What Are Reflecting Telescopes?

Are you confused by the different types of telescopes?  Or are you just beginning your stargazing journey and wondering what type of telescope is right for you? 

This post spotlights one type of telescope, the reflector, and aims to educate on both the pros and cons of this type of telescope so that you can make an informed decision whether a reflecting telescope is right for you as you embark on or continue your astronomy journey.

Contents

• What is a reflecting telescope?
• How do reflecting telescopes work?
• Types of reflecting telescopes
• Advantages and disadvantages of reflecting telescopes
• Choosing the right reflecting telescope for you

What is a Reflecting Telescope?

A reflecting telescope (also called a reflector) uses mirrors to collect and focus light from distant celestial objects. Isaac Newton invented this design in 1668 as an alternative to the refracting telescope, which instead uses lenses to transmit and focus light.

Very Large Telescope (VLT)

Many backyard stargazers and professional astronomers prefer to use reflector telescopes. Famous telescopes used in astronomy research today, such as the Hubble Telescope and the Very Large Telescope, are reflectors.

How Do Reflecting Telescopes Work?

The basic design of a reflector is simple. Light travels into the telescope tube and bounces off the primary mirror at the bottom of the telescope. 

The light is focused to a point called, interestingly enough, the focal point where the secondary mirror is placed. 

The secondary mirror then reflects the light to the eyepiece where you can observe the image created of the Moon, planets or deep space objects.  

What is the role of the primary mirror?

The primary mirror functions as the heart of any reflecting telescope, gathering and focusing incoming light. This curved reflective surface sits at the lower end of the telescope tube, where it collects light rays and points them toward the focal point.

Modern primary mirrors are made from a solid glass cylinder with a carefully ground front surface, shaped into either a parabolic or spherical curve. A very thin film of metal—usually aluminum—covers the mirror's surface through vacuum deposition to create a highly reflective surface.

The primary mirror's shape is vital for proper image formation. Spherical mirrors are easier to make but have spherical aberration because light rays hitting near the edge don't join with those reflecting from the center. 

Most quality reflecting telescopes use parabolic mirrors that focus all light to a common focal point.

The focal length is the distance from the primary mirror to this focal point. This measurement sets key features of the telescope, including how much it can magnify when used with different eyepieces.

What role does the secondary mirror play?

The secondary mirror is just as important in the optical system. It redirects light from the primary mirror to a spot where you can easily observe it. Without this part, the image would form inside the telescope at the prime focus—right in the path of incoming light.

This smaller mirror connects to the optical tube through a specially shaped support called a "spider". Its size needs careful planning—a mirror that's too large blocks too much incoming light, while one that's too small can't properly capture and redirect light from the primary mirror.

Different reflecting telescope designs use various secondary mirror setups:

·       In Newtonian reflectors, a flat secondary mirror sits at a 45° angle, reflecting light to an eyepiece at the side of the telescope tube

·       Cassegrain designs use a convex secondary mirror to reflect light back through a hole in the primary mirror

·       Gregorian telescopes use a concave secondary mirror placed outside the prime focus

How are reflectors different from refracting telescopes?

Reflecting and refracting telescopes collect light in two very different ways. Refractors bend and focus light through glass lenses, while reflectors use mirrors to reflect and concentrate light.

Types of reflecting telescopes

There are several unique designs of reflecting telescopes. Each design builds on simple reflector concepts and provides different optical features that work best for specific viewing needs.

Newtonian Reflector

The Newtonian design has stayed popular since its introduction in 1668 because of its simplicity and affordability. This setup uses a parabolic primary mirror and flat secondary mirror that sends light at a 90° angle to an eyepiece on the tube's side.

Newtonians don't suffer from chromatic aberration and provide excellent value at any aperture size. Newtonian telescopes do sometimes suffer from coma—an optical aberration that makes objects at the edges of images appear distorted, with "comet-like" tails instead of appearing pinpoint sharp.

