How to Build Your Own Refracting Telescope

Select two lenses for your telescope. The one which will be the eyepiece should have a short focal length. The other which will be the objective lens should have a longer focal length, at least twice that of the eyepiece. The magnification of the telescope will be equal to the focal length of the objective lens divided by the focal length of the eyepiece.

Add the values of the focal lengths of your two lenses together. Your telescope should be a little longer than that sum. Another way to determine the length of your telescope is to simply hold one lens in each hand and hold the eyepiece to your eye and the objective lens at a distance. Look through both lenses at an object across the room and adjust the distance until the object comes into focus. Measure the distance between your two lenses and that is about how long your telescope should be.

Divide the length of your telescope in two and cut each cardboard tube to the halved length.

Slit one of the cardboard tubes down the side and trim about a quarter inch from the circumference. Then tape it back together so that it is slightly slimmer than the other tube. The smaller tube should slide easily into the larger tube without allowing in much light.

Trace the ends of each tube onto the flat cardboard and cut out the circles. Then trace the lenses onto the circles: the eyepiece on the smaller circle and the objective lens on the larger circle. Cut out those circles to make two rings.

Glue the lenses to the center of the cardboard rings then glue or tape the rings to the ends of the tubes.

Insert the open end of the smaller tube into the open end of the bigger tube. The assembly of the telescope is now finished!

Look through the telescope at a far away object. Focus the telescope by sliding the two pieces together or apart to adjust its length.