Schmidt-Cassegrain is a telescope that uses a combination of mirrors
and lenses (catadioptric optics) to fold the optics and form an image.
The light enters through a thin aspheric Schmidt correcting lens, then
strikes the spherical primary mirror and is reflected back up the tube
and intercepted by a small convex spherical secondary mirror which
reflects the light out an opening in the rear of the instrument where
the image is formed at the eyepiece. These catadioptrics are the most
popular type of instrument, with the most modern design, marketed
throughout the world in 3-1/2" and larger apertures.
A Schmidt-Cassegrain telescope's optical plan.
Schmidt-Cassegrain (SCT) Advantages
- Best all-around, all-purpose telescope design. Combines the optical advantages of both lenses and mirrors while canceling their disadvantages.
- Excellent optics with razor sharp images over a wide field.
- Excellent for deep sky observing or astrophotography with fast films or CCDs.
- Very good for lunar, planetary and binary star observing or photography.
- Excellent for terrestrial viewing or photography.
- Focal ratio generally around f/10. Useful for all types of photography. Avoid faster f/ratio telescopes (they yield lower contrast and increase aberrations). For faster astrophotography, use a reducer/corrector lens.
- Closed tube design reduces image degrading air currents.
- Most are extremely compact and portable.
- Easy to use.
- Durable and virtually maintenance free.
- Large apertures at reasonable prices and less expensive than equivalent aperture refractors.
- Most versatile type of telescope.
- More accessories available than with other types of telescopes.
- Superior near focus capability compared to other types of telescope (approximately 20 feet or 6 meters).
The Celestron C-8 SCT.
Schmidt-Cassegrain (SCT) Disadvantages
- More expensive than Newtonians of equal aperture.
- It is not what people expect a telescope to look like.
- Slight light loss due to secondary mirror obstruction compared to refractors.