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Your eyepieces will handle the longer focal ratio which a barlow offers better. The field of view will be sharper nearer the edge using a barlow. Standard plossl eyepieces become increasingly difficult to peer into when they get shorter than10mm. This means that the20mm with the barlow will be more comfortable to use than the 10mm on its own. Going shorter than10mm using a single eyepiece that has good eye relief means such an eyepiece would most likely have a barlow type lens in it anyway. The only significant problem you are likely to encounter with a barlow is when observing the moon, internal reflections may interfere. I have Meade 2x long barlow that i eliminated such reflections by flocking it. I believe the Celestron excell 2x barlow gets around the internal reflections problem by having a larger internal cavity.
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