Reflector and refractor are similar words. While refractor and reflector telescopes aim to achieve the same thing, they do so in different ways. In this article, we look at the differences between a refractor vs reflector telescope. Importantly, we will detail which type of telescope is best for different purposes.
Refractor telescopes were the first telescopes developed in the early 17th century. Galileo is believed to be the first to point one at the night sky. With this type of telescope, Galileo significantly advanced our understanding of the universe’s workings.
In the middle of the 17th century, Isaac Newton devised the first reflecting telescope. These telescopes provide many advantages over refracting telescopes, especially when large apertures are required. For this reason, all modern scientific telescopes are of this design. However, refracting telescopes are still popular for amateur astronomers.
Choosing a telescope for someone looking to buy their first one can be bewildering due to the large number of options. Your first aim should be to decide if a refractor or reflector suits your desires better. This article will help you drastically cut the number of options by helping you decide which type of telescope is best for you.
To begin our look at the difference between a refractor vs reflector telescope we will discuss refractor telescopes followed by reflector telescopes. Later sections will compare the two and detail which telescope fits your needs best.
Refractor Telescopes
Refractor telescopes are the type of telescopes most people think of when they are mentioned. People imagine a long tube with a viewing position at the rear of the tube.
While the image that comes to mind appears simple, some magic occurs inside the tube.
How Do Refractor Telescopes Work?
Refractor telescopes do indeed come in a long tube. However, it is the contents of the tube that performs the magic.
How a refractor telescope works
Refractors use a large lens (or, more typically, a lens group) at the front of the telescope. This lens is the objective lens.
A smaller lens is located at the rear of the telescope. Again, this is more typically a group of multiple lenses. This is called the eyepiece. Eyepieces are interchangeable to adjust the magnification of the telescope.
Located near the eyepiece (and between it and the primary lens) is a focusser. This is adjusted to generate a sharp image for viewing.
The lenses produce the magic of refractor telescopes. Light is bent when it hits and enters a lens at an angle other than 90 degrees. The primary mirror of refractors is shaped such that the light is focussed to a point further up the telescope.
The eyepiece also bends the light to form an image. There are many lens designs used in eyepieces. Some of these designs use lenses that diverge light rather than bringing it to a point.
In the last section, it was mentioned that lenses bend light. We glossed over an important detail that we should address.
When a lens bends light, it discriminates against some light. Not all light is equal!
The amount of bending a lens achieves is related to the wavelength of the light. The wavelength of light is what distinguishes one color from another. Therefore, some colors are bent more than others.
As the light we see in our telescopes is a combination of many colors, they are not brought to focus at the same point. For example, blue light is bent more than green light, which is bent more than red light. As a result, blue light is brought to a focus closer to the objective lens than red light.
When we focus our telescope, we can not focus for all colors, and color fringing is observed. The fringing occurs along the edges of the bright objects. Quite often a halo of blue surrounds bright objects.
This effect is called chromatic aberration. Chromatic is just a fancy name for color.
Chromatic aberration is rectified in refractor telescopes by using multiple lenses within an objective lens cell. Two common lens cell designs are used in amateur telescopes.
Achromatic lens designs (also called doublet) combine two lens elements. This lens arrangement does not fully correct for chromatic aberration. Achromatic lenses are used in starter refractor telescopes.
Apochromatic lens designs (also called triplet) are more costly to produce but provide better correction. They provide better correction of chromatic and spherical aberration than the achromatic design. As a result, they are better suited to more serious observation.
Pros of Refractor Telescopes
Simple design makes them beginner-friendly
Low maintenance due to closed tube and lens design
Collimation is not required
Images produced are the right way up
Superior image quality and contrast
No obstruction in the path of light
Cons of Refractor Telescopes
Larger apertures are expensive
Chromatic aberration (unless corrected for)
Typically smaller apertures leading to less light being collected
Reflector Telescopes
If you are considering purchasing a telescope, you should consider whether a reflector is a better option. A reflector is often a better option for an adult user than a refractor. The reason is that their light-gathering ability is often superior to that of a refractor. You may not yet know it, but collecting as much light as possible is the ultimate purpose of a telescope.
How Do Reflector Telescopes Work?
How does a reflector telescope work
Whereas a refractor uses an objective lens, refractor telescopes use a primary mirror. The mirror is constructed of a glass (or other material) coated with a reflective layer. As light is not transmitted through the material, there is no chromatic aberration.
The primary mirror on reflectors is located at the back of the tube. Light enters the open front of the telescope and is reflected towards the front of the tube. As the mirror is convex, the light converges. Before the light is brought to a focus point, it is reflected by a flat secondary mirror near the front of the telescope tube. The secondary mirror is angled such that the light exits the side of the optical tube. The light then enters the eyepiece to form an image. As with a refractor, a focuser is located close to the eyepiece and mounted on the optical tube’s side.
The eyepieces used with reflector telescopes are the same as those used with a refractor.
