A 2x barlow lens doubles the magnifying power of your eyepiece by doubling its effective focal length. For example, an 18mm eyepiece used with a 2x Barlow lens would have the magnifying power of a 9mm eyepiece. Barlow lenses mount into either your telescope’s focuser or visual back.
Astro Basics 2x Barlow Lens – 1.25″ Fitting
2x Barlow lens doubles eyepiece magnification
1.25″ (31.75 mm) Standard astro fitting