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Choosing a Telescope - the Ultimate Guide

Posted by Ryan DeLange on

With the huge variety of telescopes on the market, the new astronomer faces a daunting task to decide which one to buy. This guide will help you cut through the hype. Here’s what you’ll learn:

Key telescope specifications - and why they’re important

The three types of telescope optical designs - and who they’re good for

Mount types - and their functions

Key Telescope Specifications

There are three important specifications to consider when choosing a telescope. You’ll see these numbers in advertisements and dealers will use them. So, it’s important to understand them.


Aperture is the diameter of the front opening (objective) of the telescope. It is the probably the most important specification because it determines how much light enters. If a telescope can gather a lot of light, then it can create bright, high resolution views. Think of it as a limiting factor. If a telescope is advertised as having huge magnification power, but there's not enough light available, then you're just going to be magnifying a blurry blob.

Focal length

Focal length refers to the distance between the objective opening of the telescope and the point where the light comes to a focus where the eyepiece is. For some telescopes, the focal length is approximately the distance from one end of the telescope to the other. But for other telescopes, the light travels back and forth inside the tube so the focal length is actually longer than the telescope. We'll explore that later in the Optical Design section.


This number (sometimes referred to as ‘power’) indicates how many times larger the telescope can make an object appear. For example, a telescope with a magnification of 250X can potentially magnify an object to 250 times the size that it appears with the naked eye.

Determining the magnification

Telescopes with longer focal lengths have more magnification power. In fact, you can easily calculate the magnification power of a telescope by dividing its focal length by the focal length of the eyepiece that you are using. For example, if your telescope has a focal length of 1000mm and you use a 25mm eyepiece, the magnification is 40x.

A note of caution about magnification claims!

Most experts caution consumers about the extraordinary claims that manufacturers make about the magnification power of their telescopes, warning that they are trying to mislead or even scam buyers. I agree and disagree.

It's true that if you are hoping to view faint, deep-space objects, and you buy a telescope with a small aperture and a high advertised magnification power, then you will be disappointed with the fuzzy image you get. These faint objects need a wide aperture to gather enough light to be able to magnify them clearly.

But you might be more interested in viewing brighter objects like the moon and some of the planets. These objects are so bright that they can be viewed with fairly small apertures and still be magnified quite effectively. In this case, the bold magnification claims don't disappoint.

Here’s a rule of thumb. To estimate the ‘maximum useful magnification’ for a telescope, multiply it’s aperture (in inches) by 50. For instance, for an 80mm (3 inch) telescope, 3 x 50 = 150. So, on a very clear night, the upper limit of magnification where you could still see objects clearly would be 150x. If the package screams ‘400 times magnification!!’, then perhaps you should move on.

Types of telescope optical designs

There are three categories of telescopes, and knowing their benefits and drawbacks is key to a good decision.

Refractor Telescopes

refractor telescope diagramThe refractor telescope design was the one that Galileo used. The light enters the front opening (objective) of the telescope and immediately passes through a convex objective lens which refracts (bends) the light so that it converges at a focal point at the back of the telescope. Here the eyepiece is positioned for viewing the image.


  • These can be rugged and low maintenance, because the lenses rarely, if ever, need adjustment and dust cannot enter the closed system
  • It produces an erect image, which makes terrestrial viewing and locating night sky objects easier.
  • Images are a little bit clearer and higher contrast compared to other designs at the same aperture. For this reason, higher end refractors make excellent astrophotography telescopes.
  • Air cannot travel inside the system, so temperatures are stable, making the images steadier


  • Refractors cost more per inch of aperture because lenses are more expensive to produce than mirrors.
  • Chromatic aberration is an issue because different kinds of light refract differently through the lens. Manufacturers compensate for this by using more complex mirror designs. Most medium-priced refractors are achromatic, which means much of the discoloration is corrected (fine for visual observation). Premium refractors are apochromatic, indicating that the problem is virtually non-existent.
  • The closed system can take some time to adjust to the ambient air temperature.

Who should buy a refractor telescope?  Smaller refractors make a good gift for a child or new astronomer because they are rugged and require no maintenance. Also, if you want to view terrestrial objects, choose this design, since its the only one that keeps the image upright. High quality achromatic and apochromatic telescopes are the best choice for many astrophotographers.

Reflector Telescopes

reflector Newtonian telescope diagramA reflector telescope, on the other hand, has no objective lens, so the light passes directly to the back of the telescope, where it meets a concave ‘primary mirror’. This mirror reflects the light back up the telescope tube toward a smaller, ‘secondary mirror’. 

