Schmidt-Cassegrain Reflectors

Schmidt-Cassegrain telescopes use a combination of lenses and mirrors to fold a long focal length into a short and light optical tube.  Some say Schmidt-Cassegrains are the best all-around telescope for amateur astronomers.

SCT telescope

SCT telescope (credit: Meade Corp.)

Shortly after Newton developed his reflector, an obscure French Catholic priest named Laurent Cassegrain invented a reflector that used two mirrors to fold a long optical path into a shorter tube. Now nearly all reflectors use a variation of the Cassegrain telescope.

Not until 1930 did Bernard Schmidt add a new twist. He combined a simple spherical mirror with a specially-figured lens at the front of the tube to correct for spherical aberration. At the focal plane, he placed a piece of film. This layout is a Schmidt camera. It’s used for imaging wide-field views of the sky.  Finally, in 1946, an architect and artist named Roger Hayward placed a convex mirror behind the corrector lens to send light out the back of the tube to an eyepiece or a camera. Celestron built on this design and developed manufacturing techniques to produce SCT’s in large quantities that revolutionized amateur astronomy.

Schmidt-Cassegrains have something for everyone. Telescope manufacturers love them because the spherical mirrors and corrector lenses are easy to make compared to parabolic mirrors for Newtonians.  Casual observers love them because they are portable AND they have a relatively large aperture to 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 additional telecompressing optics, to wide field imaging.

A schematic of the folded light path of a Schmidt Cassegrain

Schmidt-Cassegrain telescopes 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, it has a narrow field of view, which is a big drawback if you like rich-field views of star clouds. 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

Schmidt-Cassegrain Telescope 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