A Barlow lens or focal extender fits between the focuser and the eyepiece and multiplies the telescope's effective focal length, so every eyepiece you own gives a higher magnification. The factor sets the job: 1.5x to 2x models such as the Antares twist-lock 1.5x and Celestron X-Cel LX 2x fill gaps in an eyepiece set, while 3x to 5x units such as the Takahashi Ortho Extender 4X and Explore Scientific 5x Field Extender suit high-power lunar and planetary work, and the barrel size, 1.25-inch or 2-inch, should match your focuser. For eyepieces to pair with them, see Eyepieces.
