jambouk Posted July 11, 2022 Share Posted July 11, 2022 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 1 Link to comment Share on other sites More sharing options...
robin_astro Posted July 11, 2022 Share Posted July 11, 2022 (edited) 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 July 11, 2022 by robin_astro clarification 2 Link to comment Share on other sites More sharing options...
malc-c Posted July 12, 2022 Share Posted July 12, 2022 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. 2 Link to comment Share on other sites More sharing options...
Alien 13 Posted July 12, 2022 Share Posted July 12, 2022 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 1 Link to comment Share on other sites More sharing options...
ollypenrice Posted July 13, 2022 Share Posted July 13, 2022 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 Link to comment Share on other sites More sharing options...
Macavity Posted July 13, 2022 Share Posted July 13, 2022 (edited) 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 July 13, 2022 by Macavity Link to comment Share on other sites More sharing options...
robin_astro Posted July 13, 2022 Share Posted July 13, 2022 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 Link to comment Share on other sites More sharing options...
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