A Barlow is an enormously valuable accessory which when inserted into the telescope's focuser before the eyepiece 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 to avoid duplication. For example a 10 mm eyepiece is largely redundant as a 20 mm and a 2x Barlow will offer the same power. A barlow can also be used in conjunction with other devices such as cameras and webcams.The higher power barlows (eg 3x) are particularly useful when used with a webcam for planetary imaging.
As well as effective magnifying power Barlow lenses also come in 1.25 and 2 Inch sizes.
The Explore Scientific 5x Barlow - Focal Extender 1.25" is Explore Scientific's most powerful optical amplifier, delivering 5x magnification with excellent quality image and flat field characteristics.