The different types of telescopes explained
Last Updated: October 14, 2022
Since ancient times, humans have stared at the night sky, marveling at the stars above.
But our eyes, even with perfect 20/20 vision, are only so good, designed for viewing things here on Earth and not light-years away in the heavens.
Over the years, we developed telescopes to magnify our vision of the objects in the night sky. While the options may seem overwhelming, there are actually only three basic types of telescopes.
Refracting Telescopes
In 1608, Hans Lipperhey, an eyeglass maker in the Netherlands, filed a patent for a device “for seeing things far away as if they were nearby”. A year later, Galileo Galilei decided to direct it to the night sky, and thus the Refracting telescope was born.
A refracting telescope utilizes a lens (often called the primary/ objective lens) to collect and direct light along a tube to an eyepiece at the other end which the viewer looks through.
While the lens does focus and magnify the image it is pointed at, the bigger factor is actually the aperture: the diameter of the main optical component, in this case, the lens. Our eyes’ aperture is only about 7 mm (0.28 in), meaning even a small telescope at 70mm gathers 100 times more light.
Refracting telescopes/ Refractors started out fairly simple, but drastically sharpened our view of the night sky. Galilei used the telescope to make observations that proved the Earth actually orbited around the Sun and not the other way around.
Advantages:
- Intuitive Design: it is basically what you think of as a telescope, making it easy to use
- Reliable and easy: since the objective lens is protected inside the tube, upkeep is minimal
- Misalignment of lenses is uncommon
- The lens is protected from the environment meaning less cleaning is required
- Typically smaller and more portable than other types
Disadvantages:
- Quality lenses and the tube length required mean refracting telescopes quickly become expensive; 8 inches is typically the maximum
- Visible color fringing/ chromatic aberration is common as the colored light wavelengths are split through the lens and arrive at slightly different angles. Modern telescopes typically address this in one of two ways.
- Achromatic refractors: use two lenses that greatly reduce, but not eliminate, color fringing; a cheaper option common in low- to mid-range modern refractors
- Apochromatic refractor: a more advanced (and expensive) version utilizing specialized glass and possibly a third lens (making this version sometimes called a “triplet” versus the “doublet” achromatic) creating a truer color image.
- Typically more expensive than reflectors
This short refracting telescope is mounted on a basic altazimuth mount.
Reflecting Telescopes
In contrast to Refractors, reflecting telescopes utilize mirrors, reflecting the light at various angles within the tube, lengthening the path of light, meaning these telescopes can be shorter than Refractors for similar-sized apertures. This aspect, paired with the fact that mirrors are cheaper to manufacture than lenses, is the reason why reflectors are often seen to provide the most bang for your buck, or rather the most telescope aperture for your dollar. A 6-inch refractor can cost as much as 10x more than a 6-inch reflector.
The most common type of reflector is called the Newtonian after its creator, the famous Isaac Newton. This type utilizes a curved, dish-shaped mirror at the bottom of the telescope which reflects light to a secondary mirror located at the top of the tube, which then reflects it to an eyepiece on the side of the tube.
With the primary/ objective mirror supported along the bottom of the telescope, it’s easier to make bigger sizes and have suitable mounts. A Dobsonian reflector utilizes a rocker box mount and a limited tube design, a frame that is covered with a cloth to create a lightweight telescope for the size.
Advantages:
- No chromatic aberration/ color fringing
- Easier to scale up than refractors as bigger mirrors are easier and cheaper to make
- Cheaper in comparison to similar-sized refractors
- Bigger apertures
Disadvantages:
- More upkeep and maintenance:
- Mirrors require precise alignment with each other to reflect properly and they can easily fall out of alignment (collimation), requiring consistent realignment
- Due to its exposed tube, the mirrors also need frequent cleaning
- Upside-down image: Due to using mirrors, the image seen through an eyepiece will be upside down. While this doesn’t matter for astronomical objects, if you intend to use this telescope for more earthly endeavors such as birdwatching, it won’t work well without an image corrector.
- The upside-down image can make locating that small object in the big sky through the telescope difficult and disorienting. One of two attachments is recommended:
- A finder scope: essentially a mini refractor on the side of your telescope to help you find your target and then fine-tune adjustments with the telescope.
- A red dot finder: a laser pointer made for astronomy to help narrow down the search area for your telescope. Many modern telescopes come with one.
- The upside-down image can make locating that small object in the big sky through the telescope difficult and disorienting. One of two attachments is recommended:
Dobsonian telescopes are easy to use and an ideal choice for beginner astronomers.
