Telescopes 101

Beginner Tips on Selecting Your First Telescope

Refractor Telescope

A refractor telescope is a dioptric telescope that uses a long, narrow tube that permits light to pass in a straight line between the front objective lens and a rear-mounted eyepiece. Originally designed for use in spy glasses and astronomical telescopes, refractor telescopes are also used in binoculars and long or telephoto camera lenses.

Advantages of a Refractor Telescope

• A simple design contributes to reliability and ease of use
• Little or no maintenance is required
• Good for distant terrestrial viewing, along with lunar, planetary, and binary star observing

Disadvantages of a Refractor Telescope

• Cost per inch of aperture is more expensive than reflector or catadioptrics designs
• Heavier, longer, and bulkier compared to equivalent-aperture reflectors and catadioptrics
• Practical aperture limitations make it less suited for viewing faint and small deep-sky objects

For more information on selecting your first refractor telescope, see this article.

Reflecting Telescope

A reflecting telescope is an optical telescope that uses a single curved mirror or a combination of curved mirrors that reflect light to form an image. Invented in the 17th century as an alternative to a refracting telescope, it's designed for very large diameter objects. Nearly all major telescopes used in astronomy research are reflectors.

Advantages of a Reflecting Telescope

• Brighter images are delivered with few optical aberrations
• Ideal for viewing faint, deep-sky objects such as nebulae, galaxies, and star clusters
• Compact and portable

Disadvantages of a Reflecting Telescope

• Ultraviolet light doesn't pass through the lens at all
• It's difficult to make a glass lens with no imperfections inside the lens
• The glass lens sags under its own weight

For more information on selecting your first reflector telescope, see this article.

Schmidt-Cassegrain Catadioptric Telescopes

A Schmidt-Cassegrain Catadioptric Telescope combines a folded optical path with a corrector plate to create a compact astronomical instrument. The telescope design uses a spherical primary mirror and a Schmidt corrector plate for spherical aberration correction. From the Cassegrain, it inherits the convex secondary mirror, perforated primary mirror, and a final focal plane behind the primary.

Advantages of a Schmidt-Cassegrain Catadioptric Telescope

• Optics provide razor-sharp images over a wide field
• Ideal for observing deep-sky objects or for astrophotography
• Very little maintenance needed

Disadvantages of a Schmidt-Cassegrain Catadioptric Telescope

• Unusual look surprises some people
• Higher in cost compared to reflectors of equal aperture
• Obstruction by the secondary mirror causes a slight light loss

For more information on selecting your first Schmidt-Cassegrain Catadioptric telescope, see this article.

Maksutov-Cassegrain Catadioptric Telescopes

A Maksutov-Cassegrain Catadioptric Telescope employs a full diameter corrector plate (also called meniscus lens) to resolve problems of off-axis aberrations such as coma found in reflecting telescopes while avoiding chromatic aberration. The telescope's Cassegrain smaller secondary mirror provides slightly better resolution for celestial observing.

Advantages of a Maksutov-Cassegrain Catadioptric Telescope

• Can focus on a nearby object
• Fixing the alignment of the secondary mirror eliminates the need for a spider
• Closed tube design eliminates image-degrading air currents

Disadvantages of a Maksutov-Cassegrain Catadioptric Telescope

• Thick correcting lens takes longer to reach thermal stability at night
• Corrector lens requires more material than that of a Schmidt Cassegrain
• A slight light loss occurs due to the secondary mirror

For more information on selecting your first Maksutov-Cassegrain Catadioptric telescope, see this article.