Schmidt-Cassegrain Reflectors

You’ve seen them in the astronomy magazines. Glossy blue or black tubes, short and wide, set on sturdy mounts and aimed hopefully at the sky. These are Schmidt-Cassegrain (SCT) telescopes made famous by Celestron and Meade. Are they the best all-around telescope for amateur astronomers? And is an SCT the best telescope for you?

The Basics

• A Newtonian telescope has a big drawback: it uses a single reflection from a curved mirror to send light to an eyepiece. That means the length of Newtonian is roughly equal to the focal length, so you get a long and heavy telescope with a big and awkward mount.

• But 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.

A Deeper Look

• 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

Good To Know

An 8-inch SCT like those sold by Celestron, Meade, and Orion have a focal length of 80 inches packed into a 12-pound tube less than 18-inches long. Pretty impressive.

Personal View

SCT’s are not perfect at anything but they’re pretty good at everything. The biggest advantage is portability… an SCT packs a lot of aperture into a small package. However, it has a narrow field of view… 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 and SCT as you would with a refractor. And an SCT is twice the price of a Newtonian of the same aperture.