# Focal length

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With a basic refractor, how is the focal length measured? In optics, the focal length if the distance from the lens axis to the principal focus as shown in the diagram below by the grey line. But reading online, people talk about measuring the focal length by pointing a refractor at the Moon and moving card at the back to bring the Moon into focus, and then measure the distance from the objective lens to the card... This would be a very different value to the former method.

Which is correct?

Thanks.

James

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They are the same thing. You remove the eyepiece and focus the moon on the card. For objects at infinity (like the moon) the plane where the image is in focus is the principle focus. (rack the focuser all the way in so the focal plane is outside the tube)

Cheers

Robin

Edited by robin_astro
clarification
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My understanding is that it has always been the "prime" focus point.  This may explain how the optics are related to the human eye when we look through a scope.  As you can see the objective image is at the focus point of the objective which if a piece of card is placed at that point would produce a small focused image.

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As above, determining the focal length of a SCT/Mak type scope is far more difficult as you have two optical surfaces, one fixed the other movable..

Alan

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It seems to me that the diagram in James' original post is confusing. Why does the point of focus lie so far inside the tube?

Olly

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Ah, "Ray diagrams" - Kudos re. above! Also the bane of many a Physics Student's life? 😅
(As with many things, can be "ignored" later!) But interesting... To include the EYEBALL?
Nothing is trivial? I guess THIS shows how we, the short sighted, can re-focus the scope!

Otherwise, the magnification is: (minus!) fo/fe... as D tends to infinity? (wibble) 😏

Edited by Macavity
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14 hours ago, ollypenrice said:

It seems to me that the diagram in James' original post is confusing. Why does the point of focus lie so far inside the tube?

It doesn't in most "real" refractors (well it might do if you rack the focuser fully out.)  If it did  prime focus astrophotography would not be possible.

Cheers

Robin

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