Askar 91F - 91mm f/6.7 Quadruplet Flat-Field APO Refractor - 91F (Pre-owned)Used, in excellent condition Askar 91 mm Quad ED APO Flat Field Refractor Askar 71F Flat Field Quad Refractor Highlights The Askar 91F adopts a quadruplet air spaced APO lens design, including one piece of ED glass, which can effectively reduce chromatic aberration. The Askar 91F supports 44mm full frame imaging with great performance of the surrounding star points. The Askar 91F is a self flattened design that allows users to avoid calculating the