Celestron’s Omni 2x Barlow comes in a sleek, silver package, with the two-element optics being fully multi-coated, and the edges of the lenses are blackened to cut down on internal reflections. The 1.25” barrel will fit in standard 1.25” focusers and diagonals, and the Barlow will accept 1.25” eyepieces. A metal thumbscrew locks the eyepiece firmly in place.
The barlow lens is inserted into your focuser first and then you insert the eyepiece into the open barrel of the barlow. Let’s say you have two eyepieces in your accessory case, a 10 mm and a 25 mm. Add a Celestron Omni 2X Barlow Lens to the mix, and now you have four magnifications to choose from. Combining this lens with the 25 mm provides double the magnification, or the equivalent of a 12.5 mm eyepiece, while the 10 mm gives you the power of a 5 mm eyepiece.
Eye relief is the distance your eye has to be from the first lens in the eyepiece in order to see the full field of view. When you use a Barlow lens with an eyepiece, you get double the magnification, but the eye relief is not affected. This is a good thing, because normally the shorter the focal length of the eyepiece, the less eye relief you have. In other words, a standard 10mm eyepiece, as a general rule, will have better eye relief than a 5mm eyepiece. Bonus!