Types of Telescopes

Chapter index in this window —   — Chapter index in separate window -- Please support this website

This material (including images) is copyrighted!. See my copyright notice for fair use practices. Select the photographs to display the original source in another window.

There are two basic types of telescopes, refractors and reflectors. The part of the telescope that gathers the light, called the objective, determines the type of telescope. A refractor telescope uses a glass lens as its objective. The glass lens is at the front of the telescope and light is bent (refracted) as it passes through the lens. A reflector telescope uses a mirror as its objective. The mirror is close to the rear of the telescope and light is bounced off (reflected) as it strikes the mirror.

Refractor Telescopes

The refractor telescope uses a lens to gather and focus light. The first telescopes built were refractors. The small telescopes sold in department stores are refractors, as well as, those used for rifle scopes.

Advantages

  1. Refractor telescopes are rugged. After the initial alignment, their optical system is more resistant to misalignment than the reflector telescopes.
  2. The glass surface inside the tube is sealed from the atmosphere so it rarely needs cleaning.
  3. Since the tube is closed off from the outside, air currents and effects due to changing temperatures are eliminated. This means that the images are steadier and sharper than those from a reflector telescope of the same size.

light path for refractor telescope

Though excellent refractors are still made, the disadvantages of the refractor telescope have blocked the construction of very large refractors for use in astronomical research.

Disadvantages

  1. All refractors suffer from an effect called chromatic aberration (``color deviation or distortion'') that produces a rainbow of colors around the image. Because of the wave nature of light, the longer wavelength light (redder colors) is bent less than the shorter wavelength light (bluer colors) as it passes through the lens. This is used in prisms to produce pretty rainbows, but can it ruin an image!
    different colors focus at different points

    There a couple of ways to reduce chromatic aberration. One way uses multiple compensating lenses to counteract chromatic aberration. The other way uses a very long objective focal length (distance between the focus and the objective) to minimize the effect. This is why the early refracting telescopes were made very long.
  2. How well the light passes through the lens varies with the wavelength of the light. Ultraviolet light does not pass through the lens at all.
  3. How well the light passes through decreases as the thickness of the lens increases.
  4. It is difficult to make a glass lens with no imperfections inside the lens and with a perfect curvature on both sides of the lens.
  5. The objective lens can be supported only at the ends. The glass lens will sag under its own weight.
Because of these disadvantages, the largest refractor telescope built is the one at Yerkes Observatory. It has an objective 1.02 meters (40 inches) across at one end of a 19.2-meter (63 feet) tube. The two largest refractors are shown below. The first picture is the 40-inch refractor at Yerkes Observatory. Clicking on the image will lead you to more information about this telescope (and better pictures!) in another window. The second picture shows an astronomer (Kyle Cudworth) next to the objective to give you an idea of the size of the telescope. Notice the size of the people in the first picture! The third picture is the 0.91-meter (36-inch) refractor at Lick Observatory. Notice the astronomer at the lower left. Clicking on the image will take you to the Lick Observatory webpage about the telescope in another window. The last picture is E.E. Barnard at the eyepiece of the Lick 36-inch.

40-inch telescope tour astronomer at objective

photo of 36-inch Lick refractor E E Barnard and Lick 36-inch.

previousGo back to previous section -- next Go to next section

Go to Astronomy Notes home -- Please support this website

last updated: January 7, 2011

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel