Schmidt-Cassegrain Telescopes

A Schmidt-Cassegrain telescope (credit: Celestron)

A Schmidt-Cassegrain telescope (credit: Celestron)

In the most recent article on telescopes, you had a look at Newtonian reflectors, the oldest type of mirror-based telescope for astronomy. Newtonians, as you learned, have one big drawback: they are big. Because they use a single primary mirror to direct light back to a flat secondary mirror, which in turn reflects light to the eyepiece at the side of the tube, the physical length of a Newtonian is roughly equal to its focal length. So a 12″ aperture f/6 Newtonian, for example, is at least six feet long and more than a foot wide. But in 1672, shortly after Issac Newton developed his famous design, an obscure French Catholic priest named Laurent Cassegrain invented a reflector that used two mirrors to fold a long optical path into a much shorter tube. Now many reflectors, and nearly all professional astronomy telescopes, use some variation of the Cassegrain design.

The light path of a classical Cassegrain telescope (credit: Szőcs Tamás)

The light path of a classical Cassegrain telescope (credit: Szőcs Tamás)

The image above shows the light path of a “classical” Cassegrain telescope. Like a Newtonian, this design uses a parabolic mirror at one end of the tube. The light from the primary reflects to a small curved secondary mirror, then back through a hole in the primary mirror to an eyepiece. The curvature of the secondary is usually hyperbolic in modern Cassegrains. The effect of the two curved mirrors is a folding of a long optical path into a tube that’s much shorter than the focal length of the primary.

Cassegrain’s design was by no means an instant hit. The design was immediately criticized by the great 17th-century Dutch scientist Christian Huygens. William Herschel, who in the 18th-century became the first to seriously use reflectors for astronomy, stuck with the Newtonian design. Few Cassegrains were made and used even into the 20th century. In fact large reflectors of any type were uncommon until the early-20th century when glass technology advanced sufficiently to allow the casting of large mirrors relatively inexpensively.

In 1930, as reflectors were coming back into vogue, the ingenious German optician Bernard Schmidt added a new twist to the Cassegrain design that made it useful for astronomical photography. He combined a simpler spherical primary mirror with a specially-figured lens, called a ‘corrector plate’, at the front of the tube to correct for spherical aberration.  At the focal plane, instead of a secondary mirror, he placed a holder for a piece of film. This layout was called a Schmidt camera, and it’s still used for imaging wide-field views of the sky.

The schematic of a Schmidt-Cassegrain telescope (credit: Celestron)

The schematic of a Schmidt-Cassegrain telescope (credit: Celestron)

Then in 1946, an architect and artist named Roger Hayward placed a convex mirror behind the corrector lens of a Schmidt camera to send light out the back of the tube to an eyepiece or a camera, much like Cassegrain’s early design. This turned the camera back into a telescope. A company called Celestron built on this design and developed manufacturing techniques to produce what’s now called Schmidt-Cassegrain telescopes (SCT) in large quantities.  This revolutionized amateur astronomy in the 1970’s because stargazers could now purchase relatively large-aperture but compact telescopes at a reasonable price.

Schmidt-Cassegrains, which are now primarily made by Celestron and their competitor Meade, have something for everyone. Telescope manufacturers love them because the spherical mirrors and corrector lenses are easy to make compared to the parabolic mirrors of Newtonians. Casual observers love them because they are portable and they have a relatively large aperture to help see faint deep-sky objects. And astrophotographers love them because they’re easy to mount and guide, they lend themselves to narrow field imaging and, with an additional optical element called a telecompressor, to wide field imaging.

A 16" Schmidt-Cassegrain telescope on an altazimuth fork mount (credit: Meade Corp).

A 16″ Schmidt-Cassegrain telescope on an altazimuth fork mount (credit: Meade Corp).

SCT’s are not perfect at anything but they’re pretty good at everything. The biggest advantage is portability: an 8-inch f/10 SCT packs a lot of aperture and a 2000 mm (80”) focal length into a tube about 430 mm (17”)  long. It weighs about 13 lbs (without the mount).  However, with a focal ratio of f/10, such a scope has a narrow field of view, which is a big drawback if you like rich-field views of star clouds along the Milky Way. Because of the secondary mirror, you won’t get the same sharp contrast on the Moon and planets with an SCT as you would with a refractor.  The secondary mirror occasionally requires alignment, though not as often as a Newtonian.  And an SCT is twice the price of a Newtonian of the same aperture.

Schmidt-Cassegrain Telescope Pros

  • Compact and versatile
  • Very little chromatic aberration
  • Large aperture compared to refractors

Schmidt-CassegrainTelescope Cons

  • More expensive than Newtonians for the same aperture
  • Require occasional minor alignment
  • Narrow field of view

Schmidt-CassegrainTelescopes are best for

  • All around observing of the Moon, planets, double stars, and narrow-field views of deep-sky objects
  • Observers with a larger budget who still want aperture but who favor portability
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