A 3x barlow lens tripples the magnifying power of your eyepiece by doubling its effective focal length. For example, an 18mm eyepiece used with a 3x Barlow lens would have the magnifying power of a 6mm eyepiece. Barlow lenses mount into either your telescope’s focuser or visual back.
Astro Basics 3x Barlow Lens – 1.25″ Fitting
3x Barlow lens tripples eyepiece magnification
1.25″ (31.75 mm) Standard astro fitting