Catadioptric/Compound Telescopes
Catadioptric/ Compound telescopes utilize both lenses and mirrors for maximum effect. Two common types available to amateur astronomers are the Schmidt-Cassegrain telescope, SCT, and the Maksutov-Cassegrain telescope, MCT.
Both utilize a lens at the front as a corrector plate, a rear primary mirror with a hole in the center, and a secondary reflector in the center of the front corrector plate which then directs the light through the hole in the primary mirror at the back to the eyepiece. This maximizes the light path within a smaller tube.
Advantages:
- Compact design with tubes two to three times as long as they are wide makes them fairly portable
- Combines the benefits of both lenses and mirrors
- Medium amount of maintenance
- Infrequent Collimation: While it does utilize mirrors that will need collimation, this will be much less frequent than reflectors. A well-maintained compound telescope can last months before requiring a collimation
- Less cleaning required: Similar to refractors, the tubes are sealed, keeping out dust and dirt/
Disadvantages:
- They are generally more expensive, due to the intricate nature of the optical system inside their optical tube assembly (tube).
- They require precise alignment and adjustments, which can be difficult for beginners.
- They are not as portable as other types of telescopes.
- They are more difficult to maintain and clean than other types of telescopes.
- They are more fragile and require more careful handling than other designs.
- They are more expensive to fix if anything breaks or a part needs to be replaced.
Thanks to their long focal length, Catadioptric telescopes are fantastic for observong deep sky objects such as galaxies and nebulae.
So, which one is the one for you?
You have to consider your abilities and your intentions. Will the telescope be used primarily from your backyard or do you hope to travel with it? Will frequent collimations be a struggle? Are you primarily interested in astronomical viewing or astrophotography?
- Go for the biggest you can reasonably afford BUT factor in your ability to carry and store it. Having a beautiful, massive telescope won’t matter if you can’t lug it out or only use it once because it’s uncomfortable.
- If you are interested in astrophotography, particularly of deep sky objects and wide-field views of galaxies and nebulae, a wide-field apochromatic refractor is a good option.
- Compound telescopes provide great flexibility in terms of portability, what you can view, and how you view it. They also tend to be great choices for planets and smaller Deep Sky Objects.
- An 8” Dobsonian reflector is typically recommended for beginner amateur astronomers due to its large aperture, its practical user-friendly format, and its affordable price. However, remember that you will need to be comfortable with frequent collimation adjustments.
- For travel scopes, short tube refractors and smaller compounds work best.
- A stable mount is vital to a telescope so don’t forget to factor it into your decision and budget.
- But all types will provide a better view of the night sky than your eyes, particularly of the moon and the planets, even Deep Sky Objects.
There is no one-size-fits-all telescope and many amateur astronomers have multiple telescopes (i.e. a portable one for travel and a bigger one for astrophotography or Deep Sky Objects).
If you’re not sure, there are several ways to explore the night sky and explore your options before committing. A pair of binoculars will greatly improve your view of the night sky. They’re easily portable, typically cheaper than a telescope, and require less upkeep.
Many local amateur astronomy groups host skywatching events where you can try out the different types. They are also happy to discuss all the details to help you decide which option would work best for you. Or did you know some libraries let you check out a telescope just like you would a book?
Conclusion
So, now that we’ve discussed the different types of telescopes, we hope you have a better idea of your options and how they work. if you’ve been considering a telescope to enhance your view of the night sky, take some time to evaluate what you want out of it to determine whether you want a refractor, reflector, or a catadioptric/ compound telescope.
Choosing a telescope really depends on your specific needs and wants. If you want to get into astrophotography, then a short apochromatic refractor, a Schmidt-Cassegrain or a Maksutov Cassegrain might be a good option for you. But if you just want to do some casual observing of the night sky, then a Dobsonian or an entry-level refractor telescope might be a better choice.
I started off my stargazing journey with a Dobsonian telescope and I definitely have a soft spot for that type of telescope. I became familiar with the night sky due to the user-friendly nature of this telescope and I still use it today.
But whatever type of telescope you choose, make sure that it is something that you are comfortable using. After all, the best telescope in the world won’t do you any good if you can’t use it properly. A good mount is an important investment and it can make a big difference in your stargazing experience.
Thanks for reading, and clear skies!
Sarah Hoffschwelle is a freelance writer who covers a combination of topics including astronomy, general science and STEM, self-development, art, and societal commentary. In the past, Sarah worked in educational nonprofits providing free-choice learning experiences for audiences ages 2-99. As a lifelong space nerd, she loves sharing the universe with others through her words. She currently writes on Medium at https://medium.com/@sarah-marie and authors self-help and children’s books.
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