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for comfortable viewing when scope is pointed at, or near, overhead zenith. Resulting image is right-side up and will be reversed

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objects that are at a 90° angle from the direction the telescope is pointing.  This allows comfortable viewing when the telescope is at or near the zenith(directly overhead).Celestron's erect image diagonals are Amici prism designs, meaning that images are upright and laterally correct.  This design, however, instead renders images which are laterally reversed-but are brighter than an erect image diagonal.&3160 When maxing out your magnification capacity given your telescope and seeing conditions, that bit of brightness is pretty important.