How different are refracting and reflecting telescopes from each other?
Is one type better for a specific telescope use?
Which type is more affordable?
Clear the air with a solid understanding of the optical designs of refractors and reflectors. If you know an inch in knowledge, you could possibly achieve your astronomy goals by a mile.
This isn’t a race or a boxing match between the two types of telescopes but instead is an informative presentation of the differences between the two. You’ll learn how they can balance each other out when one falls short.
Since no one telescope is flawless, why not learn about both and own one of both?
Let’s scope right into it.
Refracting Optical Design
Refracting telescopes have a glass objective lens that collects light. As light passes through this lens, they are split into their respective colors or wavelengths. They continue to travel and eventually meet at a focal point within the tube while parallel lightwaves fall onto a focal plane. Additional telescope accessories are used to acquire a formed image.
Not all the wavelengths will be capable of meeting at the same focal point unless additional glass elements are used. When this happens, an optical aberration called chromatic aberration (CA) is seen. It appears as false color on bright objects such as the white moon with a fuzzy, purple or blue edge. Additional glass elements can help to minimize this aberration for this inherent design flaw. Therefore, we have refractors at different price points with various levels of optical quality.
The viewing assembly (focuser, diagonal, and eyepiece) are mounted at the rear end of the tube that is directly within the optical path. However, without a diagonal, it would be extremely uncomfortable to look through the telescope as you would have to be very low to point the tube up towards the sky. To make it easier and more comfortable to look through, a diagonal diverts that light path at a 45 or 90-degree angle to see the formed image through the eyepiece.
Reflecting Optical Design
Reflecting telescopes rely on internal reflections using mirrors versus the glass lenses of a refractor. A large primary mirror reflects incoming light waves towards a smaller secondary mirror suspended at the front, open end of the tube. The secondary mirror is tilted on a 45-degree angle that reflects the light up towards the viewing assembly. Therefore, the viewing assembly is found at the top of the tube versus the bottom as seen on a refractor.
Because mirrors do not split light waves like glass, they are known to have good control over chromatic aberration. However, because of where the secondary mirror is, it blocks all the possible light that could reach the primary mirror. As a result, a large portion but not all ever reaches the primary mirror for reflection, so the reflector cannot perform to its full aperture potential the same way a refractor can. This is an inherent design flaw known as secondary mirror obstruction. It’s a good thing a reflector can be made larger in size to acquire more light-gathering benefits cheaper than a refractor.
Refractor VS Reflector
It’s easier to compare the differences when you can see why they’re different. This section isn’t necessarily to pit one type against the other but to show how their differences may be beneficial or detrimental to telescope uses that you may be considering.
The aperture is the diameter of the objective lens in the refractor and the primary mirror in the reflector. The aperture is an important telescope feature to consider as it determines how much light-grasp a telescope can achieve. With more light collected fainter objects can be seen with more resolution.
Refractors come in small aperture sizes from 50 mm to 127 mm. Larger refractors are very expensive as objective lenses require more precision to make. Reflectors come in larger sizes starting at 4” and can be made incredibly large, however a maximum of 16” is usually sufficient for an amateur.
Reflecting telescopes also offer more bang for the buck because mirrors are cheaper to make even at larger sizes. So, they offer more value per inch in aperture versus refractors.
This is a biggie because budget is important. Both types of telescopes come at multiple price points starting in the entry-level market all the way to the high-end spectrum. Refractors are also popular standalone buys (tube-only) because the optical system may be very advanced and therefore very expensive.
Additionally, it’s not just the telescope type that determines the overall cost. Optical quality and mount type are influential factors. Both can come with computerized mounts, advanced technology, and upgraded mechanical features.
If you’re looking for the cheapest telescope possible, it’s likely going to be a refractor for under $100. Reflectors will be on tabletop mounts and will start at $100. However, at this price point, the reflector offers the larger aperture on a more reliable mount.
Portability is about more than just size and weight as overall quality is just as important. Imagine traveling the world with your portable telescope or even a few hours out of town. Something breaks such as a plastic bearing on a cheap telescope mount – you’re done for. You’ve also gone all this way and you finally have sights on a hard-to-see galaxy. With a tiny aperture, it appears as a grey smudge. If you have the luxury of hauling a larger telescope, that smudge may actually show fantastic details making the drive or the flight worth it. So, there a few things to consider here.
But, most people are thinking about if it fits in their trunk or their carry-on case for flight travel, or if it’s light enough to get from point A to point B.
Refractors tend to have the upper hand here as they’re smaller in aperture which generally means a light weight ideal for travel. They’re very easy to setup and acclimate to temperatures instantly. Small reflectors on tabletop mounts offer portability benefits, too. However, the larger the reflector, the more unwieldy the tube becomes to transport and mount alone, and they take longer to cool down. They will also require collimation when you finally get to point B.
