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By design, the Barlow lens has to be positioned inside the prime focus, by roughly half is focal length.
(How do you measure the focal length of a negative lens like a Barlow? Well measure the free lens diameter say 25mm and then draw a circle on a card twice this diameter (50mm) point the lens towards the Sun and when the image fills the circle measure the distance from the card - this the the focal length of the lens)
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