Compound Telescopes

 
Compound telescopes, also called Catadioptric telescopes, use both lenses and mirrors to gather light. The use of both lenses and mirrors can give a greater focal length with a shorter tube. There are various types of comound telescopes, described below, but they all have this same basic configuration. Today, the Schmidt-Cassegrain design, which was invented in the 1960s, is one of the the most popular type of telescopes available. The  Maksutov-Cassegrain is another popular design.

 As you can see in the picture on the above, the light

1. Enters the tube through an objective lens.
2. Reflects off a primary mirror at the base of the telescope tube,
3. Reflect off a secondary mirror.
4. Travels through the tube again to the focal point at the base of the tube.

This traveling through the tube increases the focal length of the telescope without increasing the length of the telescope tube. A longer focal length will give you a higher power for a given eyepiece. The increased focal length in a compact format has made compound telescopes very popular with armature astronomers.

In the Schmidt-Cassegrain design 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.

In the Maksutov-Cassegrain design 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.

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.