
    or
    FL_EP = EP*FL_scope/Aperture

    All in the same units (mm or inches).

    It is easy to measure the EP on a bright object--you can use graph paper and the moon at night--just move the paper until the EP is smallest and use the rulled lines to detemine the size of the EP.

    Anyway you should be able to determine EP within 10%, and this will put you in the right ball park for FL_EP.
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    Default Re: How to find out the focal length of an eyepiece?

    Thanks for the awesome advice everyone, I'll try these out.

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    Default Re: How to find out the focal length of an eyepiece?

    You could always drift test it and then do the math, this will give you a very precise f/l (i.e. 4.7mm vs 5mm). Not sure of the formulas, but basically the time it takes the star/object to go from end to end in the eyepiece, is then put into a set of formulas based on your scopes f/l and speed (and other things) that will tell you what the eyepiece f/l is. Another suggestion--post pics of the eyepiece and someone will probably recognize it....
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    Default Re: How to find out the focal length of an eyepiece?

    Quote Originally Posted by Zilladude View Post
    You could always drift test it and then do the math, this will give you a very precise f/l (i.e. 4.7mm vs 5mm).
    Drift gives you True Field of View. So a 26mm Plssl (50 dFoV) and a 13mm Ethos (100dFoV) will have the same drift count in seconds on the same scope.

 

 

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