Catadioptric telescopes

Catadioptric, or mirror-lens telescopes, are compact and offer high optical performance. These telescopes are relatively expensive, but they’re worth the investment due to the high-quality images they produce. Many models can be equipped with computerized mounts. Catadioptric telescopes can feature mirrors up to 300 mm in diameter while still remaining quite compact.

The optics of catadioptrics can be designed within either an open or a closed tube. A closed catadioptric system is slightly heavier but protects the lenses and mirrors from dust and damage. Open designs, on the other hand, adapt more quickly to the ambient temperature — useful when preparing for outdoor stargazing, for example.

Because the light path is folded multiple times within the optical tube before reaching the eyepiece, these telescopes can have very long focal lengths. For instance, a 90 mm catadioptric telescope can have a focal length of about 1000 mm. However, despite all these advantages, image distortions are still present to some extent.

How a catadioptric telescope works

Essentially, a catadioptric telescope is a hybrid between a refractor and a reflector: it uses both mirrors and lenses in its optical design. This approach allows for the creation of relatively affordable telescopes with large apertures. More importantly, it delivers high-quality imaging.

The two most common catadioptric optical designs are the Schmidt-Cassegrain and the Maksutov-Cassegrain.

The Schmidt-Cassegrain system is based on spherical mirrors housed in a short, sealed tube. A full-aperture Schmidt corrector plate eliminates spherical aberration. This type of telescope offers a wide field of view (up to 6°), which is excellent for observation. Residual distortions include field curvature and coma.

The Maksutov-Cassegrain system uses a special meniscus lens for aberration correction. This design significantly reduces nearly all aberrations and can deliver sharper images than the Schmidt-Cassegrain. However, its drawbacks include longer thermal stabilization time and heavier weight. Telescopes using this design tend to be heavier than other types.

Light path in catadioptric telescopes

As mentioned earlier, catadioptric telescopes achieve long focal lengths within a compact tube. But how exactly does light travel through this system of mirrors and lenses?

Light path in a Maksutov-Cassegrain

The Maksutov-Cassegrain design is considered the best among cadioptric layouts. Its light path follows these steps:

1. Light enters the telescope and passes through the meniscus lens, which corrects spherical aberration;
2. It reflects off the concave primary mirror located at the back of the optical tube;
3. It then reaches a small convex secondary mirror, mounted on the back of the meniscus, and reflects back toward the primary mirror with an offset toward the center, where there’s a hole;
4. Light passes through the hole in the primary mirror and exits the optical tube;
5. The beam either enters the eyepiece directly or reflects at a 90° angle from a diagonal mirror before entering the eyepiece.

Light path in a Schmidt-Cassegrain

The Schmidt-Cassegrain design is also widespread. Here’s how light travels through it:

1. Light from the target object enters the optical tube through the Schmidt corrector plate, which corrects spherical aberration;
2. It hits the concave primary mirror at the back of the tube and reflects forward;
3. It reflects off a convex secondary mirror mounted at the center of the Schmidt corrector plate (on its inner side);
4. Light then travels through a hole in the primary mirror;
5. It goes into the eyepiece either directly or via a diagonal mirror.

Image quality depends not only on the optical design but also on the quality of the lenses, mirrors, and alignment. All optical designs are susceptible to some distortions, but overall, catadioptrics provide very good image quality. This type of telescope is great for deep-sky observation and astrophotography.

Maksutov-Cassegrain vs. Schmidt-Cassegrain

What catadioptric optical designs exist?

• Volosov, Slevogt: Complex to manufacture, hence expensive. Effectively corrects spherical aberration and coma.
• Maksutov-Cassegrain: Spherical mirrors + large achromatic meniscus lens. Secondary mirror can be part of the meniscus. Minimal distortion.
• Klevtsov: Spherical mirrors + meniscus + Mangin mirror. Open tube design. Notable distortions due to the secondary mirror being mounted on struts.
• Schmidt-Cassegrain: Aspherical base + spherical mirrors. Schmidt corrected spherical aberration, but coma remains.

Catadioptric or refractor?

Which is better: a refractor telescope or a catadioptric one? The truth is, they serve different purposes, so neither is universally better or worse than the other.

Refractors are generally more suitable for beginners and are great for observing planets and double stars. They’re user-friendly, require minimal setup, and quickly adapt to the surrounding temperature. Mid-sized models are convenient for city use, but taking them out into nature can be cumbersome, especially for high-powered versions. In refractors, light travels in a straight path, so the power is directly tied to length — the higher the magnification, the longer and heavier the tube.

Refractors produce crisp, high-contrast images, but inexpensive models often suffer from chromatic aberrations (colored halos around bright objects).

Advantages of refractors:

• User-friendly and ideal for beginners
• Great for observing the Moon, planets, and double stars
• Sharp, high-contrast images

Disadvantages of refractors:

• Not ideal for deep-sky objects
• High-power models are bulky and heavy
• Budget models have chromatic aberrations

Mirror-lens telescopes are better suited for more experienced users who want advanced capabilities. They are more expensive than refractors but offer greater optical power in a compact form. They’re equally good for viewing both planets and deep-sky objects. Portable and well-suited for trips into nature, they are also excellent for astrophotography. However, they take longer to thermally stabilize and require periodic collimation, which can be a challenge for beginners.

Advantages of catadioptric telescopes:

• Powerful models are more affordable than equivalent refractors or reflectors
• Good for both deep-sky and planetary observation
• Closed-tube models are protected from dust
• Very compact, even high-powered models
• Excellent for astrophotography

Disadvantages of catadioptrics:

• Long thermal stabilization time
• High cost
• Require precise collimation — better for experienced users

Astrophotography with a catadioptric telescope

Catadioptric telescopes deliver excellent results in astrophotography. They provide sharp, detailed images of the Moon, planets, and compact bright deep-sky objects — although they are not ideal for wide-field shots. For those, an apochromatic refractor is a better choice. Catadioptrics can also capture decent images of galaxies, though it requires long exposures and autoguiding. Using a focal reducer is recommended to increase light-gathering power.

Moon imaging: High detail when shooting craters, seas, and ridges even with focal lengths of 130–150 mm.

Planets: Excellent photos thanks to high magnification. Jupiter’s bands, Saturn’s rings, and Mars’s polar cap are visible. Both stills and video recording are possible.

Deep-sky: Narrow field of view; long exposures and guiding needed. Light gathering is limited, so a focal reducer is recommended.