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It really depends, in theory any amount of extra glass will impact the image, but in practise it's not that big of a difference compared to the reduced brightness of the magnification. A good quality barlow will be essentially invisible. In fact some eyepieces incorporate a Barlow into their design, I believe the Televue Naglers do this. The last question is a little simpler if it's the regular Televue barlow they probably claim to use better glass and multicoatings to justify the price. If instead you are talking about a Powermate, they are a little different as they have extra lenses in them to make them telecentric, that way the eye relief isn't increased along with the eyepiece magnification.
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