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Ok, so here's the other side of the argument :-)
As you increase the magnification, the airy disk gets larger (assuming a non-infinite mirror diameter). So the same amount of light is spread out over more area and it gets dimmer.
Imagine taking this to an extreme where the airy disk as captured on the ccd was 20 pixels across - each individual pixel is now a lot dimmer than before.
This only becomes apparent when the magnification reaches a point so that the airy disk is larger than the pixel size of the camera of course...
cheers, Bird
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