Barlow Lenses

While the Barlow is not strictly an eyepiece, our discussion would not be complete without mentioning this enormously valuable accessory. A Barlow is inserted into the telescope's focuser before the eyepiece and instantly doubles (or triples, in some instances) the magnification. At the expense of a small loss of light, this very useful device can double your eyepiece investment by making each perform at two powers. If you are contemplating purchasing a Barlow with a set of eyepieces, give some thought to staggering the focal lengths so that they are not simple multiples of one another: a 10 mm eyepiece is largely redundant if you use a 20 mm and a Barlow. Look for "achromatic" and "multi-coated" specifications when shopping around.