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How to Choose a Telescope: What to Look For When Buying a Telescope

What telescope should you buy?  What features should you consider and what do they mean? Knowing these things helps when you are searching for that first telescope. Otherwise, it can be frustrating understanding the technical jargon, when you are a beginner looking for scopes for astronomy. The following describes the features of telescopes to help you to better understand the telescope reviews and product descriptions when choosing that telescope to buy.

What This Covers…

Buying A Telescope Guide

How do I look for a good astronomical telescope? In this telescope guide, I start with five main features that differentiate amateur telescopes and then expand with a glossary of terms.

What Telescope Should I Buy?

There are many types of telescope, so this depends on what you are wanting to get out of buying a telescope. You may want it for viewing planets or for taking with you on an excursions outdoors (See my list of easy trails) or you might be after a gift for a child, for example.

As I said above, the following is about helping you make an informed choice in buying a telescope by better understanding the terms used in telescope reviews and product descriptions.

1. Types of Telescopes

The three main types of optical telescopes are reflector, refractor, and catadioptric.

About these, the main difference between them is the geometrical optics:

  • Reflectors use mirrors (catoptrics)
  • Refractors use lenses (dioptrics)
  • Catadioptric combine lenses and mirrors

The following describes the main selection of telescopes for astronomy, available for the amateur and beginner astronomer.

> The Reflector Telescope

The reflector type (also called a reflecting telescope) uses mirrors, which have a glossy surface to gather light and then reflect the image. The Newtonian reflector type typically transmits 77–80% of light collected.

In this, there is a mirror at the back where light is collected and concentrated onto a smaller mirror that redirects the light into the eyepiece. The main mirror has a parabolic surface and this removes spherical (or color) aberration as it focuses the light rays to a common focus point. What’s aberration? That’s when different colors come into focus or there is blurring or distortion.

>> Dobinsonians

The Skywatcher 10″ Dobsonian – Click image for details

This is a reflector. A Dobsonian (named after John Dobson) is a Newtonian telescope that has an alt-azimuth mount (moves around on two axes). It requires manual adjusting using finder charts and star maps.

Dobsonians are the most economical in terms of light gathering power and are often recommended for beginners. Amateur astronomers love them because with modern Dobsonians they can explore the depth of the universe that only professionals could once reach.


> The Refractor Telescope

The refractor (also called a refracting telescope) uses a series of lenses to capture light and reflect the image. They typically transmit 90% or more of the light they collect.

The benefits of refractors: Low maintenance and long-lasting. Cons: Spurious colors at times, such as the pale violet halo around bright stars, the limb of the Moon, and the planets.

> Catadioptric Telescope

The catadioptric typically transmits 64-75% of the light collected. A Maksutov-Cassegrain (also known as Maks) is a catadioptric type that has a spherical mirror and a slightly negative meniscus lens (usually full-diameter).

Slightly less expensive, a Schmidt-Cassegrain (also known as SCTs) is also a catadioptric type. It uses a thin correcting lens (Schmidt), which is a spherical main mirror and another small mirror (no negative curvature) to concentrate the light through the eyepiece at the rear. They shouldn’t suffer color aberration. The SCTs are manufactured in much larger sizes also. This is a jack of all trades.

> Maks

The advantage of Maks is that they don’t require collimation (alignment of the optical elements) and are fairly rugged. So, they are suited to being on the move. Maks are good for planet viewing and lunar observations. Cons: Suffer from aberration and not for deep sky objects.

> GoTos

These are high tech and require some tinkering to set up, but with their onboard computer, they automatically take you to that object in the sky. You don’t need much knowledge of the sky with this telescope to see stars. Smartphones, tablets, and Wi-Fi feature in their use. Price wise, expect to pay a tad more.

As a guide, you will pay the same price for a well-equipped GoTo as you would for a Dobsonian with a larger aperture (3x that of the GoTo).

2. Aperture Size 

One of the main features to consider in the telescope range is the aperture size, which is the diameter of the primary light-gathering lens or mirror. The sharpness and brightness of the view improve with size.

As a guide, the following telescope comparison indicates the advantages of different aperture sizes among the three main telescope types.

>> For a Newtonian reflector telescope

A decent look at the details of planets requires at least a 6-inch (150 mm) mirror (you should be able to see celestial objects like Jupiter in greater detail). This size is the minimum for any decent astrophotography as well.

>> For a refractor telescope

A the details of planets is likely clearer with at least a 4-inch refractor.

>> For a catadioptric (Maksutov-Cassegrain) telescope

The details of planets possibly requires at least a 3.5-inch catadioptric.

3. Focal ratio (f/x)

This is another key feature to consider in a telescope. It represents the speed of the telescope’s optics. The longer the focal length generally the higher the focal ratio (the focal length divided by the aperture in mm). You’ll find the focal length is usually marked somewhere near the eyepiece focuser. The value can lie between 400 and 3000 mm.

The f/3 to f/5 types are in the category of fast. These have lower power and are best for wide field observing, i.e. observing large faint objects like galaxies. Suited for deep space photography.

The f/6 to f/10 types fall in the category of slow. These have higher power and are best for narrower field viewing, like for the planets and binary stars. Suited for photography of the moon. These are most useful for observing brighter objects like planets and observing small features on the Moon.

4. Telescope Mounts

There are two main types of mounts: an equatorial and an alt-azimuth.

>> Equatorial mounts (EQ)

An equatorial mount is a single axis mount that runs parallel with the Earth’s axis of rotation. The advantage is that there is only one axis to move when compensating for Earth’s rotation when focusing on celestial objects. This type is useful for astrophotography.

