Important Features to Consider 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 the right telescope for you. Otherwise, it can be frustrating for you, especially if you are a beginner interested in obtaining 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 telescopes

There are three main types of telescopes: reflector, refractor, and catadioptric.

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

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


  • Spurious colors at times, like the pale violet halo around bright stars, the limb of the Moon, and the planets.

The last type is catadioptric, which typically transmits 64–75% of the light collected. Maksutov-Cassegrain is a catadioptric type.

  • Compact and great for portability.
  • Use both lenses and mirrors to capture light and reflect an image. T
  • Ideal for lunar and planetary observation with large focal ratios.

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.

  • 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

EQ (Equatorial mount): A single axis mount that allows the telescope to follow a particular object with the Earth’s rotation. Useful for astrophotography.

Altazimuth: 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) and thus, not for 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.

5. Other features

Barlow lens: A concave lens that can increase the eyepiece magnification.

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.

Highest useful magnification:

50 or 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, which is the size the image can get before light is lost outside the observer’s pupil (width of 6 mm for older adults and 7 mm for young adults). Thus, it is unwise to use an eyepiece providing 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.