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If you are looking at getting a new Telescope, you will notice that there are two main types choose from Reflecting Telescopes and and Refracting Telescopes. Each have their own uses, applications, advantages and disadvantages. If you are wondering how they work and the main differences between them then this article will provide you with all that you need to know.
Difference Between Reflecting and Refracting Telescopes
I’ve conducted some extensive research around these two primary types of telescopes. I will first begin with Refractors, before moving swiftly on to the Reflecting Counterpart.
The the first reported use of telescopes appeared in Europe during the early years of the 1600s. They were designed using lenses to enlarge distant objects.
This type of telescope, that utilizes lenses alone, is what we now know as and refer to as a Refracting Telescope.
The first known Refracting telescopes worked using a design known as a Galilean Refractor.
In this design, a convex lens objective and a concave lens eyepiece are used to magnify distant objects and their apparent diameters.
These telescopes are relatively easy to use because they produce an image that is shown the correct side up left to right.
Today, Galilean Refractors are not typically used; they are primarily sold as collectors items. Sometimes you will seem them sold under the name “Spy Glasses”.
If you are to purchase a modern Refracting Telescope today, it will most likely be constructed using a slightly adapted design, a Kaplerian Refractor
Kaplerian Refractors still use a convex lens objective but instead have a longer focal length with a convex lens eyepiece set behind the focal plane.
How A Refractor Works: The purpose of the objective lens, which is at the opposite end to the eyepiece, is to collect the light from an object you are observing in the sky, e.g. a star. It then bends that light into a single point of focus. The second lens (which is the eyepiece) has a purpose to then enlarge your focused image for your eyes; in many ways it is just like a magnifying glass. In other words, imagine the focused light being collected from the first lens as an insect, and then think of the eyepiece like a Magnifying Glass. This is pretty much how it works.
However, it is important to note that this method of magnification creates an image that is inverted both vertically and horizontally. However, the Kaplerian Design is still preferred over the Galilean because it allows for a larger apparent field of view (see more at any one time).
Therefore, it is important to note that if you purchase a Refractor Telescope, your images will be inverted and appear upside down. This can take some getting used to, but if you wanted to correct this, then you would need to apply a Erect-Image Prism Diagonal to correct and re-orientate the image
Keplerian Refractors are good if you are a beginner in astronomy and are commonly used. These are some of the benefits and as to why they are used:
Advantages of Refractor Telescopes
The three main positive of a Refractor Telescope Are:
- Easy Maintenance, Light and Transportable.
- Better Performance in Weaker Conditions – Images Appear Steadier
- Near permanent optical Alignment.
Easy Maintenance: Due to the design of a Refractor Telescope, the glass surface is protected in the tube and is therefore closed from the atmosphere. This means that it barely needs cleaning.
Better Performance in Weaker Conditions: A secondary benefit to the design, is that the lenses are less affected by changing temperatures. This results in steadier and sharper images than can be typically achieved from a Reflector telescope (of the same size).
Optical Alignment: Refractor telescopes only really require a one-off alignment. This is because their optical system is more resistant to misalignment than the reflector telescopes.
However, refracting telescopes do have their limitations and its good to be cognizant of these:
Disadvantages of Refractor Telescopes
The main three negatives of a Refractor Telescope are:
- Spherical Aberration
- Chromatic Aberration and
- Lens Sag
Spherical Aberration happens due to the use of spherical lenses. These are unable to bring parallel light rays into a perfect focus as each part of the lens refracts light somewhat differently. The result is that this causes a slight blurring of images. Thankfully, this can be corrected by using an objective lens that is parabolic rather than spherical.
Chromatic Aberration occurs because simple glass lenses are only able to refract shorter wavelengths of light over longer wavelengths. This problem can be mostly corrected by using compound lenses. These are lenses made with two different types of glass each with different refractive properties.
Lens Sag happens only in the largest Refracting telescopes. However, it is the principal limiting factor of Refracting telescopes as they get larger. Glass is not perfectly equal so as lenses are made larger and larger they become too heavy to retain the correct shape lens. The result is that the lens becomes distorted due to its own weight.
Unfortunately, there’s no real way to fix Lens Sag, apart from refraining from using a lens to capture and focus the light. Enter the Reflecting Telescope.
The Reflecting Telescope was discovered by Isaac Newton. He noticed that by using a concave mirror as your objective, your image is free from Chromatic Aberration.
Plus, as mirrors can be supported from behind, they can be made larger than lenses without the Sag problem.
Remember: the larger the objective the more light that can be gathered. And with more light comes more power and more visibility into the night sky.
The initial Newton design is still primarily used in most Reflecting Telescopes.
A concave mirror is placed at the end of a large tubular housing. The other end of the tube is open to let in the light. Within the tube a small flat mirror is placed at a 45 degree angle. This small mirror reflects light out through an opening in the side of the telescopes housing containing a convex eyepiece.
How A Reflector Works: Unlike a Refractor, a Reflector uses two mirrors instead of two lenses. These mirrors are used to manipulate the the light that enters the telescope from an object (like a Star). As the light enters the telescope, the first curved mirror (positioned at the end of the tube) reflects the light onto a smaller, flatter mirror (positioned in the center of the tube). From there the image is reflected to the eyepiece.
Advantages of Reflecting Telescope
The three main positives of a Reflecting Telescope are:
- Size and Light Collection
Size and Light Collection: As mirrors are easier to build compared to lenses, it is possible to make them larger and more durable. And as only one mirror is required to collect the and focus the light, the opposing side can easily be positioned on a surface for extra support. This enables the mirror to be constructed a lot larger than that of a lens. As a larger device means more light can be collected, the more you will ultimately be able to see.
Cost: Another advantage of the user of mirrors is that they do not cost as much to manufacture. This means a Reflecting telescope is a lot cheaper to produce than that of a Refractor. The result is that a stronger performing Reflector telescopes is available for less money than a Refractor equivalent. This means you can purchase more magnification power for less.
Disadvantages of Reflector Telescopes
- Use of Mirrors can cause Loss of Light
- Mirrors Curving
- Usage and Preference
Use of Mirrors can cause Loss of Light: Due to the nature of mirrors, some loss of light is to be expected. This can slightly reduce the surface area of the objective. As you can imagine, both the small secondary mirror and the opening in the objective reduce the effective surface area of the objective mirror, but just like the Newtonian design, this reduction is rather small.
Mirrors Curving: Depending on how the Telescope was constructed, transported etc, the Mirrors may not be a perfect curve. This can lead to image being reflected not coming to and forming a perfect point. Consequently a dragging effect may be observed; where a single point instead is seen as a line or cross.
Usage and Preference: Some people find it awkward to look sideways at objects. One way to fix this latter problem is by using a different method to get light into the eyepiece.
The Cassegrain Reflector uses a small mirror that is not angled but is instead convex, reflecting light back towards an opening in the center of the objective mirror that contains the convex lens eyepiece.
In general, Reflective telescopes are preferred by professional astronomers because the larger objective allows for more light to be captured. This way, dimmer objects can be seen with more detail and resolution.
The result is ultimately you can see more.
However there’s absolutely nothing wrong with refracting telescopes. They’re great for beginners as they
do a pretty good job and don’t require much maintenance at all.
I hope you’ve enjoyed learning about the differences between refracting and reflecting telescopes.
If you are looking to get a new telescope, be sure to read our Best Telescopes Buyers Guide to help you make an informed decision.
Ultimately, there are many different specific designs available. This includes several variations of the designs discussed here. But hopefully this has given you some more information to help you understand what scope is best for you.