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A barlow supplies several benefits. The first is that a 2x barlow doubles
the focal length of whatever scope it is used with, reducing optical
aberrations that most scopes suffer from. Secondly, it improves the eye relief of the EP that you are using, which in the case of some short
focal length ones, is very welcome. Thirdly, it effectively doubles your
available range of magnifications. It is especially useful with
achromatic refractors, which suffer from chromatic aberration.
There only a few possible downsides; namely, it will make a fast scope
slow for imaging; a poor quality one can reduce the quality of your
viewing or imaging experience, and lastly, it makes the optical train
physically longer, which doesn't matter when using the scope visually,
but can cause a heavy camera to sag somewhat, so that it's sensor is not perpendicular to the light path.
raymo
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