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P.S. if your fully illuminated field size is to small for a given eyepiece size objects towards the out side will drop of in brightness.
The 75% drop off is used as anything above that 75% to 100% you won't notice the difference in brightness.
Below 75% things will suffer in brightness.
Imaging the 75% is noticable so you need to fully illuminate that CCD chip.
If żour only going to use the Newt on the planets and the largest eyepiece is say a 12mm for high power than you could work on only a 6mm FIF but i would not be going to that extreme.
Mark
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