The Tele Vue PMT-5126 4-element 5x Barlow lens offers a unique imaging quality Extension factor 5x 1.25" barrel with filter thread 4-element lens design for highest sharpness on the axis and in the field Sharp edge image even with fast telescopes The extension factor is retained even if the working distance is changed. Multi-coating on each glass-air surface for a bright image and high contrast Ideal for astrophotography when good sharpness is required throughout the field Is often used for H-Alpha telescopes to get the optimal aperture ratio Adapter and eyepieces are gently and firmly clamped by a compression ring