Dobsonian Reflector

John Dobson popularized this telescope design in the 1960s. Dobsonians are basically Newtonian reflectors placed on a simple altazimuth mount. This design stresses large aperture, easy transport, and low cost to help observers see faint deep-sky objects.

The altazimuth mount usually has a three-piece construction with Teflon blocks that allow smooth repositioning of the telescope. Dobsonians are sometimes referred to as "light buckets", perfect for newcomers who want large aperture instruments without breaking the bank.

Cassegrain Reflector

The Cassegrain's clever design uses a concave primary mirror with a convex secondary mirror that bounces light back through a hole in the primary. This folded light path creates a longer focal length in a more compact system compared to Newtonian or Dobsonian telescopes. 

Ritchey-Chrétien Reflector

The Ritchey-Chrétien telescope eliminates coma by using hyperbolic mirrors for both primary and secondary elements. This design was developed in the early 1910s by George Willis Ritchey and Henri Chrétien. Research professionals love this configuration, with it used in both the Hubble Space Telescope and Very Large Telescope.

Dall-Kirkham Reflector

Engineers created the Dall-Kirkham design in 1928 with a concave elliptical primary mirror and convex spherical secondary. This mirror combination is easier and cheaper to make compared to classic Cassegrains, and it also makes collimation easier. However, this design leads to off-axis coma that is much worse, limiting wide angle viewing.

Gregorian Reflector

The Gregorian telescope combines two concave mirrors—a parabolic primary and ellipsoid secondary—with the secondary placed beyond the primary's focal point. Though designed before Newton's reflector, astronomers couldn't build it successfully until 1673. Many modern solar telescopes still use this setup.

Advantages and Disadvantages of Reflectors

Every telescope design comes with its own set of trade-offs. Reflector telescopes shine in certain areas but face unique challenges that might affect your stargazing experience.

Benefits: no chromatic aberration, great value, large apertures

Reflecting telescopes eliminate chromatic aberration— rainbow-like halos around objects seen in refracting telescopes. Mirrors reflect all wavelengths of light equally, so reflectors create clearer images without color fringing. This makes them excellent tools for planetary and deep sky observation.

Reflectors offer great value for money. Mirrors need to be perfect only on the front surface, while glass lenses must be flawless throughout. Reflectors with large apertures are less expensive compared to refractors of similar size.

Mirrors are structurally supported from the back allowing reflectors to reach impressive sizes. The practical lens size of refracting telescopes maxes out around 1 meter, but reflecting telescopes can stretch beyond 10 meters in diameter. This size advantage means better light-gathering power—a vital feature for viewing faint objects like distant galaxies.

Drawbacks: collimation, dust sensitivity, image distortion

Mirror alignment (collimation) stands as the biggest challenge for reflector owners. Collimation needs to be checked often—before each viewing session for some telescopes. 

Temperature changes or small bumps can throw the optical system out of alignment. New users may find this process intimidating at first, but most quickly become skilled at the process.

The open tube design of reflectors does allow dust to accumulate on the mirrors within.  This degrades image resolution, reducing contrast between bright and dark areas of images. Mirrors need to be cleaned regularly to ensure the best viewing experience.

 

Reflectors can suffer from optical issues, such as coma which leaves stars near the field's edge looking like comets. The mechanisms involved include spherical aberration, pinched optics, field curvature, and vignetting. Many of these issues can be reduced with corrective accessories or by choosing more premium reflector designs.

Choosing the Right Reflector for You

Beginners: Small Newtonian or Dobsonian

Smaller Dobsonians and Newtonians serve as perfect gateway instruments for beginner stargazers taking their first steps into the hobby.

These compact light buckets offer remarkable bang-for-buck in the aperture department, typically providing enough gathering power to reveal lunar craters, Jupiter's cloud bands, Saturn's rings, and even some brighter deep sky objects like the Orion Nebula and Andromeda Galaxy. 

The simplicity of the Dobsonian mount (essentially a box with a pivot point) eliminates the intimidation factor of working with other mount systems. Newtonian reflectors on simple alt-azimuth mounts bring similar advantages with their straightforward setup - just plop down and start observing within minutes.

