The Basic Telescope Types
If you're new to the world of astronomy it's important to learn the basic telescope types. Most any telescope you get will fall into one of three catagories: refractors (which use lenses to bend light into focus,) reflectors (which use mirrors that reflect light into focus,) and catadioptric or compound scopes, (which use a combination of both lenses and mirrors.) While you might hear other specific terms for telescope types mentioned, such as “Dobsonian” or “Schmidt-Cassegrain,” most of these other types are actually variants of the basic three, such as a Dobsonian being a type of reflector or a Schmidt-Cassegrain being a type of catadioptric.
Refractors utilize specially designed lenses to focus the light into an image. They are usually long relative to their size, as the light must flow in a straight path through the telescope tube to the eyepiece. The larger the lenses in a refracting telescope, the longer the optical tube has to be to bring the image into focus. This combined with the fact that large lenses can be difficult and expensive for glassmakers to manufacture at high quality means that larger refractors can get rather expensive. For this reason, most refractors available for purchase tend to be smaller than other types, making refracting telescopes one of the most portable types of telescopes on the market.
Depending on the type of lenses used for the optics, you may encounter visible color fringing at high magnifications. Also known as chromatic aberration, color fringing is when the various colored wavelengths of light get split from each other and arrive at slightly different angles, showing up as an image with distinct coloration at the edges. Most low-cost refractors are “doublets,” which may have color fringing, whereas “triplet” refractors are designed to eliminate this issue.
Still, whether a doublet or triplet, refractors are solidly built scopes. Their non-movable lenses make for a sturdy design that doesn’t need much maintenance beyond the occasional cleaning.
Reflectors use mirrors, which causes light to reflect at various angles within the optical tube, extending the overall light path. This often causes reflectors to be shorter than refractors of the same aperture, as the light doesn’t need to flow in a straight line to move the same distance. When combined with how manufacturing large mirrors is often cheaper than manufacturing large lenses, it’s fairly common for reflectors to be much less expensive than refractors at larger apertures. Additionally, reflectors are not susceptible to color fringing in the same way that doublet refractors are. If you’re looking for a lot of bang for your buck in terms of aperture, reflectors are a great way to go. This is especially true for Dobsonians which come with their own easy-to-use rocker-box mount.
Reflectors can be a great value with many conveniences. They can also come in a variety of sizes and can get quite large. With this in mind, purchasing the largest reflector you can afford is a great low-cost way to get a high-aperture scope. Just make sure you are able to store and transport it safely.
There are some things you should consider about the reflector design. By default, the image you see through a reflector’s eyepiece will be upside down. For this reason, you’ll want to use your scope’s finder to line it up with the objects you want to see before looking through the eyepiece. Most modern reflecting telescopes come with a finder scope or a red dot finder, so you most likely won’t have to make an additional purchase to acquire this.
Additionally, a reflector will sometimes require a process called collimation, which consists of adjusting the reflector’s mirrors to make sure they stay in proper alignment with each other.
When properly maintained, a large reflector is a great way to view smaller or far away objects with great clarity, and is a great value for achieving high-aperture viewing.
Catadioptrics combine the optical benefits of both lenses and mirrors into a compact, convenient package, being smaller and more portable than either refractors or reflectors of the same aperture. This is made possible by the corrector plate that folds the light path and the curved secondary mirror that magnifies the light internally. There are many variations on this design, including Schmidt-Cassegrains and Maksutov-Cassegrains.
Since it uses mirrors much like a reflector, a catadioptric scope will require collimation. However, unlike with reflectors, this procedure needs to be performed far less frequently. If well taken-care of, a compound scope can go for years without requiring collimation.
The catadioptrics’ small size and portability give them a convenience not found in reflectors of the same class, making them a great investment for both beginners and experienced astronomers alike.
Picking the Right Scope for You
So which scope is right for you? In most cases you’re going to want the largest telescope that you can reasonably afford and carry. However, you might have specific needs that could make one scope or another more worthwhile for you personally.
If you want to do deep-sky observing or wide-field astrophotography of distant galaxies and nebulae, you might want to consider picking up a large reflector.
If you want a high degree of flexibility in what you observe, a Schmidt-Cassegrain or other compound telescope will give you a lot of options for how to target your viewing and imaging experience.
If you carefully weigh their benefits and considerations, while also keeping your budget in mind, you should be able to choose a scope that will help you get the most enjoyment you can out of the night sky.
If you have any questions or want to know more, you can contact us at OPT and we will be glad to help you out. Happy observing!