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Thread: Magnfication in Punch and Judy language.

  1. #1
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    Default Magnfication in Punch and Judy language.



    In my local public observatory people often ask me how this "magnification thing" works. It is like magic..
    A simple explanation like focal length of the objective divided through focal length of the eyepiece mostly doesn't ring a bell. Oh yes, they can copy the little arithmetic:

    F objective : 1000 mm
    f eyepiece : 25 mm
    Magnification: 1000/25 = 40X

    But I can still see the question marks in their eyes: "Yes,......but how does it work?"

    Most of the time I am on the observation terrace with them when they ask and I have no pen, paper or complicated optical drawings at hand. To tell you the truth, they aren't waiting for that too. They just want a plain Punch and Judy answer.

    This is what I tell them.

    The normal standard near point of sharp vision is 250mm. ( reading, microscopes, although there are 160mm microscopes too)
    You can't see an object sharp at 25mm. This distance is 10X closer than normal vision. An eyepiece with 25mm focal distance does that for you. Which means you can see an object 10 times closer, enlarged and sharp, compared to normal vision. This is easily demonstrated.

    Than I point at the telescope. The telescope has a focal length of 1000 mm. That is 4 times more than normal nearpoint vision. When pointed at the Moon I remove the eyepiece and they can actually see the Moon 4 times enlarged.

    Now comes the trick:
    With you 10 times enlarged vision trough the eyepiece you are looking at a 4 times enlarged image of the Moon. How much magnification does that give? 10 X 4 =? " 40 times", is the answer. They smile and see the logic of it.


    So: 1000/25=40

    Normally they thank me and step up to the next telescope, enriched with their newly achieved knowledge.

    Punch and Judy explanations are not always so bad. ( Jack and Jill? ) In Dutch: Jip en Janneke.
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    Default Re: Magnfication in Punch and Judy language.

    Clever method.

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    Default Re: Magnfication in Punch and Judy language.

    I'm just more confused now. So, Punch was the serial wife beater who evaded the police and got away with murder, right? Anyway, enough of the this commedia dell'arte lol. I usually choose magnifications to match exit pupils on a specific scope these days. I just find it simplifies things and I don't have to worry about the whole maths of it. There again, I'm pretty simple.
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    Default Re: Magnfication in Punch and Judy language.

    Hello John,

    this is an excellent explanation of the telescope magnification.
    Now, I understand why the Dutch have been the first nation making the telescopes, while the Germans have been still using glass for making beer mugs.

    Moreover, your explanation correlates with the labelling of the microscope eyepieces of today:
    10x written on the eyepiece tube means: 25cm/10=2.5cm focus length

    Still learning,

    JG
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    Default Re: Magnfication in Punch and Judy language.

    Very nice, John. I shall use this during my next outreach attendance.
    Bryan

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    Default Re: Magnfication in Punch and Judy language.

    Quote Originally Posted by Shorty Barlow View Post
    (...)I usually choose magnifications to match exit pupils on a specific scope these days. I just find it simplifies things and I don't have to worry about the whole maths of it. )...)
    Thanks about your remarks. Finding the right exit pupil for the object. The one that brings out contrast best; given the instrument, filters, darkness of the background and your eye is what it is all about. That is advanced business. It is what you and I do. Starters always seem preoccupied by magnification. Online sellers know this and direct their advertisements to it

    Quote Originally Posted by j.gardavsky View Post
    Hello John,

    this is an excellent explanation of the telescope magnification.
    Now, I understand why the Dutch have been the first nation making the telescopes, while the Germans have been still using glass for making beer mugs.

    Moreover, your explanation correlates with the labelling of the microscope eyepieces of today:
    10x written on the eyepiece tube means: 25cm/10=2.5cm focus length

    Still learning,

    JG
    Thanks, I am glad Gutenberg can't read your second line any more. In 1439 Dutch monks were still writing by hand (and drinking home brewed beer from German mugs).
    As a matter of fact the 25cm viewing / projecting distance in most microscopes is what brought me to think of this explanation.

    Quote Originally Posted by bladekeeper View Post
    Very nice, John. I shall use this during my next outreach attendance.
    Thank you. I certainly hope so. Demystifying magnification brings our hobby and the universe closer to the public.
    Last edited by John Baars; 03-17-2019 at 08:48 AM.
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    Default Re: Magnfication in Punch and Judy language.

    Nice explanation John.

    Surprisingly, the question never came up for me. Typically, people just ask what magnification the scope set to, but never ask how this accomplished.
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