In Newtonian reflectors, the light is then reflected sideways towards an eyepiece placed on the outside of the tube. One of the most popular types of telescope amongst amateurs is the Dobsonian. This is a large Newtonian telescope mounted on a swiveling base for easy maneuvering. 


  • Reflectors cost less than refractors because mirrors are more cost-effective than lenses. This means for the same price you can buy a reflector with a larger aperture.
  • No chromatic aberration because the light is reflecting rather than refracting.
  • The profile of a reflector is relatively stable and manageable. Since the light is traveling back and forth inside the tube, the telescope can be half the length of a reflector and still have nearly the same focal length. That means you can have a very powerful telescope that sits easily in the back of your car and rests gracefully on your mount.
  • The eyepiece position on the side of the tube makes for a comfortable viewing position.


  • Periodic maintenance may be necessary, as the mirrors need adjustment (this is called collimation). Also, dust can enter the system.
  • Newtonians suffer from a problem called coma distortion, where stars at the edge of the field of view have a tail effect, similar to comets. While this isn’t a concern for most visual astronomers, a device called a coma corrector can fix the issue.
  • Reflectors usually are not suitable for serious astrophotography without modification. Recently, however, a few imaging Newtonians designed specifically for astrophotography have entered the market.

Who should buy a reflector telescope?  Newtonian reflector telescopes are loved by the visual astronomer who wants as much light gathering power as their budget will allow. Dobsonian telescopes are affectionately called ‘light buckets’ because their apertures are so large. These are a great choice, not for young children, but certainly for enthusiastic teenagers and adults who want to see deep space delights that may be fainter.

Catadioptrics telescopes (compound telescopes)

Catadioptric Cassegrain Telescope diagramWe have seen that refractors use lenses, while reflectors use mirrors. Well, catadioptric telescopes use a combination of both lenses and mirrors. This allows manufacturers to adjust for some of the minor image aberrations common to either refractors or reflectors. This also allows them to ‘fold’ the light path, which means more magnification compared to other telescopes of similar size.

The most common types of catadioptric telescopes are called Schmidt-Cassegrains and Maksutov-Cassegrains.


  • Because the light path is folded, catadioptric telescopes are much more compact than refractors or Newtonians.
  • For the same reason, they often have very long focal lengths (coupled with relatively large apertures), making them powerful telescopes.
  • Very little maintenance is required compared to Newtonians, as the mirrors and lenses rarely need adjustment.
  • They make wonderful astrophotography telescopes.


  • The longer focal length means a narrow field of view, which isn’t suitable for viewing some of the large deep space objects - great for planetary viewing though.
  • These can be relatively expensive.
  • The closed system can take some time to adjust to ambient air temperature.

Who should buy a compound telescope?  These telescopes do a lot of things very well. If you want a low-maintenance, very portable telescope for both visual and photographic use, and you don’t mind paying a bit more, these telescopes are a great pick. Also, if you want a planet viewing specialist, this is the one.

Types of Mounts

The type of mount you have is just as important as the optical design of your telescope. This is not an exaggeration, so read on.

Altazimuth Mounts

This is the most common type of mount for beginners. Its most similar to a camera mount in that it adjusts the orientation of the telescope in two directions, straight up and down (altitude) and left to right horizontally (azimuth). This makes it easy to move the view of the telescope around the sky as you desire.

Equatorial Mounts

Since stars don't move straight up and down or left to right, the equatorial mount was invented to follow their actual paths. This kind of mount is for more advanced or ambitious users since it must first be aligned with the North Celestial Pole and it adjusts along two different axes, namely the Declination and Right Ascension axes. This movement puts the telescope in awkward positions, so a counterweight keeps the center of balance over the mount. Many equatorial mounts are driven by a motor along the Right Ascension axis, which keeps the image in view for better photography and visual observation.


This is actually a type of altazimuth mount because it also moves horizontally and vertically. However, it looks very different and is used only with Newtonian telescopes. Instead of using a tripod, it uses a swiveling base, much like a Lazy Susan. This makes easy work of maneuvering larger telescopes.

GoTo Mounts

GoTo Mounts are completely computerized mounts that can find objects in the sky and track them automatically. They exist in all the above forms: Altazimuth, Equatorial and Dobsonian mounts. They are very popular for astrophotographers who require long-exposure images, but they also make visual observation easier and even more social. Imagine inviting friends over for a star party and having your mount take you all on a guided tour of the sky tailored to the specific date and time.

Congratulations! Now you can identify the different types of telescopes and their mounts. You understand the specifications. And, you can make an informed decision about which one to purchase. If you have any questions, feel free to contact us. We’re happy to help.

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