The Schmidt-Cassegrain telescope (SCT) 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 3.5 in and larger apertures.
SCT optical plan
Advantages of the Schmist-Cassegrain optical design:
- 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, making it 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 ft or 6 m)
The classic Celestron C8 SCT
Disadvantages of the SCT optical design:
- More expensive than Newtonians of equal aperture
- Short-tube appearance is not what people expect a telescope to look like
- Slight light loss due to secondary mirror obstruction compared to refractors