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It is the design of the eye piece that allowes object to be focused at certain focal lenghts, a 31mm nagler with an 82deg field gives a mag of 59 times and an aparent field of view 83.4 arc mins whilste a 13mm nagler with a 82 deg field and magnification of141 times gives 35.00 arc mins, so it comes down to the design of the eyepiece
astroron
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