This is Vixen AX-103S (f/5.6 with x0.7 focal reducer), 4-inch apochromatic refractor from Vixen. Unlike an achromatic refractor, whose objective consists of two optical elements, it has a three-element objective. The central lens boasts extra-low dispersion (ED) glass designed to reduce chromatic aberration (colors focusing at different points) and improve image contrast. Also, a fourth optical component in included as a field corrector lens positioned toward the rear of the tube.
