Most people, looking for their first telescope, will have an image in their mind of a long tube with a lens on one end and an eyepiece in the other. That describes a refractor telescope. This is the type that was made popular by Galileo in the early 1600s.
They start their shopping only to discover that there are also reflector telescopes that are based on mirrors rather than a lens. The mirror sits in the bottom of a tube and the eyepiece sticks out the side. This type was developed by Sir. Isaac Newton in the mid-1600 and so it is often referred to as a Newtonian or Newtonian reflector.
Which is better for a first-time telescope buyer? What’s the difference between reflector and refractor telescopes?
I am going to address that question and hopefully enable you to purchase the best telescope with confidence.
First, let’s define some terms that I will be using.
- Aperture – The diameter of the front lens or the rear mirror in inches or millimeters.
- Focal Length – The length of the path that the light travels in the optical tube.
- Focal Ratio – Focal length/aperture usually expressed as F# such as F5 or F10
When we say telescope we are really talking about two parts – the optical tube assembly and a mount. This discussion will be focused on the optical tube, refractor or reflector. Just know that there are a variety of styles of mounts and either type of optical tube can go on any kind of mount. For the purpose of this discussion, we will speak of telescopes and optical tubes interchangeably as we can assume the optical tube will be on some kind of mount.
Telescope aperture is often reported in units of inches or millimeters. One inch equals 25.4 mm. For convenience, you can round that to 25. So if you think of a 4-inch aperture as 100 mm, you will be close enough for this discussion.
What is A Refractor Telescope?
A refractor telescope has a curved lens on the front. As it gathers light it bends it and concentrates it to a focal point within the optical tube.
You could have an eyepiece directly in the path of the light, like a pirate’s spyglass. However, if the eyepiece were at the end of the tube and you had the telescope pointed high in the sky you would have to be down on your knees to look through the eyepiece.
You see an example of a refractor telescope in the picture. This one is mounted on a tripod.
Astronomical refractor telescopes usually have a diagonal in the light path that bends the light through a 90-degree angle to place the eyepiece at a more convenient position. The eyepiece is usually inserted into the diagonal. There are diagonals that use a 45-degree angle but these are better for daytime spotting scope use. For astronomy, you want a 90-degree “star diagonal”.
A refractor with a diagonal presents an image that is correct up and down but reversed left and right. For astronomy purposes, this left-right flip is of little importance as there is no left and right in space. If you want to use your refractor during the day to view boats on the lake or go bird watching, you can get a diagonal that has a prism to correct this left/right flip. This is how spotting scopes and binoculars are made, which are based on the refractor design.
In this design, we have an open-ended tube. At the bottom of the tube is a curved mirror, called the primary mirror. This gathers the light from the sky and focuses it toward a flat secondary mirror that is set part way up the tube. This secondary mirror is set at a 45-degree angle so the light can be directed to the focuser on the side of the tube. The focuser holds the eyepiece. There is no diagonal as the secondary mirror serves this purpose.
The picture shows a Newtonian reflector design on a tripod mount. Light enters from the left, goes to the mirror which is inside the tube on the right, then back up and out to the eyepiece near the front of the optical tube.
The reflector design produces an image that is inverted. This is not a big issue for astronomy purposes, although some people are bothered by this. However, this does render this type of scope impractical for daytime use. It would show the boats on the lake hanging upside down.
Because the Newtonian reflector design scales up so well the optical tube can also get quite long. An 8” primary mirror that was at a focal ratio of 10 would need a telescope tube 80 inches long, almost 7 feet. Clearly, this would not be a very convenient size telescope for the average person. So focal ratios of F6 and lower are common. An 8” F6 reflector would be about 48 inches long which would fit in most cars.
As their aperture gets bigger reflector designers often work to lower and lower focal ratios to keep the optical tube size manageable. This would also be true for refractors but you simply don’t see them as big as reflectors. Reflectors of 16” aperture are common and 25” are readily available. Focal ratios of F3 to F5 are common in these big Newtonian reflectors. The largest privately owned reflector telescope has a 75” mirror, is mounted on a trailer and is pulled by a truck.
The second thing you see with Newtonian reflectors is truss designs. As shown in the picture, the solid tube is replaced with poles or struts. The primary mirror is contained in the box on the ground. The poles are attached to form a support for a ring where there secondary mirror and focuser reside.
The picture shows a Dobsonian type mount where the mirror box sits on a rotating base and rockers so you can move it left and right, up and down to point the telescope. If there is a lot of ground light, a shroud can be put over the framework to keep that stray light out of the light path.
Since a truss Newtonian reflector can be taken apart you can get a huge telescope into your car where a solid optical tube would need a trailer for transport. A 14” truss Newtonian will fit in the typical sedan and a 25” truss Newtonian will fit in the typical SUV.
Advantages and Disadvantages Of Each Design
All optical devices represent a set of compromises. As such, there is no best design. Each has its strengths and weaknesses, advantages, and disadvantages. I am going to get into a refractor vs reflector debate so you can take these into consideration when you make your purchase.
A key advantage of the refractor is that there is no central obstruction. The secondary mirror of the reflector blocks some of the light coming into the tube which reduces the effective aperture. In telescopes with 5” or less aperture the refractor is typically considered to have about a 1-inch advantage. This means that a 5” reflector and a 4” refractor would be considered about equal in light gathering ability, a key measure of the power of a telescope.
