A barlow is a negative or diverging lens tube which finds its home in a telescope's focuser before adding an eyepiece. At one end of the tube is concave lens which increases the telescope's effective focal length, which boosts the magnification power of the eyepiece from two to three times. The eyepiece is then inserted in the open tube end and held in place by set screws.We know a good barlow lens can double - or triple - or amplify any eyepiece's magnification. That makes owning a barlow or PowerMate a great cost-effective addition to your telescope accessories.