- Vixen (telescopes)
Vixen is a Japanese company that makes
telescopes, binoculars, spotting scopes and accessories for their products.
They have created two unusual varieties of
catadioptrictelescopes both with open tube design, rather than the usual meniscus.
This design is a modified Maksutov-Cassegrain design. It has an open tube rather than a corrector plate and provides correction of aberrations via a two element miniscus corrector lens in front of the secondary. This design was originally envisaged by G. I. Popov with a practical implementation by Yu. A. Klevtsov. The 8 inch employs a refractor style rack and pinion focuser while the larger apertures move the primary mirror as in most other Cassegrain designs. It is produced in 8, 10.25 and 13 inch
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 in wide field applications. The design results in a image that is free of coma and astigmatism. These characteristics have led to the VISAC being referred to as a "Poor man's Richtey-Chretien". In fact, given that is has no astigmatism and field curvature, it performs better than a true RC, though with the downside of the potential for chromatic aberration due to the refractive elements though little to none is observable.
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 with 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.
* [http://www.vixen.co.jp/English/index_e.html Vixen's global website]
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