Vixen is a Japanese company that makes

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...

spotting scope A spotting scope is a compact lightweight portable telescope optimized for detailed observation of distant objects. They are used as tripod mounted optical enhancement devices for various outdoor activities such as birdwatching, skygazing and ...

s and accessories for their products. Among many other more mainstream products they have created two unusual varieties of

catadioptric A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses ( dioptrics) and curved mirrors ( catoptrics). Catadioptric combinations are used in focusing systems such as searchlig ...

telescopes with an open tube design instead of the full-

aperture In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

corrector plate of the Schmidt–Cassegrain and most

Maksutov–Cassegrain 250px, A 150mm aperture Maksutov–Cassegrain telescope The Maksutov (also called a "Mak") is a catadioptric telescope design that combines a spherical mirror with a weakly negative meniscus lens in a design that takes advantage of all the sur ...

designs.

Klevtsov–Cassegrain

This design is based on the

. It provides correction of aberrations via a two-element

meniscus Meniscus may refer to: *Meniscus (anatomy), crescent-shaped fibrocartilaginous structure that partly divides a joint cavity *Meniscus (liquid) In physics (particularly liquid statics), the meniscus (: menisci, ) is the curve in the upper sur ...

-shaped corrector

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

in front of the secondary

mirror A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...

. 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 A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and ...

style

rack and pinion rack and pinion is a type of linear actuator that comprises a circular gear (the '' pinion'') engaging a linear gear (the ''rack''). Together, they convert between rotational motion and linear motion: rotating the pinion causes the rack to be d ...

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 Petzval field curvature, named for Joseph Petzval, describes the optical aberration in which a flat object normal to the optical axis (or a non-flat object past the hyperfocal distance) cannot be brought properly into focus on a flat image plane ...

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 is a type of refractive error due to rotational asymmetry in the eye's refractive power. The lens and cornea of an eye without astigmatism are nearly spherical, with only a single radius of curvature, and any refractive errors ...

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

chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion, color aberration, color fringing, or purple fringing, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the ...

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, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1839, but it was no ...

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

External links

Vixen's global website
{{Authority control Telescope manufacturers