Vixen is a Japanese company that makes telescope

Telescope

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

s, binoculars

Binoculars

Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...

, spotting scope

Spotting scope

A spotting scope is a small portable telescope with added optics to present an erect image, optimized for the observation of terrestrial objects...

s and accessories for their products.

Among many other more mainstream products they have created two unusual varieties of catadioptric

Catadioptric

A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses and curved mirrors . Catadioptric combinations are used in focusing systems such as search lights, headlamps, early lighthouse focusing systems, optical telescopes,...

 telescopes with an open tube design instead of the full-aperture

Aperture

In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

 corrector plate of the Schmidt-Cassegrain and most Maksutov-Cassegrain designs.

Klevtsov-Cassegrain

This design is based on the Maksutov-Cassegrain. It provides correction of aberration

Aberration

An aberration is something that deviates from the normal way.Aberration may refer to:In optics and physics:*Optical aberration, an imperfection in image formation by an optical system...

s via a two element meniscus

Meniscus

The meniscus is the curve in the upper surface of a liquid close to the surface of the container or another object, caused by surface tension. It can be either convex or concave. A convex meniscus occurs when the molecules have a stronger attraction to each other than to the material of the...

-shaped corrector lens

Lens (optics)

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 in front of the secondary mirror

Mirror

A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

. This design was originally envisaged by G. I. Popov with a practical implementation by Yu. A. Klevtsov. It is produced with apertures of 8, 10.25 and 13 inches. The 8 inch model employs a refractor style rack and pinion

Rack and pinion

A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby...

 focuser while in the larger designs the primary mirror is moved as in most other small to medium sized Cassegrain designs.

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

Astigmatism

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...

 and field curvature, it performs better than a true RC. There is the downside of the potential for chromatic aberration

Chromatic aberration

In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...

 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

Astrophotography

Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...

either at the native f/9 or using the optional focal reducer at around f/6.3.

External links

• Vixen's global website