Cassegrains

Cassegrains

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The Cassegrain is a type of reflecting telescope which employs a combination of a primary concave mirror and a secondary convex mirror into its design.  Its combination of parabolic and hyperbolic mirrors was first conceived by Laurent Cassegrain in 1672 – nearly three and a half centuries ago!   While James Gregory had proposed the configuration, he never carried through with his plans and when Isaac Newton developed his reflector telescope, the Cassegrain was put aside.  It emerged many years later as one of the most popular designs in the world because it offered major improvements over original refractor designs.

 

How a Cassegrain Works

 

In the classic Cassegrain telescope a parabolic primary mirror has a hole placed in its center.  While this may seem like a very strange thing to do, the incoming electromagnetic waves are actually captured by the parabolic mirror’s edges and re-directed to the hyperbolic secondary mirror where they converge.  The light (or wave) is then refocused back towards the hole where it converges to a focal point – the eyepiece.  This “folding” technique makes for a very compact design which delivers longer focal lengths and permits higher magnification factors.

 

Cassegrain Designs

 

In staying true to classic configurations, two types follow the original Cassegrain design – the Ritchey-Chretien telescope and the Dall-Kirkham Cassegrain telescope.  When optician Bernhard Schmidt developed an astronomical camera that employed the Cassegrain to direct its light onto a photographic plate, another idea was born.  Using a corrector plate, or lens, at the front of the Cassegrain telescope further improves its focusing abilities.  Although many believe it was Schmidt who developed the Schmidt-Cassegrain telescope, it was actually James Gilbert Baker who created the first working models.  Other variations on the Cassegrain design include the Maksutov-Cassegrain telescope and the Argunov-Cassegrain telescope.

 

Cassegrain Uses

 

The Cassegrain telescope is highly prized for its wide field photographic abilities and high magnification performance.  Many believe it produces unrivaled views of Solar System studies and double stars.  For astrophotography, a camera, or a light detector such as a CCD, is placed at the focal point instead of an eyepiece.  The mirrors in this type of Cassegrain telescope are designed and arranged to share a single focal point.  Still other Cassegrain models make use of a tilted arrangement called “off axis”.  This allows the image to focus without the shadow of the secondary mirror.  Many radio telescopes are off-axis Cassegrain designs.  A key selling point is their extreme portability and "best of both worlds" views.  On the average, they are about twice as expensive as a comparative reflector and yet cost far less than an equal aperture refractor.  Their sharp, high contrast images on any subject make the Schmidt-Cassegrain Telescope (SCT) or Maksutov Telescope (Mak) a good choice for the intermediate to more experienced observer.

 

Maksutov Cassegrain Design

 

The Maksutov-Cassegrain is a specific telescope design invented in 1941 by Russian optician Dmitri Maksutov. It is patterned somewhat after the Schmidt-Cassegrain telescope as far as general appearances go, but the Maksutov-Cassegrain is an entity unto itself.   Unlike the traditional catadioptric style which uses a flat lens across the front (the meniscus), the Maksutov utilizes a positive lens arrangement for the meniscus in conjunction with a spherical primary mirror.  This large, full diameter lens structure is great at correcting off-axis and chromatic aberrations.  While the Maksutov stops coma and false colors, it gets very expensive and is prone to lens sag because of the size and weight of the lens itself.  It simply doesn’t apply itself well to large apertures for precisely the same reasons that large refractors aren’t common.  The folded optical path in conjunction with a secondary silvered “spot” on the convex side of the meniscus facing the primary mirror makes an excellent choice for a durable and portable telescope.  It has a simple construction.  By incorporating the silvered spot instead of a secondary mirror, it gives it the advantage of fixing the alignment of the secondary and eliminates the need for a ‘spider’ that would cause diffraction spikes.

 

Maksutov Cassegrain Astronomy Applications

 

With its long, folded optical path, the Maksutov-Cassegrain offers up ridiculously high focal ratios, making it a superb instrument for high magnification studies.  While they offer a very restricted field of view, they also excel at lunar and planetary imaging.  This makes the Maksutov-Cassegrain telescope especially well designed for certain astronomical applications.  Since the optical train is “fixed”, the telescope can be sealed away from exterior conditions.  This makes it well suited to tracking, remote viewing, and radar calibration and boresighting where instruments are subjected to severe environments and high g-forces are encountered.

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