Indian Astronomical Supplies

Catadioptrics












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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 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.
 

Maksutov-Cassegrain

The Maksutov 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.