The student will learn to identify the parts, functions, magnification, and light gathering power of the classroom refracting telescopes, and learn how to operate the classroom telescopes.


  1. Sketch the refracting telescope. Label the parts: objective lens, eyepiece, focus knob, telescope tube, viewfinder, declination and right ascension fine-adjustment knobs, declination and right ascension lock levers. (When finished, check your answers with the key.)

  2. Give the function for each part labeled:
    Objective Lens 
    Focus Knob 
    Fine-Adjustment Knobs 
    Lock Levers 

  3. Draw a schematic of the telescope and show the path that light takes, from the objective lens to your eye.

  4. The magnification of the telescope:
    1. What is the focal length (fobj ) of the objective lens of the telescope?

    2. What is the focal length (feye) of the eyepiece of the telescope?

    3. What is the magnification of this telescope using this eyepiece?
        magnification = fobj/feye =

    4. What would be the magnification of the refracting telescope if you used an eyepiece with feye = 10 mm?
        magnification =

    5. In words, summarize how we determine the magnification of a telescope.

  5. How does the light gathering power of the telescope compare to that of the Keck telescopes? To do this exercise you will need to measure or find the diameter of the objective lens of each telescope and then ratio the squares of the two diameters, since the area of a circle depends on the radius squared (since we are taking a ratio, the "2" in "2*radius = diameter" cancels).

    The 8-meter Keck telescope in Hawaii is one of the largest in the world. Compare its light gathering power to our telescope (make sure you use the same units for all quantities -- 1 meter = 100 centimeters).

© 2003 Weber State University
Revised: 24 April, 2003