How Telescopes Work


This is a set of filters for viewing, including a light pollution filter (left) and colored filters for enhancing contrast in planetary images.

 What type of telescope should I buy?

The type of telescope you should buy depends on the observing you want to do. Many amateur astronomers own more than one telescope, each specialized for a different type of observing. If you are a beginner, though, you might want to look for a versatile model that will work for several different activities. Each type has advantages and disadvantages with respect to optical quality, mechanical performance, maintenance, ease of use and price. Generally, refractors are good for lunar and planetary observing, while reflectors are good for deep-sky observing. Compound telescopes are good general observing instruments.


How big should my telescope be?

The telescope's ability to collect light is directly related to the size or diameter (aperture) of the objective lens or primary mirror. Generally, the bigger the lens or mirror, the more light the telescope collects and brings to focus, and the brighter the final image. Aperture is probably the most important consideration when buying a telescope, but it is not the only consideration. You want to buy as much aperture as you can reasonably afford; however, avoid "aperture fever." You should also consider other factors like size, weight, storage space, portability and sky conditions. The biggest telescope is not always the best one for you!

How powerful should my telescope be?

This consideration is perhaps the most misleading to novice telescope buyers. Often, manufacturers of "cheap, department store" telescopes will display "200x power or more" on the boxes of their products. The magnification or power has little to do with the optical performance of the telescope, and is not a primary consideration. The telescope's ability to magnify an image depends upon the combination of the lenses used, usually a long focal length objective lens or primary mirror in combination with a short focal length eyepiece. As the magnification of an image increases, the field of view and the brightness of the image decrease. A general rule about magnification is that the telescope's maximum magnification is 40x to 60x (average = 50x) per inch of aperture. Since any magnification can be achieved for almost any telescope by using different eyepieces, aperture becomes a more important feature than magnification. Besides that, most astronomical objects are best viewed on a low magnification or power to gather the most light possible.

What kind of telescope would be good for a child?

Before you buy a telescope for your child, take him or her out skywatching for some time. Let them learn their way around the night sky by identifying the constellations for each season.

Some good telescopes for children:

  • Small refractors - many children like to see the moon and planets. These telescopes provide good views of these objects.
  • Rich-field reflectors - these telescopes provide bright, low-power, wide-field images of many types of objects. They are usually easy to aim, and the wide-field view makes them easy to use when searching for objects.

Regardless of the type of telescope that you buy, consider that children should have a lightweight telescope that is easy to carry, set up and use. Consider the following in a mount:

  • The mount should be not be too high for a child, so he or she can view through the eyepiece while standing.
  • The mount should be sturdy so that it does not vibrate.
  • The mount should have a low center of gravity so that it does not tip over easily.

Finally, remember that a child's first telescope need not be the only telescope they will ever use. They should be able to use it by themselves and enjoy it. Later, they can graduate to another, more advanced model.

What is the f-number?

The focal ratio or f/number relates to the brightness of the image and the width of the field of view. The focal ratio is the focal length of the objective lens or primary mirror divided by the aperture. The focal ratio concept comes from the camera world, where a small focal ratio meant a short exposure time for the film, and was said to be "fast." Although the same is true for a telescope, if a "fast" and a "slow" telescope are compared at the same magnification for visual rather than photographic viewing, both telescopes will have the same quality image. Generally, the following information about focal ratios should be considered:

  • f/10 or higher - good for observing the moon, planets and double stars (high power)
  • f/8 - good for all-around viewing
  • f/6 or lower - good for viewing deep-sky objects (low power)

What type of mount should I have for my telescope?

The type of mount that you use will depend upon your observing needs. The two types are alt-azimuth and equatorial. Alt-azimuth mounts are simpler, easy to use and cheaper than equatorial mounts. You set the horizontal and vertical coordinates of the object when sighting it, and then lock it in. You must readjust the horizontal and vertical coordinates as the object moves out of the field of view due to the Earth's rotation.

Equatorial mounts are more complicated, require some set-up, and are more expensive than alt-azimuth mounts. Equatorial mounts must be aligned with the Earth's poles. They often have counterweights to balance the weight of the telescope. Once the mount is aligned with the poles, you can set the coordinates of the target object (right ascension, declination). An equatorial mount will track an object's motion across the sky, and makes it easier to keep an object in the field of view. If you wish to do astrophotography, an equatorial mount is necessary.

How much does a telescope cost?

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

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

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

  • refractors: about 3 inches / 80 millimeters
  • reflectors: 4 to 8 inches / 10 to 20 centimeters
  • compound telescopes: 6 to 8 inches / 16 to 20 centimeters

How many eyepieces do I need?

