Catadioptrics are very popular amateur instruments. The design was popularized by Celestron in the late 1960's in the form of the Schmidt-cassegrain. Catadioptrics include at least one lens and one mirror. The two most popular designs are the Schmidt-cassegrain and Maksutov-cassegrain. A cassegrain telescope is a purely reflective telescope composed of two mirrors where the image emerges through a hole in the primary mirror at the opposite end from the entrance aperture. The Schmidt and Maksutov modifications to the cassegrain design add a lens at the entrance aperture. Because these two types of catadioptric are by far the most popular and for this level of discussion are essentially identical, the rest of the discussion will focus on the attributes of these two types of catadioptrics which are:
- Medium cost for a given aperture
With regard to the optical assembly only, these telescopes have a middle of the range cost. Typical amateur models will be sold with a "goto" mount, however, which pushes up the cost. They're still pretty good deals. - Very compact size for a given aperture.
Probably the most obvious feature of these designs are the shortness of the optical tube. This is the result of the fact that the optical path is folded on itself. Light enters the telescope on one end and hits the primary mirror on the opposite end of the tube. The primary mirror focuses the light at a point that is beyond the front edge of the optical tube, but before the light can escape it is intercepted by a secondary mirror that reflects the light back to the primary. At the center of the primary is a hole that allows the focused light to escape the tube. - Long focal ratios
These telescopes have focal ratios in the range of f/10 - f/15 which is quite long. This results in narrow fields of view and high magnifications. It also means that lower quality eyepieces produce more acceptable images than they would in a faster telescope. - Obstructed optical path
Because of the secondary mirror discussed above, the optical paths in these cassegrains are obstructed. This has the same deleterious effect that it has in the Newtonian reflector discussed previously. Additionally, the secondary obstruction in cassegrains tends to be larger than in Newtonians which further degrades the image to a greater degree. - Secondary Mirror
- Obstructed optical path
Catadioptrics have a small mirror that redirects the light path out the back of the telescope for viewing. This has a couple of effects optically.
- The first, and probably least significant, is that some of the light entering the optical tube does not make it into the final image. So compared to a design with an unobstructed optical path of the same aperture, the Newtonian's image will be somewhat dimmer.
- The more significant effect, is that anytime something is in the optical path it degrades the image. The obstruction in these catadioptrics is typically between 30-40%. Conventional wisdom is that if the secondary mirror is 20% or less (by diameter) than the primary mirror the effect is negligible. There are surely some that would argue with that position (just as there are probably some that would say 25% is OK). Just keep in mind that as far as optical quality is concerned the smaller the secondary obstruction the better.
- Collimation
In order for the telescope to form images the secondary and primary mirrors must be aligned. This process is called collimation. Some newtonians will need to be collimated every time they are used and others will not. Many people find the process intimidating, but it can be learned without too much difficulty. It does require special tools that will most likely not come with your telescope.
- Closed optical tube
While temperature differentials between a telescope and its environment can cause air currents within the closed tube of a telescope, the fact that the air cannot flow freely between the two environments typically results in a greatly reduced effect on the image when the temperature of the telescope is equalizing with the outside air. - Balanced aberrations
While these designs can be quite good, the examples typically available are designed prioritizing cost above optical quality. This results in a scope with high coma distortion, but at a level that's equal to or less than a typical Newtonian of the same aperture. These scopes will also possess a minimal amount of chromatic aberration, but not to the degree of an achromatic refractor. Other aberrations are present, but not necessarily to a greater or lesser degree than found in refracting or reflecting telescopes.
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