Vixen Sixth-Order Aspheric Cassegrain (VISAC)
Vixen produces an 8 inch aperture modified Cassegrain design (VC200L) they refer to as a VISAC (Vixen Sixth-Order Aspheric Cassegrain) that is based on a Cassegrain design with a primary mirror that is "sixth order aspheric" - somewhat like a hyperbolic mirror but able to be manufactured using mass-production techniques. To compensate for the aberrations the mirror design introduces, Vixen adds a "field corrector lens" - actually a three element corrector in the draw tube of the focuser which also reduces field curvature for wide field applications. The design results in an image that is free of coma and astigmatism. These characteristics have led to the VISAC being referred to as a "Poor man's Ritchey-Chrétien". In fact, given that it has no astigmatism and field curvature, it performs better than a true RC. There is the downside of the potential for chromatic aberration due to the refractive elements, but it is hardly noticeable.
This particular design is also unusual in that it is a Cassegrain design but has a fixed primary and refractor style rack and pinion focuser which removes the image shift issues seen with other catadioptric designs.
These features together make for a telescope that is very well suited to astrophotography either at the native f/9 or using the optional focal reducer at around f/6.3.
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