Telescopes

Background Reading: Stars & Planets, p. 385-392 and the telescope-related sections of your ASTR 110 text.

Summary

Telescopes consist of two main optical components: the objective (in our telescopes it's a mirror) and the eyepiece (here a compound lens). The objective gathers the light and forms an image, which the eyepiece magnifies so you can see its detail. Every lens or mirror is described by its focal length and its diameter. The focal length is the distance from the lens or mirror at which the image of a distant object is formed. Objects that are not a long distance away will form an image farther from the mirror or lens. The angular magnification of a distant object in a telescope is the ratio of the objective focal length to the eyepiece focal length.

Report

Write your report based on the following questions and exercises.

  1. With one sentence describe each of the following telescope parts. Draw a line to the appropriate location on the figure.
    Base
    Tube
    Primary mirror
    Secondary mirror
    Spider (secondary mirror support
    Eyepiece
    Focuser
    Finder scope
    Filter
  2. In a sentence describe each of the following distinctions:

    1. Reflector vs. refractor telescope

    2. Alt-az vs. equatorial mount

  3. On our Dobsonian 8-inch telescopes, which part is 8 inches?

  4. Focal lengths and magnification

    1. Carefully take the eyepiece holder out of the focuser tube and point your telescope at a distant light source. Look down the focuser tube with your eye and sketch or describe what you see. Using a piece of paper, find the position where the primary mirror produces an image of the distant source. What is the approximate focal distance? Do the same measurement for a nearby light source. How has the focus position changed?

    2. Each eyepiece has a number in millimeters written on it. This is the focal length of the eyepiece. Which eyepiece will produce the greatest magnification? How much? What do you thihk the advantage and disadvantage of a greater magnification is when observing with a telescope? What gets smaller when you use a higher magnification?

    3. Now hold the eyepiece in one hand while you look through it and try to magnify the image of a distant light formed by the primary mirror. You'll have to move the eyepiece and your eye around until you find the image, since the image is beyond the range of the focuser tube. Describe the differences you see, with a sketch, and explain why in terms of the eyepiece properties.

mickey@ifa.hawaii.edu

 
 
 

Last modified: January 20, 2005
http://www.ifa.hawaii.edu/users/mickey/ASTR110L_S05/Telescopes.html