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  1. #1
    greg1234's Avatar
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    Default Eyepiece focal length



    is there a way to measure the focal length of an eyepiece directly? I have an eyepiece that is not labelled. I think it is either a 25mm or a 32mm, not sure which.
    6" Dobsonian Reflector
    10" Meade Starfinder Equatorial
    25mm Kellner, 12mm Meade MA, 9.7mm Plossl, 9mm Meade MA, 8mm Televue Plossl, 6mm Plossl eyepieces, 2X barlow.


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    Hi Greg,
    I have never dealt with it in practice but it's what I would do:
    I would take a tungsten bulb (the old type one, or the car one) and use the EP to project the image of the spiral onto a piece of paper. the distance from the lens to the paper would be the focus distance of the EP.
    Cheers,
    Michael

  3. #3
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    Default

    Hi Greg,
    I have never dealt with it in practice but it's what I would do:
    I would take a tungsten bulb (the old type one, or the car one) and use the EP to project the image of the spiral onto a piece of paper. the distance from the lens to the paper would be the focus distance of the EP.
    Cheers,
    Michael

  4. #4
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    Default

    Greg, try this:

    Put the eyepiece (EP) on your scope and focus on a distant day time object. Now step back a little from the EP, you will see a bright circle at the middle of the EP eye lens, this is called the "exit pupil." Measure that in millimeter, like I tried it out with an EP and the exit pupil is 3.5mm.

    Next, measure the diameter of your telescope's objective lens/mirror (if you know the specs from the manufacturer, better) in mm's. Like the small refractor I used for measuring the exit pupil above is 66mm.

    Divide the objective diameter by the exit pupil, 66/3.5 = 18.8 which is the magnification.

    Next, if you know the focal length of the telescope you're using, divide this by the magnification above, like for my scope, its focal length is 388mm/18.8 = 20.6mm which is the derived focal length of the eyepiece.

    The eyepiece in this example is a Telvue 20mm plossl so the result is quite near. Hope this helps.

    Best,

    Hernando
    Last edited by ibase; 09-22-2010 at 05:08 AM.

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  6. #5
    greg1234's Avatar
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    Default

    Quote Originally Posted by ibase View Post
    Greg, try this:

    Put the eyepiece (EP) on your scope and focus on a distant day time object. Now step back a little from the EP, you will see a bright circle at the middle of the EP eye lens, this is called the "exit pupil." Measure that in millimeter, like I tried it out with an EP and the exit pupil is 3.5mm.

    Next, measure the diameter of your telescope's objective lens/mirror (if you know the specs from the manufacturer, better) in mm's. Like the small refractor I used for measuring the exit pupil above is 66mm.

    Divide the objective diameter by the exit pupil, 66/3.5 = 18.8 which is the magnification.

    Next, if you know the focal length of the telescope you're using, divide this by the magnification above, like for my scope, its focal length is 388mm/18.8 = 20.6mm which is the derived focal length of the eyepiece.

    The eyepiece in this example is a Telvue 20mm plossl so the result is quite near. Hope this helps.

    Best,

    Hernando
    Hey Hernando..i just tried this. It worked like a charm!! First i tested it with my 12mm Meade MA. The exit pupil measured approx. 1.7mm, which made the magnification 90, which estimated the eyepiece FL at 13.3 (pretty close to 12).

    Then i measured the UNKNOWN eyepiece exit pupil at 3.0mm. This put the magnification at 50, which estimated the eyepiece at 24. So, it must be a 24 or 25...definately NOT a 32.

    Thanks
    6" Dobsonian Reflector
    10" Meade Starfinder Equatorial
    25mm Kellner, 12mm Meade MA, 9.7mm Plossl, 9mm Meade MA, 8mm Televue Plossl, 6mm Plossl eyepieces, 2X barlow.


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    Default

    Interesting bit of information here.
    Thanks guys..
    Declan.
    Celestron 8"Edge HD SCT.

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