Next to the telescope itself and the mount, the eyepieces will be your most important purchase. Most telescopes come with one eyepiece (low power), but some telescopes come with none. Therefore, you may have to purchase eyepieces so that you can vary the magnification of your telescope.

Eyepieces come in many designs:

  • Huygens
  • Ramsden
  • Orthoscopic
  • Kellner and RKE
  • Erfle
  • Plossl
  • Nagler
  • Barlow (used in combination with another eyepiece to increase magnification 2 to 3 times)

See How They Work for a discussion of each type. Generally, you should have a low-power eyepiece, and some intermediate or high-power eyepiece. Remember that the telescope's maximum magnification is 40x to 60x per inch of aperture.

What does a finderscope do?

Finders are devices used to help aim the telescope at its target, similar to the sights on a rifle. Some finders come standard on telescopes, while others are sold separately. Finders can come in three basic types:

  • peep sights - notches or circles that allow you to line up the target.
  • reflex sights - a mirror box that shows the sky and illuminates the target with a red LED diode spot similar to a laser sight on a gun.
  • telescope sight - a small, low magnification (5x to 10x) telescope mounted on the side with a cross hair reticle, like a telescopic sight on a rifle; unlike the other two finders, this type presents an upside-down image to your eye.

A telescope turns the image upside down. Should I buy a device to turn it right-side up?

This is not a problem when observing astronomical objects, but is rather annoying for terrestrial observations like bird-watching. To correct this, an erecting prism or Porro prism, either straight through or angled, is used to turn the image right side-up.

I live in a city with lots of lights. Can I still observe the sky? Yes, you can probably still get good views of the moon and planets from a city park. Try to position your telescope so that trees or buildings can block out major sources of light. You may also want to consider buying a light pollution filter to block the wavelengths of light emitted by street lights.

Can I observe the sun with my telescope?

Yes, but NEVER LOOK DIRECTLY AT THE SUN THROUGH A TELESCOPE! To safely view the sun, you should use a projection system or solar filters that fit over the end of a telescope to block most of the light from the sun. They are made of aluminum. (See Observing the Sun for details).

What does a color filter do for me?

Filters are pieces of glass or plastic that you place in the barrel of an eyepiece to restrict the wavelengths of light that come through in the image. Filters can be used to:

  • enhance the viewing of faint sky objects in light-polluted skies
  • enhance the contrast of fine features and details on the moon and planets

What do I need if I want to take pictures of the moon, planets and stars?

Photographs of deep-sky objects, the moon and planets can be taken with conventional film cameras, CCD devices/digital cameras, and even video camcorders. Photography can be done without a telescope, with the camera "piggybacked" onto the telescope (i.e. telescope is used to guide the camera) or with the telescope as the camera's lens (prime focus photography). If you wish to do astrophotography using the prime focus method, you will need the following:

  • 35-mm camera (with manual capability), video camcorder, or CCD device/digital camera
  • camera or "T" adapter
  • manual cable release for 35-mm camera
  • off-axis guider
  • laptop computer or personal digital assistant (PDA) (for CCD use only)

The camera or CCD device acquires the image. The camera or T-adapter hooks the camera to the telescope's eyepiece holder. The off-axis guider is a combination camera adapter and eyepiece holder, letting you guide the telescope's movement with the object while acquiring the image with the camera. The off-axis guider splits the light coming from the object so that you can look at the object, usually with an illuminated reticle eyepiece, and the camera can capture the light on film/CCD. The laptop or PDA has the software to acquire, display and store the image. Image processing is usually done later, away from the observing site.

Besides my telescope, what else do I need for observing?

First of all, dress warmly when you go out at night! When the sun goes down, temperatures fall and moisture condenses. You would be surprised how cold you can feel even on a hot, summer night. I have been observing on summer nights in North Carolina and needed a sweater and jacket even when the outside temperature is 60 degrees Fahrenheit. If you are not comfortable, then you will not enjoy observing.

Next, you will need a red flashlight so that you can see things while keeping your eyes adapted to the dark. If you do not have one, cut a piece of brown paper bag, place it over the end of the light and fasten it with a rubber band.

Typical set of observing supplies.

Other supplies include binoculars for sweeping the skies, filters, star charts, field guides and eyepieces. Finally, do not forget to take a snack and something to drink. You would be surprised at how hungry you can get during hours of observing.

Can I do real science with my telescope?

Yes, many amateur astronomers contribute to the science of astronomy. Amateurs have much more time to spend on the "little things" than professionals do. Also, the price of large aperture telescopes has come down so much over the years that many amateurs now have equipment that rivals the stuff used by professional astronomers. Amateurs can contribute in many areas, such as variable star observing, meteor counting and comet hunting.