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By doubling the focal length it's a double edged sword, in that you Field of View of the width and height of your sensor is halved.
Also each pixel will only see one quarter of the light than before.
Lets say a stars photons covers 2 pixels x 2 pixels without the Barlow, with the Barlow installed the star now spreads out to 4 pixels x 4 pixels. Or in area, 4 square pixels vs 16 square pixels.
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