The reflector telescope described above is called a Newtonian refractor. It is a simple design. There are more complicated designs available. These types of telescopes are called Catadioptric telescopes.
Catadioptric reflectors have their eyepiece at the rear of the optical tube, similar to refractor telescopes. To achieve this, the primary mirror reflects the light to the secondary mirror. Rather than being angled to the path of the light, the mirror is perpendicular to it. This reflects the light towards the primary mirror. A hole in the primary mirror’s center allows the light to pass to the eyepiece.
The optical tube of these telescopes is closed and are focussed by moving the primary mirror.
A benefit of the design is that the optical tube is shorter; however, they tend to be optically slower (i.e. have a larger f-ratio) than Newtonian reflectors have.
There are several designs and some use a lens on the front of the tube to correct for aberrations.
Reflector Telescope Optical Aberrations
Reflector telescope mirrors can be spherical or hyperbolic.
Spherical mirrors are those where the curve of the mirror is constant. This design can lead to spherical aberration, where areas away from the mirror’s center are brought to focus at a different point. At this point, we need to deviate to explain a telescope feature before continuing.
Telescopes have different ‘speeds’. A fast telescope collects more light than a slower one over the same period at a given aperture. This feature is called f-ratio. The mirror on a fast telescope must have more curvature than a slow telescope. For more information, see the Native Focal Ratio section on our Telescope Calculator page.
Getting back to mirror shapes. A slow telescope with a spherical mirror can produce acceptable images. However, for fast telescopes, the aberration will produce unacceptable results.
Hyperbolical mirrors are utilized for fast reflector telescopes. Hyperbolic mirrors are precisely and accurately shaped to bring all light to focus at the same point. To do so, the center of the mirror is more curved than towards the edges. As a result, these mirrors do not produce spherical aberration.
However, hyperbolic mirrors suffer from coma. This is inherent to the design and results from the mirror’s shape. In the eyepiece, stars appear to be stretched towards the nearest edge in the field of view.
For visual observations coma is perfectly acceptable. It only becomes an issue for those interested in astrophotography. It can be corrected using a special lens called a field flattener.
A refractor is the best option for the best optical properties. However, refractors come at a very high price for larger apertures. Since every astronomer’s dream is always for a larger aperture, this is a serious negative for refractors.
That said, a small aperture is not a hindrance for bright objects. This means that a refractor is a very good option for viewing the Moon and planets. Better optical qualities are needed to resolve features on the Moon and planets. For this purpose and for younger children, a small refractor is a good option for a first telescope.
Those interested in astrophotography may also like to consider a refractor. They produce wonderful, sharp, wide-field images. I have used approximately 110mm aperture refractors to produce some wonderful images. The image to the right is one such image of the Pleiades cluster.
A reflector is the best option for an adult interested in observing various celestial objects. They provide good apertures at an affordable price. Unless you know that you will get into astrophotography fairly quickly, a refractor on a Dobsonian mount is a good option for a beginner. I suggest an 8″ (200mm) Newtonian reflector on a Dobsonian mount.
Comparing Refractor vs Reflector Telescope Summary
Refractor Telescope
Reflector Telescope
Uses only lenses
Uses a primary mirror
Eyepiece located at the rear
Eyepiece located on the side
Produces superior image quatity
Image quality is lower
Expensive for larger aperture
Larger aperture more affordable
Smaller aperture gathers less light
Large aperture gathers more light to view dim objects
Closed tube require less maintance
Open tube allowing contaminants to enter
Does not require collimation
Must be collimated periodically
Best for bright objects
Best for dim objects
Final Thoughts
When considering a refractor vs reflector telescope, it is essential to remember that they operate differently to achieve the same goal. The primary purpose of any telescope is to collect light and form an image for viewing.
However, as we have seen, many factors must be considered before deciding which design suits our needs. To decide which to purchase, consider who will use the telescope and what type of celestial targets will be primarily observed.
A younger child will be more interested in viewing bright objects like the Moon and planets. Hence, a refractor is a good option. Adults interested in astrophotography should also consider refractors but should be prepared to pay a premium for larger apertures.
For most beginner adults, a Newtonian reflector is a good option. My personal suggestion is an 8″ (200mm) aperture reflector on a Dobsonian mount. We have some recommendations at Best Dobsonian Telescopes.
Hopefully, you now have a clearer idea of what type of telescope suits your needs.
For a general overview of telescopes and other equipment, see our general post on Telescopes.
I found astronomy while working in dark rural locations. Initially, I explored the night sky and learnt the constellations before purchasing a pair of binoculars to further my knowledge of the sky.
My first telescope was a 200 mm Newtonian reflector on an equatorial mount. I found that this telescope had a steep learning curve but was a rewarding experience.
As time progressed, I became interested in astrophotography. This resulted in purchasing a 110 mm refracting telescope and a dedicated monochrome-cooled astronomical camera. This resulted in another very rewarding steep learning curve that far surpassed the experience with my first telescope.
I have joined Telescope Guru to share my knowledge of telescopes and astronomy.
