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CATADIOPTRICS
Catadioptrics use a combination of mirrors and lenses to fold the optics and form an image. There are two popular designs: the Schmidt-Cassegrain and the Maksutov-Cassegrain.
In the Schmidt-Cassegrain 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 secondary mirror which reflects the light out an opening in the rear of the instrument where the image is formed at the eyepiece. Catadioptrics are the most popular type of instrument, with the most modern design, marketed throughout the world in 3 1/2" and larger apertures.
Schmidt-Cassegrain, or catadioptric, telescopes use a combination of mirrors and lenses to fold the optics and form an image. Incoming light enters through a thin aspheric Schmmidt correcting lens, then strikes the spherical primary mirror and is reflected back up the tube. The light is then intercepted by a small secondary mirror which reflects the light out an opening in the rear of the instrument, where the image is formed at the eyepiece.
The Maksutov-Cassegrain design is a catadioptric (using both mirrors and lens) design with basically the same advantages and disadvantages as the Schmidt. It uses a thick meniscus correcting lens with a strong curvature and a secondary mirror that is usually an aluminized spot on the corrector. The Maksutov secondary mirror is typically smaller than the Schmidt's giving it slightly better resolution for planetary observing.
The Maksutov is heavier than the Schmidt and because of the thick correcting lens takes a long time to reach thermal stability at night in larger apertures (over 90mm).
The Maksutov optical design typically is easier to make but requires more material for the corrector lens than the Schmidt-Cassegrain.
These scopes are the most versatile telescopes and have the best all around, all purpose design. The Schmidt-Cassegrain & Maksutov-Cassegrain scopes are very portable. They utilize a large aperture in a very compact tube. Away from urban sprawl, with reasonably dark skies, an 8" SC provides excellent views of the Moon, planets, and faint deep-sky objects (clusters, galaxies, nebulas, comets etc.) and is well suited for astrophotography. You will pay over $1000 for the most basic models (and hundreds more to outfit it for astrophotography) with an equatorial mount. With a hefty tripod and mount, the larger models (10" & 12") can be a bit cumbersome for one person. There are more accessories available for these scopes than the other two. These telescopes are gaining popularity and can be completely computer-controlled, giving it what's called "GO TO" capability to any object in the sky, if you want to pay the price.
Schmidt-Cassegrain 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 CCD's.
- 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.
- Best near focus capability of any type telescope.
Schmidt-Cassegrain 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.
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CATADIOPTRIC REPORT CARDS
(used in excellent seeing conditions and with no light pollution; adapted from Astronomy Magazine):
E = excellent; VG = very good; G = good; F = fair; P = poor.
Small aperture (3.5" to 5") Schmidt- and Maksutov-Cassegrains:
Price range: $500-$4300
Portability: E
Ease of setup: VG
Ease of use: G+
Performance on the Moon: E
Performance on comets: F
Performance on double stars: G
Performance on galaxies and nebulas: F
Performance on planets: G
Medium aperture (6" to 8") Schmidt- and Maksutov-Cassegrains:
Price range: $1000-$4000
Portability: VG
Ease of setup: VG
Ease of use: VG+
Performance on the Moon: E
Performance on comets: G
Performance on double stars: VG
Performance on galaxies and nebulas:, G
Performance on planets:, G
Large aperture (9" to 16") Schmidt- and Maksutov-Cassegrains:
Price range: $1700-$20,000 and up
Portability: P+
Ease of setup: F
Ease of use: G
Performance on the Moon: VG
Performance on comets: VG
Performance on double stars: VG
Performance on galaxies and nebulas: VG
Performance on planets: VG |
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