There should be very little user interference with the optical components of a telescope. While there are optical tools that allow for cleaning the objective lens on a refractor if you must, trying to clean inside of the tube regardless of telescope type can cause damage and possibly void your warranty.
A refractor scope requires very little user maintenance. It has less moving parts and collimation is rarely necessary. Hence, they’re convenient telescopes if you’re looking for ownership ease.
Reflectors on the other hand are somewhat high maintenance. They have exposed optics because the tube is open. This can cause dust, dirt, and debris to enter the tube. Mirror coatings can tarnish and degrade over time.
But, one of the most important maintenance procedures to consider is collimation. The primary mirror can come out of place that causes it to be misaligned. This can interfere with optical performance. Being able to collimate it to acquire optical alignment is essential. You can’t be afraid of collimation if you own a Newtonian, especially a large one.
Why are telescope images upside down?
You may have found that your specific telescope was broken and realized with your replacement that it has an upside-down image too. Hm, must be the scope, right? It sure is but your understanding is not quite there yet. All telescopes are going to show an upside-down image and others will also be mirror-reversed.
Both refractors and reflectors will have an inverted (upside-down) image. This is inconsequential when looking at astronomical objects as left/right, up/down does not matter.
Since image orientation for astronomical use is a non-issue and both types present an image that is at least upside down, it’s a tie.
So, your telescope shows an image that is upside down and you want to “fix” it.
There’s little you can do to modify image orientation with a reflector, and you will find that they are consistently not recommended for land use. However, it’s different with a refracting telescope.
The image orientation can be made to be “correct” with use of an erect image diagonal such as a prism. One of the most popular prisms you may come across in the amateur market is an Amici prism. Star diagonals with a refractor will correct it right-side up but it will present a mirror-image. A 45-degree diagonal is more comfortable and better suited to terrestrial observation while a 90-degree diagonal is better suited to astronomical observation.
With what we’ve learned above about image orientation, it’s clear that refractors can offer dual-purpose telescope use. This means that a refractor can be used with the appropriate type of diagonal to achieve image-correct orientation suitable for land viewing.
You can observe the landscape and scenery, wildlife, birds, and much more. Low power and wide field of view eyepieces will offer the best performance for terrestrial observation. You will find that mirage may interfere with the quality of your visibility but that is a phenomenon that is out of your control to minimize.
All telescopes can “see” our planets, but what if you wanted to see more? You are missing out on being able to see some incredible details if you don’t nail down the specifics required of a telescope to see these planetary features. What you want is a planetary telescope.
Planets are bright but their features are faint. While you don’t need a huge aperture to see the planets, it doesn’t hurt as it helps with resolution and making the most of high magnification. The key feature you want to look for is a slow focal ratio of f/10 or higher which both refractors and reflectors offer.
The slower optical speed narrows the field of view, but it makes it easier to see the planets and home in on surface features and other details. A Dobsonian could work very well for visual use. A refractor could also work but the chromatic aberration will degrade color fidelity and reduce contrast. Catadioptric telescopes are very good options for planetary observation and imaging.
To see more of deep space, bigger is better. The larger aperture allows for more light gathering. This allows for faint deep-sky objects (DSO) to be seen which is usually the goal. When it comes to these objects, especially point sources, there is no substitute. Reflectors, specifically Dobsonians, are excellent DSO hunters.
However, you can also consider the focal ratio to determine how good the telescope will be for stargazing. The faster it is, the better. A focal ratio of f/8 and lower will be wide enough to fit these large objects within the field of view. Since fast telescopes aren’t always the best at making use of high magnification, they do very well with low to medium power to achieve optimal fields of view with good visibility quality.
You can try imaging with any telescope type. With imaging planets with a planetary camera or taking photos of small planetary nebulae, a slow focal ratio rules. For imaging DSOs, a wide field of view and large aperture rules.
In this context, we will focus on imaging DSOs with an astrophotography telescope. Due to the negative effects of CA on imaging, semi-APO and APO telescopes are the popular choice for astrophotography with a refractor. Imaging quality is good, they’re easy to use and setup, and they cool almost instantly compared to reflectors. However, while a fast refractor will capture images faster at the compromise of a smaller image scale, a slower telescope will have a larger image scale, but it will take long exposure times.
A reflector has other considerations but the most obvious is that fast Newtonians will require precise collimation and a coma corrector, and very large models will likely need autoguiding. However, it can capture more in a given exposure with better resolution over a small refractor.
What really makes or breaks imaging is the mount. Are you using guiding equipment? Are you taking long exposures? Computerized GEM mounts provide the best parameters for imaging.
Which is Best for You: Refractor or Reflector?
By now, you should be able to determine which type works best for your needs to achieve your astronomy goals.
They each provide the same results which is a magnified view of an astronomical object, but they work differently to acquire it. Because of their optical designs, they offer different benefits and drawbacks.
Instead of looking at what you can’t do with one telescope, aim to own at least one of each type to acquire maximum performance for every need.