Important Features to Consider When Buying a Telescope

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What telescope should you buy?  What features should you consider and what do they mean? Knowing these things helps when you are searching for the right telescope for you. Otherwise, it can be frustrating for you, especially if you are a beginner looking for a telescope for night sky viewing. The following description of different features of telescopes will help you with what to look for in a telescope.

By the way, if you are wondering: what is the best telescope to buy? A small telescope that’s simple and easy to set up is much better than a large sophisticated one 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.

1. Types of Optical Telescopes

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

The main difference between the three is the geometrical optics:

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

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 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.

A Dobsonian (named after John Dobson) telescope 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.

See the latest price of the Sky-Watcher 10″ Collapsible Dobsonian Telescope

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

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. SCTs are manufactured in much larger sizes also. Shouldn’t suffer aberration. 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 these. 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 get a Dobsonian with an aperture 3x that of the GoTo.

2. Aperture Size 

One of the main features to consider in a telescope 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 gives an example of the size advantage with different telescope types.

For a Newtonian reflector telescope

A decent look at the details of planets requires at least a 6-inch (150 mm) mirror, such as in the Orion 9827 AstroView 6 (you should be able to see objects like Jupiter in greater detail). This size is the minimum for any decent astrophotography as well.

For a refractor telescope

A decent look at the details of planets possibly requires at least a 4-inch refractor, such as in the Sky-Watcher ProED 100mm Doublet APO.

For a catadioptric (Maksutov-Cassegrain) telescope

A decent look at the details of planets possibly requires at least a 3.5″ catadioptric, such as in the Celestron NexStar 90SLT Mak Computerized Telescope (Black).

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.

  • f/3 to f/5 = fast, but lower power; best for wide field observing  and deep space photography
    • √  observing large faint objects like galaxies
  • f/6 – f/10 = slow, but higher power; best for narrower field viewing, photography of the moon, the planets, and binary stars
    •  √ observing brighter objects like planets
    • √ observing small features on the Moon

4. Mounts

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

Equatorial mount (EQ)

A single axis mount that allows the telescope to follow a particular object with the Earth’s rotation. Only one axis to move for Earth’s rotation. Useful for astrophotography.

Cons: Less portable, complicated, and can mean the eyepiece of the Newtonian telescope gets into a hard to access position. Unlike the EQ type, they do not allow for setting circles to find known objects using coordinates.

Alt-azimuth

Less expensive and lighter than the EQ.

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.

Cons: 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 not ideal for pointing at objects near the zenith. 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.

6. Other Features

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.

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 powerful it is giving you a larger image and smaller field of view.

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 (20 times in practical terms with atmospheric turbulence) the telescope’s aperture in inches, 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 can get before the light is lost outside the observer’s pupil (width of 6 mm for older adults and 7 mm for young adults). Best not to use an eyepiece giving less than the lowest useful magnification of the telescope.

Limiting stellar magnitude:

Indicates the faintest celestial object you will observe with the telescope (higher indicates fainter). It is dependent on the aperture size. Star brightness is measured in magnitudes with the brightest stars having the lowest magnitude. The higher the limiting stellar magnitude, the fainter the objects that the telescope can detect.

Moon filter: Placed on the base of eyepieces, to reduce glare and show more surface detail.

Star diagonal mirror or prism: Adds comfort with viewing from a direction that is at right angles to the usual eyepiece axis.

Related: Where to go to find the best places to stargaze in the world