This lack of a secondary obstruction may also give the refractor a slight advantage in image sharpness. Naturally, this will depend, to some degree, on the quality of the manufacture of the telescope.
Refractors tend to hold the alignment of the elements in the optical path because the front lenses are rigidly mounted. Refractors are fairly maintenance free making them popular for new astronomers. This may be another reason that the refractor design is incorporated into binoculars and most spotting scopes.
The primary disadvantage of the refractor is chromatic aberration or CA. As light passes through the primary lens it breaks up into its color components just as a prism casts a rainbow on the wall by breaking white light into its various colors.
As the light reaches the focal point the various colors do not arrive at exactly the same time creating a tendency to cause some color fringing on bright objects. To address this chromatic aberration, CA, refractor designers have created two variations of the refractor.
The lower cost refractor design is called an achromatic refractor, also called an acrho and sometimes a doublet due to the primary lens having two lens elements. This is the typical design seen in entry-level refractors. With two lens elements that are made of different glass, this type of primary lens can reduce the amount of chromatic aberration as compared to a single primary lens element.
The shorter, the lower the focal ratio of the achromat optical tube the more chromatic aberration you are likely to see. The higher the focal ratio, the greater the reduction of the chromatic aberration. An achromat with a focal ratio of F5 will show some color fringing around bright objects, like the Moon. An achromat with a focal ratio of F10 will show a lot less and F15 may show almost none at all.
Achromats are usually for visual use, not astrophotography. Some people are bothered by the CA of low focal ratio achromats while others hardly notice it. However, if you take a photo through the eyepiece of an achromat you will likely see color fringing around the Moon and other bright objects.
The more expensive refractor design is the apochromatic refractor, also called an A P O. Through the use of special glass and usually including an additional primary lens element, chromatic aberration is virtually eliminated. These are also often called triplets due to the third element. APO refractors are usually preferred by those engaged in astrophotography where chromatic aberration would seriously compromise the image. However, they are much more expensive and much heavier than achro refractors.
- Newtonian Reflector
The advantage of the Newtonian reflector design is that it is less expensive to manufacture quality mirrors than lenses, especially as they get large. In the range of 2”/50 mm to about 5”/127mm, the cost difference is not particularly noticeable. But when we get above 5” this cost advantage starts to become apparent. In the hobby market, it is unusual to see a refractor larger than 8” while Newtonian reflector designs easily scale to 25” and larger. Virtually all optical tubes larger than 16” are based on a reflector design.
Mirrors also provide light that remains true to color. There is no splitting of the light into its color components as you get with the refractor, so chromatic aberration is not an issue.
Reflectors do introduce an aberration called coma, especially in lower focal ratio designs. Coma results in stars that are near the outer edge of the field of view having a comet-like tail. In scopes that have a focal ratio of F6 and higher, this is significantly reduced. Below F5 an additional lens is often added to the light path which is called a coma corrector to control or eliminate coma.
The other factor with reflectors is the need for regular collimation. The typical Newtonian reflector has a primary mirror that is mounted in such a way that the mirror can be moved to align the optics. The larger the mirror the heavier it is, the more subject to being knocked out of alignment by bumps during transport. And the larger the mirror the more it will be affected by thermal expansion and contraction which can affect alignment. So there are adjustments incorporated.
To a smaller degree the secondary can be put out of alignment, but because it is much smaller and lighter it is much less subject to loss of collimation. Still, it should be checked from time to time.
Collimation is a maintenance process that you will need to learn with most Newtonians over 4” in aperture. Some of the smaller ones have the primary set permanently but the larger ones are usually adjustable. It is a simple procedure that only takes a few minutes once you have done it a few times. And it doesn’t have to be done every time you use the telescope. Still, some newbies shy away from reflectors for this reason.
Once you get past 6”, the Newtonian reflector is the king of price performance as measured by cost per inch of aperture. When mated with a low-cost Dobsonian mount you get the best price performance telescope on the market.
Which Would Be Best For You?
Either will serve you well. Both are available in low-cost packages and higher priced, more capable packages for those with a higher budget.
The more aperture you get the more and the dimmer things you can see in the sky. More aperture lets you apply more magnification and will reveal more detail. Naturally, as the size of the aperture goes up the cost goes up and the weight goes up.
If you are looking for a scope of 4” or less and are concerned about learning collimation, a refractor is a good choice. They are easy to use and rugged. They travel well on family vacations and smaller ones can be carried in your carry-on luggage on a plane. If you add a 45-degree correct image diagonal, you can use a refractor as a daytime spotting scope. At night you just switch to the 90-degree star diagonal which is much better suited for astronomy. In this way, a refractor is more versatile.
If you want something larger than 4 inches, you will likely be looking at a reflector. The Newtonian design really becomes cost-effective at this size and larger. When mated to a Dobsonian mount you have a very cost-effective solution that is rock steady and simple to use. If you go to a “star party” where people bring their telescopes, most of the larger ones are likely to be of the Newtonian/Dobsonian design.
After years in the hobby I have seen that those who pursue astronomy long term often end up with three types of telescopes:
- Binoculars (refractor)
- A grab and go, a lightweight scope that is easy to move and set-up. Typically these are in the 70 mm to 130 mm aperture range.
- A light bucket – Typically something 8”/203 mm or larger
So, your first optical device can be any one of these. As you go forward you will likely add something in the remaining categories.