The relatively lightweight nature of these smaller instruments means easier transportation to darker observation sites, essential when escaping urban light pollution.

Maintenance remains manageable, with only occasional mirror cleaning and collimation adjustments needed.

Check out this Dobsonian or this Newtonian.

Intermediate/Advanced users: Larger Dobs/Newtonians or Cassegrains

Larger Dobsonians, with 8”-16” apertures, offer exceptional light-gathering capabilities at affordable prices, making them perfect for observing faint deep-sky objects such as nebulae and distant galaxies that remain invisible using smaller instruments.

Newtonian reflectors, particularly when mounted on equatorial platforms, provide excellent value with their large apertures while offering the tracking capability needed for longer observation sessions and basic astrophotography.

Cassegrain designs bring good versatility through their compact folded optical path, allowing powerful instruments to fit in still-portable packages.

The longer focal lengths in Cassegrains create high-magnification views of planets and lunar features that reveal incredible detail, such as the Great Red Spot on Jupiter or the Cassini Division in Saturn's rings.

Check out this Dobsonian or this Newtonian.

Budget vs performance considerations

Your astronomical goals should guide your investment:

Entry-level: $400-600 for quality 6" Dobsonians

Mid-range: $600-1000 for 8"+ Dobsonians/Cassegrains

Advanced: $1000+ for larger apertures or specialized designs

Conclusion

Reflecting telescopes are wonderful tools to explore the night sky and are the preferred telescopes for many amateur and professional astronomers.

This post explained how these telescopes capture light differently from refractors and provide much larger apertures at affordable prices. These telescopes also eliminate chromatic aberration, which makes them perfect for viewing distant celestial objects with exceptional clarity.

Reflectors excel at gathering light and offer great value, but they need regular maintenance like collimation and dust cleaning. Many astronomers find these small tasks worth the effort given the spectacular views these telescopes deliver.

Picking a telescope that fits your current needs and future goals will enhance your astronomy experience greatly. The night sky beckons - you just need the right reflector to bring its distant treasures into focus.

Key Takeaways

• Reflecting telescopes use mirrors instead of lenses, eliminating color distortion and offering larger apertures at lower costs compared to refractors.

• Beginners should start with Dobsonian or Newtonian reflectors for their simplicity, affordability, and excellent light-gathering capability.

• All reflectors require regular maintenance including mirror alignment (collimation) and dust cleaning to maintain optimal performance.

• Budget considerations range from $400-600 for entry-level 6" Dobsonians to $1000+ for specialized designs and larger apertures.

FAQs

Q1. What are the main types of reflecting telescopes?

The main types of reflecting telescopes include Newtonian, Dobsonian, Cassegrain, Ritchey-Chrétien, and Gregorian. Each design has unique characteristics that make it suitable for different astronomical purposes, from beginner stargazing to advanced astrophotography.

Q2. How do reflecting telescopes differ from refracting telescopes?

Reflecting telescopes use mirrors to gather and focus light, while refracting telescopes use lenses. This key difference allows reflectors to eliminate chromatic aberration, offer larger apertures at lower costs, and generally provide better performance for deep-sky observation.

Q3. Are reflecting telescopes suitable for beginners?

Yes, reflecting telescopes, particularly Dobsonians and basic Newtonians, are excellent choices for beginners. They offer large apertures at affordable prices, providing impressive views of celestial objects without requiring extensive technical knowledge to operate.

Q4. What are the advantages of reflecting telescopes?

Reflecting telescopes offer several advantages, including the absence of chromatic aberration, cost-effectiveness for large apertures, and superior light-gathering capability. These features make them ideal for observing faint deep-sky objects and capturing detailed images of planets and galaxies.

Q5. How much maintenance do reflecting telescopes require?

Reflecting telescopes require regular maintenance, primarily in the form of collimation (mirror alignment) and dust cleaning. Collimation should be checked before each observing session, while cleaning is typically needed monthly or bi-monthly to maintain optimal performance.

While this maintenance can be challenging for beginners, it becomes routine with practice.