Labs from YSI 95 :

Rich Kron
CARA Yerkes Summer Institute, August 1995

This is meant to be given to the students.


In this lab, we will build a telescope and quantify its magnification and field-of-view, and we will observe the phenomenon of "seeing" and consider ways to get sharper images. The project includes discovering that there is a maximum useful magnification and that smaller telescopes can sometimes out-perform larger telescopes.


1) Use a simple lens to project a scene onto a ground-glass screen.

What is the focal length of the simple lens? _____________

Why is the image up-side down? Note that different lenses give different image sizes and image brightnesses. Why? Examine the image with a magnifying lens. Then, remove the ground-glass screen.

2) You have just built a model of the 40-inch refractor.

What is the focal length of the 40-inch refractor? ____________

By what factor would the size of an image of the Moon differ, as seen by these two lenses? ____________

3) A Galilean telescope uses a negative lens for the eyepiece. A Keplerian telescope (also called an astronomical telescope) uses a positive lens for the eyepiece. Compare magnifications and fields-of-view qualitatively. What are the advantages of each telescope design?

4) Set up the target. It consists of a contact print of the Moon taken with the 40-inch refractor. The circles are marked with their diameters in inches.

The distance between the target and the telescopes is made equal to the focal length of the 40-inch refractor. When viewed from this distance, the apparent size of the picture of the Moon is identical to the apparent size of the real Moon in the sky (since the contact print is a photograph made with the same telescope).

5) Look at the target with the telescopes. Compare the size of the Moon with one unaided eye with the size of the circles with the other eye looking through one of the telescopes. Hence, estimate the magnification for that telescope.

telescope:  Galilean        magnification:  ______________
telescope: Keplerian magnification: ______________
telescope: finder magnification: ______________

Galileo had telescopes like the Galilean one we have constructed. Therefore, he could have seen what we see in the target with that telescope. One of Galileo's drawings of the Moon is available for comparison.

6) The field-of-view for each telescope can be measured in terms of the diameter of the Moon. The figure at the end of the write-up shows how this is done. In general, the larger the magnification, the smaller the field-of-view.

telescope:  Galilean    field-of-view (degrees):  ______________
telescope: Keplerian field-of-view (degrees): ______________
telescope: finder field-of-view (degrees): ______________

7) Now we point one of the MAS telescopes at some distant object, such as a tree on the golf course. First we'll use a 21 mm eyepiece (that number is the focal length of the eyepiece), and then we'll use a 12 mm eyepiece. What is the magnification with each of the two eyepieces?

magnification with 21 mm eyepiece: ____________
magnification with 12 mm eyepiece: ____________

This demonstration is supposed to show that increasing the magnification will sometimes not help to reveal any further detail. (The effect of shimmering motion of distant objects is due to light being refracted in slightly different angles. This happens because some of the air along the line-of-sight is at a slightly different temperature than other air. The effect is also visible when you look through a telescope at the Moon or anything else; astronomers call this effect seeing. A night with "good seeing" means that the images of stars are very sharp, even with high magnification.)

Put the 21 mm eyepiece back into the MAS telescope, and the 12 mm eyepiece in Prof. Kron's Famous Telescope. These two telescopes have about the same magnification. Aim them at the same target to verify this. Which one gives the clearest image? This demonstration is supposed to show that larger telescopes are not always better. Why not?


  1. What is the main purpose of telescopes?
  2. What is the advantage of telescopes with larger diameters?
  3. What is meant by "field of view?"
  4. If magnification can be changed just by using another eyepiece, what is the point of building bigger telescopes?
  5. What causes "seeing," and what steps could you take to make the effect smaller?

Important Disclaimers and Caveats: