The Uranoport Piggy-Back Barlow lens threads into the bottom of a 1.25" eyepiece, just like a filter. It provides a 1.5x magnification increase. This Barlow is optimized for four-element or less eyepieces (such as Plossls, MAs, SMAs, Kellners, etc.) and so is ideal for use with many beginner's telescope which already include two or more eyepieces. It is also great for observers with short-focal-length eyepieces who are not able to use 2x Barlows with their telescope because it may exceed the maximum useful magnification. The Piggy-Back Barlow is threaded so standard 1.25" filters may be used.