Compound Telescopes

Compound (Catadioptric) Telescopes: A Comprehensive Guide

Introduction

Compound telescopes, also known as catadioptric telescopes, combine the best features of refractor and reflector telescopes. These instruments utilize both lenses and mirrors to form images, providing high-quality views of celestial objects in a short, portable tube. This article explores the workings, history, and types of compound telescopes, along with examples of consumer-available models.

How Compound Telescopes Work

Catadioptric telescopes use a combination of lenses and mirrors. The mirrors do most of the focusing, and a thin corrector lens at the front cancels the spherical aberration of the spherical primary mirror (and, in some designs, coma). Because the corrector is weak, it adds very little chromatic aberration. They consist of:

1. Primary Mirror: A concave mirror that collects and focuses light.
2. Corrector Lens: A lens that corrects spherical aberration introduced by the primary mirror.
3. Secondary Mirror: A mirror that reflects the focused light back through a hole in the primary mirror or to the side for viewing.

The light path in a compound telescope typically follows this sequence:

• Light enters through the corrector lens.
• It reflects off the primary mirror towards the secondary mirror.
• The secondary mirror reflects the light back through the primary mirror's hole or to a side-mounted eyepiece.

This design allows for a compact telescope with a long focal length, ideal for both planetary and deep-sky observations.

History of Compound Telescopes

Lenses and mirrors had been combined in a few earlier instruments, but modern catadioptric telescopes date from the 1930s. Key milestones include:

• Schmidt Camera (1930): Invented by Bernhard Schmidt, this combined a spherical mirror with a thin, specially shaped corrector plate. It was the first widely successful catadioptric design and was used mainly for wide-field astrophotography.
• Maksutov (1941): Dmitri Maksutov (and, independently, Albert Bouwers in the Netherlands) replaced Schmidt's hard-to-make aspheric plate with a deeply curved meniscus lens with spherical surfaces, which is much easier to manufacture. In the Maksutov-Cassegrain form, the secondary mirror is often simply an aluminized spot on the back of the meniscus. Maks are known for sharp, high-contrast views, usually at long focal ratios (f/12 to f/15).

• Schmidt-Cassegrain Telescope (SCT): The design was worked out by optical designers in the 1940s, but it became an amateur favorite after Celestron, founded by Tom Johnson, began mass-producing compact SCTs, most famously the Celestron 8 introduced in 1970. It uses a Schmidt corrector plate combined with a spherical primary and a convex secondary mirror in a Cassegrain arrangement, folding a long focal length (typically f/10) into a short tube. Meade's ACF (Advanced Coma-Free) and Celestron's EdgeHD are modern variants that reduce coma at the edge of the field.

Examples of Catadioptric Telescopes

Schmidt-Cassegrain Telescopes (SCT)

• Celestron NexStar 8SE: This 8-inch SCT is renowned for its ease of use, computerized mount, and excellent optical performance. It's a popular choice for amateur astronomers.
• Meade LX200: Made in various apertures, the LX200 series introduced computerized GoTo fork-mounted SCTs to amateurs in 1992, and later versions added GPS and automatic alignment. Meade closed its US operations in 2024, so these are now found mainly used.

Maksutov-Cassegrain Telescopes

• Sky-Watcher Skymax 127: Known for its sharp and contrasty views, this 5-inch Maksutov-Cassegrain is compact and portable, ideal for both beginners and experienced observers.
• Orion StarMax 90: This smaller, more affordable Maksutov-Cassegrain gives good planetary and lunar views for its size. Orion closed its US operations in 2024, so it is now mainly found used.

Consumer-Available Refractor Telescopes

Though not compound telescopes, refractor telescopes are also popular among amateur astronomers. They use lenses to form images and are known for their sharp and high-contrast views. Examples include:

• Sky-Watcher Evostar 80ED: This 80mm apochromatic refractor offers excellent color correction and crisp images, making it a favorite for astrophotography and visual observation.
• Orion EON 115mm ED: A triplet refractor with extra-low dispersion glass, providing clear and bright images with minimal chromatic aberration (now discontinued).

Top Compound Telescopes

Celestron - NexStar 6SE

• Optics: 6-inch (150mm) Schmidt-Cassegrain, 1500mm focal length (f/10).
• Resolution: about 0.77 arcseconds (Dawes limit) and 0.92 arcseconds (Rayleigh criterion).
• Mount: Single-arm computerized GoTo alt-azimuth mount with a database of nearly 40,000 objects; works with Celestron's StarSense and SkyPortal WiFi accessories.
• Setup: Assembles in a few minutes; SkyAlign lets you align on any three bright stars or planets, even if you don't know their names.
• Good for: The Moon, planets and brighter deep-sky objects such as the Orion Nebula, in a tube compact enough to carry in one hand.

Meade 8" f/10 LX90 ACF

• Optics: 8-inch (203mm) f/10 Advanced Coma-Free (ACF) optics, 2032mm focal length, with UHTC (Ultra-High Transmission) coatings.
• Mount: Dual-arm computerized fork mount with the AudioStar hand controller (30,000+ object database) and built-in GPS; can be used alt-azimuth or tilted on an equatorial wedge.
• Power: Runs on internal C-cell batteries or an external 12-volt supply.
• Note: Meade closed its US operations in 2024, so the LX90 is now found mainly on the used market.

Celestron - NexStar 8SE Telescope

• Optics: 8-inch (203mm) Schmidt-Cassegrain, 2032mm focal length (f/10), in Celestron's familiar orange tube.
• Mount: Single-arm computerized GoTo alt-azimuth mount with a database of about 40,000 objects; works with StarSense and SkyPortal WiFi accessories.
• Setup: The tube, fork arm and steel tripod come apart for transport, and SkyAlign gets the telescope aligned in minutes.
• Good for: Planets, the Moon, double stars and brighter deep-sky objects; the 8-inch aperture gathers about 80% more light than the 6SE.

Conclusion

Compound (catadioptric) telescopes represent a significant advancement in astronomical optics, blending the best features of refractors and reflectors. Their compact design, versatile performance, and high-quality images make them a popular choice among amateur astronomers. From the Schmidt-Cassegrain to the Maksutov-Cassegrain, these telescopes continue to inspire and enable stargazers to explore the universe with clarity and precision. Whether you're a beginner or a seasoned astronomer, there's a catadioptric telescope out there to suit your needs and ignite your passion for the night sky.