The EQs, however, are less portable and can be complicated because of awkward positions that you might find yourself in trying to comfortable access the eyepiece of the Newtonian telescope.

The varying designs of EQs include the German Equatorial Mount or GEM and the Open Fork Mount (explained in glossary section).

>> Alt-azimuth mounts

Alt-Azimuth refers to altitude and azimuth (see glossary). This type is less expensive and lighter than the EQ. This type is a basic up-down and left-right operation.

Allows for manual tracking of objects with the Earth’s rotation on a two-axis basis. It moves in azimuth (about a vertical axis) and in altitude (about a horizontal axis). Thus, not a simple following of a particular object in the sky. Altazimuth mounts are economical and simple to use and can result in a good cheap telescope.

The downside for these is that the non-computerized mounts need to be slightly moved every few minutes along both axes to adjust for the Earth’s rotation. This becomes more difficult with the increase in the size of magnification. They are not ideal for pointing at objects near the zenith and are not suited for astrophotography.

5. Eyepieces

Eyepieces and the focal length work together.

The basic rule of thumb for eyepieces: the lower the number the higher the magnification. When buying extras, make sure they are good quality in the 5–35 mm range. Smaller than this (< 5 mm) will not likely produce better views since the atmosphere limits the scope.

Glossary of Terms

Altazimuth: Relates to two axes. One being the vertical axis that refers to the azimuth (compass bearing) and the other the horizontal axis that refers to the altitude (angle of elevation) of the telescope’s pointing direction.

Aperture: An aperture telescope wise, is the diameter of the telescope’s light gathering component, i.e. lens or mirror.

Barlow Lens: A concave lens that increases the eyepiece magnification. Not really a lens as such a Barlow goes inside the telescope’s focuser before the eyepiece. It immediately doubles (or triples) the magnification. There is a small loss of light, but the benefit is that this device can double the value of your eyepiece by two powers. If considering a Barlow in a set with eyepieces, avoid having eyepiece focal lengths that are multiples of one another because a 20 mm and a Barlow make a 10 mm eyepiece redundant. Consider “achromatic” and “multi-coated” specifications as options to include.

Chromatic Aberration: Occurs due to different light wavelengths being refracted at slightly different angles. Also called ‘color fringing’ or ‘purple fringing’, it’s a failure of the lens to focus all colors to the same point. An aberration meaning abnormality or irregularity.

Finderscope: a low-magnification scope with a wider field that you use with the main scope to find an object in the sky.

FL (Focal length): The telescope FL divided by the eyepiece FL tells you the power (magnification) of the telescope. The longer the focal length the more power usually – providing a brighter image but at the same time you will have a smaller field of view (FOV).

Open Fork Mount (or Fork Mount): This is an equatorial mount design with a two-pronged fork attachment. You’ll likely find this type of mount on Cassegrain optical systems, that is, the modern catadioptric reflecting telescopes (200 mm or larger diameter). The mount resembles an Altazimuth mount, with the fork sitting on a wedge so that the azimuth axis is tilted and aligned with Earth’s axis of rotation (Earth’s geographic pole). The advantages are cost, versatility, and ease of use. Unlike the GEMs, this design tracks objects continuously through the meridian.

German Equatorial Mount (GEM): This is an equatorial mount design that has a primary structure shape of a ‘T’. This scope is balanced by a counterweight. The main advantage of a GEM is that you can easily swap telescopes, i.e. undo the mounting connections, remove one scope, and replace it with another. Their disadvantages are that they are heavier than other EQs because of the counterbalance and they often need to be “flipped” (a meridian flip) at some point to continue tracking to avoid the telescope hitting the mount.

Highest Useful Magnification: The highest visual power of a telescope that is achievable before the image becomes too dim to usefully observe. Usually 50 to 60 times the telescope’s aperture in inches (20 times in practical terms with atmospheric turbulence), or two times the aperture in millimeters (mm).

Lowest Useful Magnification: Three to four times per inch of the aperture, or 6 to 7 times the aperture in mm. The size the image becomes before light is lost outside visual observing ability (means the pupil width of 6 mm for older adults and 7 mm for young adults). Thus, it is best to avoid an eyepiece that produces less than the lowest useful magnification of the telescope.

Limiting Stellar Magnitude (or Limiting Magnitude): Star brightness is measured in magnitudes with the brightest stars seen by the naked eye having the lowest magnitude and the fainter ones having a higher magnitude. Limiting stellar magnitude is a measurement used to indicate the faintest celestial object or star you can observe with a telescope. So the higher the limiting stellar magnitude, the greater the expanse of stars that the telescope can detect. This factor is dependent on the aperture size. Compare a relatively large aperture of 8″ where you can see a star of magnitude 14 to 15 with that of a small 3″ that has a limiting stellar magnitude of 11 to 13 (as shown in this table).

Moon Filter: This is a filter that is placed on the base of eyepieces to reduce glare and show more surface detail.

Star Diagonal Mirror or Prism: This accessary adds comfort when viewing from a direction that is at right angles to the usual eyepiece axis.

Final Thoughts

If you looking for the best telescope for beginners and are still wondering: what is the best telescope to buy? Consider a telescope (or even a good set of binoculars) that’s simple and easy to set up. One like this is much better than any sophisticated type sitting, collecting dust, in the closet or on the shelf. So, let’s say it’s the one that gets used and gives the best views for the money.

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