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With its unique VISAC (Vixen Sixth-Order Aspheric Cassegrain) primary mirror, the optical system is composed of a concave primary mirror, a convex secondary mirror and a three-element field corrector. Especially noteworthy is its sharpness and flatness at the edge, which far exceeds that of Schmidt- Cassegrain systems of the same aperture. It provides a pinpoint star image of less than 15 microns, even at the edge of its 42mm image circle. The open tube design eliminates the dew problem that is