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How To Choose The Right Telescope

How to choose the right telescope

AstroTelescopium Team |

Since the early 1600's, telescopes have been useful optical instruments for exploring the night sky, discovering new galaxies and allowing us to better understand the universe.

With over five centuries of development, there has been a rapid evolution among the types of telescopes to choose from. Although these various types have slightly different purposes, the main function of a telescope has not changed. That is, to collect light.

A telescope collects light by using a lens or a mirror. These elements are referred to as the primary "objective", a term used in optical engineering. The diameter of this "objective" is known as the telescope's aperture.

Since the main function of the telescope is to collect light, selecting the telescope with the largest possible aperture will facilitate the collection of the maximum of light, thus producing the brightest and sharpest image with the most detail.

Based on the above statement, it seems logical to think that the easiest way to select a telescope is to simply choose one with the largest aperture. However, that is not necessarily the case.

Here are a few reasons why you should not prematurely choose a telescope solely based on its large aperture size:

  • Larger aperture sizes are more expensive
  • Bigger objectives are heavier to transport
  • Field of view is narrower
  • Storage can be difficult


How do I choose the right telescope?

    Now that we have debunked the idea of simply choosing a telescope based on aperture size alone, you may be asking yourself, "What are the other factors that I need to evaluate?".

    In this guide, we will dive further into 3 important concepts you should familiarize yourself with in order to choose the telescope that's just right for you:


    What to know before buying a telescope

    Before you look at telescope specifications, brand details and telescope types, you should first determine whether or not you need to buy a telescope at all. Binoculars are often overlooked tools for observing the night sky quite effectively. They are typically more affordable than telescopes and can capture groupings of stars, the Moon, planets and comets. In fact, binoculars are simply two telescopes conjoined side by side. So depending on their specs, they can be a tremendous improvement beyond simple observations with just the naked eye.

    Let's say you have already decided that you would like to see farther away objects in a bigger and brighter way than binoculars alone can provide. In that case, it's a good idea to first gain a basic understanding of the layout in the sky. A little background knowledge can go a long way in improving your overall experience and reducing any initial frustrations that typically come with figuring out your first telescope. The following set of guidelines will help you get started.

    3 ways to better understand the sky:

    • Learn the main points of the celestial sphere - north celestial pole, south celestial pole, celestial equator, horizon, zenith, nadir, meridian, and ecliptic. The 12 major constellations connected along the ecliptic are: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces.
    • Based on your location, know which seasons you are able to see specific constellations - For example, in the Northern Hemisphere,
      Spring season constellations include: Cancer, Leo, Ursa Major, Canes Venatici, Coma Berenices, and Virgo.
      Summer season constellations include: Hercules, Lyra, Cygnus, Vulpecula, Delphinus, Ophiuchus, Serpens, Scutum, and Sagittarius.
      Fall season constellations include: Pegasus, Andromeda, Triangulum, Cassiopeia, Cepheus, Perseus, Aquarius, Cetus, Capricornus, and Taurus.
      Winter season constellations include: Auriga, Orion, Monoceros, Gemini, and Canis Major.
    • Familiarize yourself with some of the brightest and most recognizable stars - Polaris, Vega, Sirius, Regulus, Arcturus, Capella, Altair, Deneb, Aldebaran, Fomalhaut, etc.

    With this foundational background knowledge of the sky acquired, it's time to become better acquainted with common terms you will come across as you browse various telescopes.


    Telescope specifications

    As you begin to sift through the different models, you will see a number of technical terms that may be unfamiliar to you. In this section, we will cover 4 of the most common specifications that you will see as you browse:

    • Aperture
    • Focal Length
    • Focal Ratio
    • Magnification

    As we mentioned earlier, the aperture of a telescope is the diameter of the primary objective (main lens or mirror). The larger the aperture, the more light is able to be collected by the telescope, which provides you with a brighter, sharper and more detailed image.

    The telescope's focal length is the distance light travels in the telescope from its aperture to the focuser (the link between the telescope and the eyepiece). Focal length is important because it determines the field of view. The longer the focal length, the narrower the view. The shorter the focal length, the wider the view.

    Focal ratio is sometimes referred to as the "f-number". To figure out a telescope's focal ratio, divide the focal length by the aperture. The larger the focal ratio, the higher the magnification and the narrower the field of view. Alternatively, the smaller the focal ratio, the lower the magnification and the wider the field of view.

    Many beginners believe that magnification is the most important feature of a telescope due to the way it is typically advertised by lower priced telescopes. In reality however, magnification (or power) is only useful to a point. In fact, maximum useful magnification is typically 50x the telescope's aperture in inches. For example, a telescope with a 5 inch aperture will have a maximum useful magnification of 250x (even if it advertises a power of 500x, the image will most likely be blurry at that point). To put it simply, magnification provides the ability to enlarge an image while limiting the field of view.

    For a more detailed overview on terms, you can take a look at How To Understand Telescope Specifications.

    You should now have more awareness of the layout of the sky and a better understanding of the most common technical details you will come across when browsing telescope types. Let's now take a closer look into what those telescope types actually are.


    Primary telescope types

    There are 3 primary telescope types:

    • Refractors (dioptrics) - use an objective lens to form an image
    • Reflectors (catoptrics) - use an objective mirror to form an image
    • Catadoptrics - use a combination of lenses and mirrors to form an image

    Refractors are the most commonly recognized types of telescopes. First developed in 1609 by Galileo, refractors use a glass lens to form an image. They refract, or bend, parallel light rays to converge at a focal point.

