Build a Telescope!

Your telescope is a very simple design but also a classic.  Galileo’s telescope was very similar in design to the one that you will build (though the materials were very different).  Before assembling your scope you will test the properties of the lenses.  First check that you have all of the necessary parts:

Large Lens (Objective)                         Small Lens (Eyepiece or Ocular)

Two-Part Cardboard Tube                               Very Small Cardboard Tube (Spacer)

Red Plastic Sleeve                                            Foam Cylinder

Cardboard Washer (you will cut out)

Testing the Lenses

Before assembling check each lens for its ability to form images.  The large lens will go on the end of the telescope facing the object and is referred to as the objective lens or simply the objective.   The small lens will go on the end of the telescope facing your eye and is referred to as the ocular or simply the eyepiece.

o Each lens has a shape called plano-convex.  This means that one side of the lens is convex or curved “outward” and the other is flat or “plane shaped”.  This is obvious for the ocular but not so much for the objective.  Confirm that the objective has this shape by placing either side on a flat surface and checking gently to see if it rocks back and forth on that surface.  You should find that it rocks only on one side – the convex side.

o Try magnifying the grid with each lens and place the pane side either down or up (in other words let light travel in either direction through each lens).  Also try tilting the lens relative to the line of sight.  You will find that the magnified squares will have a distorted shape depending on the tilt of the lens and the side facing the object.  Record specific observations describing how the distortion is related to the orientation of the lens:

o Estimate the magnification factor of each lens by magnifying the grid:


ocular lens, M =                                    objective lens, M =

o Project an image of the overhead lights or the bright outside doorway onto a “screen”.  As you did when magnifying the grid, experiment with the orientation of the lens and note any distortions in the image.  You will find that the projected image will have a distorted shape depending on the tilt of the lens and the side facing the object.  Record specific observations describing how the distortion is related to the orientation of the lens:

o Compare the size of the real images projected by each lens.  Which lens produces a larger real image (projected on a screen)?  Experiment and determine how much bigger – how many times larger in width or height?

o While projecting an image onto a screen try blocking the light from one half of the lens with the edge of a piece of paper or cardboard.  Does this change the size of the image?  Does this cover up part of the image?  Describe the result below noting any changes in the observed images:

Putting it Together

Now that you have a good idea about the properties of the lenses you are ready to put the telescope together.  Refer to the diagram and follow the steps below:

1. The cardboard washer forms what is called a stop (as in “f-stop” if you are familiar with cameras).  Its purpose is to stop light from passing through the outer edges of the lens.  Place the cardboard washer on the flat side of the objective lens.  Fit the convex (curved) side of the lens snugly against the front of the outer tube, making sure it is positioned perpendicular to and centered on the tube.  Slip the plastic cap over this end of the tube so that the lens and cardboard washer are sandwiched together and held firmly in place.
2. Push the ocular lens into the foam lens holder.  Slide the spacer into the foam holder such that it pushes against the flat part of the ocular.  Push the spacer into the holder just far enough so the end of the spacer is flat with the end of the foam holder.
3. With the curved side of the lens facing toward the large lens, slide the foam holder into the end of the smaller of the sliding tubes.  The foam holder should be flat with the end of the tube.
4. Put your name on the cardboard tube.  Customize the telescope by adding decoration, designs, wording, etc. to the tubes.  Have fun, be creative!

     cap          objective                                                                                  foam           spacer

               washer                                 cardboard tubes                                              ocular   

Using the Telescope

Aim the scope, close one eye, and focus by sliding the smaller tube and adjusting the distance between the two lenses.  You should be able to focus a sharp image of the object you are viewing (unless the object is too close to the scope).  Try the following experiments:

o Try focusing on objects that are different distances away from the telescope.  In order to focus on objects nearer to the telescope what must be done to the distance between the lenses?

o What do you think causes the image to be inverted (upside down)?  Is there any way to view an image that is not inverted?

o Experiment with the stop – try viewing something with fine details, like printed letters, with and without the cardboard washer in place.  What effect does it have?  Does it improve the image in any way?  Does it lessen the quality of the image in anyway?

o Experiment with the eyepiece orientation – try viewing something with fine details, like printed letters, with the foam holder and eyepiece reversed or not reversed.  Describe any differences in the observed image.