How Telescopes Work

Practical Considerations

New York City at night
Photo courtesy NASA

There are practical matters involved when purchasing a telescope. To get the most out of your purchase, these factors should also be considered:

  • Portability
  • Maintenance
  • Storage space
  • Price


Areas of dark skies are decreasing across the United States, as demonstrated by this view of New York City at night. If you are an urban astronomer, odds are that you will have to move your telescope to a site several miles away that has moderate-to-dark skies. If so, you need to make sure that it is light enough to carry in and out of your home and car, and that it will fit inside your car or van. Finally, you may want a telescope that needs minimal assembly (optics, mount) when you reach your observing site -- trying to assemble a telescope mount in the dark can be extremely frustrating.



Some telescopes, like reflectors, require periodic maintenance. The most common maintenance with reflectors is keeping the mirrors aligned, or collimated. This can be a simple or complicated procedure, depending upon the individual telescope. Sometimes, especially with open-ended or completely open telescopes, dust may enter the tube and settle on the primary or secondary mirrors; these mirrors may have to be cleaned and re-aligned. Finally, mirror surfaces can degrade with time, and may require re-aluminizing or replacement.


When not in use, telescopes must be stored somewhere. This can be a definite problem with a large aperture telescope like a 10-inch Dobsonian reflector. You want to find a place with sufficient room, that is as dust-free and moisture-free as possible. Store the telescope covered to prevent dirt and dust from getting into it.


Telescopes vary widely in price. They can range from a few hundred dollars to several thousand dollars, depending upon the type:

  • small Newtonian reflectors (6-inch aperture or less) - $250 to $1,000
  • achromatic refractors (2- to 3-inch aperture) - $250 to $1,000
  • large Dobsonian reflectors (6- to 18-inch aperture) - $300 to $2,000
  • compound telescopes (6- to 11-inch aperture) - $1,000 to $3,000
  • apochromatic refractors (3- to 5-inch aperture) - $2,000 to $10,000

You can also consider price per unit aperture, and they would rate from high to low as follows:

  1. apochromatic refractors
  2. Newtonian reflectors, compound telescopes, achromatic refractors
  3. Dobsonian reflectors

Two things to remember:

  • No matter how good the telescope quality is, you probably won't enjoy it if you have to bankrupt your savings or remortgage your house to pay for it.
  • You will have to purchase other things to complete your observing equipment (eyepieces, finders, filters).

Generally, you should buy as much aperture as you can reasonably afford. But for most observers, the following sizes will be more than sufficient:

  • Refractors: 3 inches / 80 millimeters
  • Reflectors: 4 to 8 inches / 10 to 20 centimeters
  • Compound telescopes: 6 to 8 inches / 16 to 20 centimeters