    A form of color distortion occurred in early refractor telescopes due to the curvature of the objective lens. This occurrence was referred to as chromatic aberration. In order to fix this issue, achromatic and apochromatic lenses were later developed.

    • Achromatic lenses - bring red and blue wavelengths into focus
    • Apochromatic lenses  - bring red, green and blue wavelengths into focus

    Apart from the other type of telescopes, refractors tend to produce the highest contrast. In relation to their aperture size, refractors can be on the more expensive side of the spectrum. However, their low maintenance, light weight and ease of use make them great for both beginners and advanced users.

    Explore FirstLight Refractor

    Refractor Pros

    • Easy to use
    • High portability
    • Low maintenance
    • High image contrast

    Refractor Cons

    • Chromatic aberration (unless achromatic or apochroamtic lens is used)
    • Low aperture per dollar ratio (price increases dramatically as aperture size increases)
    • Long tube length

    Refractor Use

    • Great for lunar and planetary observations
    • Popular for deep space astrophotography

    In 1668, Isaac Newton developed the reflecting telescope. Reflectors use a curved mirror at the rear of the optical tube to collect light and reflect to a secondary mirror before forming an image at the eyepiece.

    Isaac's design is known today as the Newtonian reflector. Unlike refractors, reflectors do not encounter chromatic aberration since mirrors reflect all color wavelengths in the same way without the need for specialized lenses to correct this effect.

    Newtonians also have a more affordable aperture per dollar ratio since they do not require an expensive glass lens and only one side of a mirror needs particular designing and polishing. However, they tend to be less portable due to their bulkier size.

    Another widely known reflector is the Dobsonian. John Dobson was a former monk and astronomer who redesigned the Newtonian on an alt-azimuth mount. This alt-azimuth base allows you to direct the telescope in an altitude (up and down) and azimuth (left and right) motion. If you desire the most aperture for your money, the Dobsonian is a great telescope to consider.

    Explore Scientific Dobsonian

    Reflector Pros

    • No chromatic aberration
    • Lowest cost per inch of aperture
    • Doesn't require tall mounts (since the eyepiece is located on top)
    • Shorter optical tube

    Reflector Cons

    • Heavier and bulkier optical tubes
    • Less portability
    • Requires occasional collimation (optical system adjustment)

    Reflector Use

    • Great for lunar and planetary observations
    • Popular for visual

    Catadioptrics use both refraction and reflection to form an image. They were introduced in the mid-20th centrury when a French Catholic priest, Laurent Cassegrain, improved on the reflector by adding an additional mirror to increase the optical path. Subsequently, in 1930, Bernard Schmidt built on the Cassegrain design by adding a special lens at the front of the telescope tube to correct for spherical aberration. He also placed a piece of film at the focal plane. Today, these types of telescopes are popularized as Schmidt-Cassegrain telescopes or SCT's, for short.

    Schmidt-Cassegrains are more well-rounded than refractors or reflectors alone as they combine both the ability to use lenses and mirrors to form an image. Due to the higher complexity in desgin, catadioptrics tend to be more expensive.

    An alternative type of catadioptric telescope is the Maksutov-Cassegrain, invented by Dmitri Maksutov in the 1940's. They are similar to the Schmidt-Cassegrain. However, the Maksutov-Cassegrain's corrector lens has a simpler spherical curve and is easier to manufacture. Additionally, because it's secondary mirror is just a thin layer of aluminum on the back of the lens, no realignment is required.

    Maks are better than SCT's when viewing higher magnification lunar and planetary objects, but they are not the best when observing wide field views such as the Milky Way.

    Explore FirstLight Maksutov Cassegrain

    Catadioptric Pros

    • More error-corrective than reflectors or refractors
    • High image clarity
    • Lighter and more compact

    Catadioptric Cons

    • More expensive
    • Longer cool-down time
    • Narrower field of view due to longer focal length

    Catadioptric Uses

    • Good for overall observation of lunar, planetary and deep space objects

    Smart telescopes, or digital telescopes, are another telescope type that has revolutionized astronomy. With artificial intelligence (AI) and advanced camera sensors, smart telescopes are able to hone in on various objects in the night sky, at the touch of a button.

    Unistellar Equinox

    As light pollution gets increasingly worse, especially in urban settings, smart telescopes have drastically increased the ability for both newcomers and veterans to continue enjoying unparalleled crisp and detailed views of the universe.


    Why is the view upside down?

    if you are viewing celestial objects such as the Moon with your naked eye, and then look through your telescope to view that same object, you may notice that the image is inverted. In fact, astronomical telescopes always show views that are not right-side-up.

    The reason for this is that the telescope would require additional optics to flip the image. The extra optics would add additional cost and worsen the image quality by reducing the light.

    Fortunately, adjusting to right-side-up views adds little value to your viewing experience since the vast majority of objects viewed through a telescope only appear as tiny dots to the naked eye.

    However, when using straight-through finderscopes, make sure to adjust the star chart upside down to match your view. If you use astronomy apps instead of star charts to locate objects, this won't be an issue.


    Next Steps

    You now have a better understanding of how to choose the right telescope for you today, however, as you become more attuned with this hobby, your preferences will evolve. Maybe now you are most interested in observing the planets, but in a year from now you become more interested in deep space objects and galaxies. It is completely normal to acquire multiple telescopes for different purposes over time.

    Ultimately, the best telescope for you will be determined through actual usage and experience.

    If you have any additional questions or need more assistance, please do not hesitate to reach out to a member of our team.

    Once you've selected your next telescope, be sure to review the best practices when it comes to Telescope Storage.



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