A Barlow lens is an accessory to double, triple, or even quintuple the magnification of each of your eyepieces. While these lenses used to be a little junky, modern versions are of much higher quality. A judicious choice of Barlow lens can effectively double the number of eyepieces you have. A Barlow lens also allows you to get high magnification with a longer focal length eyepiece, which can give you a longer “eye relief”– the distance your eye needs to be from the front lens of the eyepiece. Like eyepieces, you get what you pay for. Prepare to spend at about $100 for a decent Barlow lens, or more for the more advanced Powermate lenses from Televue.