Physics/Astronomy Activity: Compiled by C. Erkal, UT Martin

This activity is adopted from “the Space Based Astronomy, Teachers Guide” and provides instructions about building a refractor telescope (NASA, 1994).

Material:

Mailing tubes (one inside tube, one outside tube-telescoping), Styrofoam tray, rubber cement, some lenses, metric ruler, razor blade knife, cutting surface, marker pen, fine grade sandpaper.

Safety Precaution:

Cutting knife should only be used during the presence og your teacher, and should be collected by your teacher after the work. NEVER LOOK AT THE SUN WITH YOUR TELESCOPE. IRREVERSIBLE DAMAGE WILL RESULT!

Procedure to Build:

1) Use the outside tube (larger diameter) to cut a short section (will be a circle) from the end to be used for tracing only. You will use this circle to trace a circle using its inside. Place the circle on a Styrofoam tray and using a marker pen, trace the inside circle. Repeat this two more times, therefore producing three different non-overlapping circles.

2) Lay the objective lens (larger one) in the center of each one of the three circles, and trace the lens’ outline on the circle. Now you produced three rings; outside circle is determined by the tube, inside circle is determined by the lens’ outline.

3) Place the Styrofoam on a safe cutting surface. Using extra care, cut out one ring so than when you cut the inside of the ring, the hole is slightly smaller than the diameter of the lens. Call this middle ring.

4) Before cutting the other two rings, draw smaller circles inside them approximately equal to 7/8 ths of the diameter of the large lens. Cut out the rings using this new inside circle along with the already marked outside circle. Call these two rings outer rings.

5) Coat both sides of the middle ring with rubber cement and let it dry a little. Coat just one side of the two other rings with cement and let it dry.

6) Insert the objective lens into the middle ring and press the other rings to either sides. Be careful to align the circles properly. Since the outer rings have have smaller diameters than the lens, the lens is firmly held in place. You have completed the objective lens mounting assembly.

7) Repeat the above steps with inside tube and use the smaller lens (eyepiece). Use the small styrofoam tray (thinner) since it is a better match for the eyepiece.

8) Lay the fine sandpaper on a flat surface and gradually sand the edges of each lens completed mounting assembly to make them smooth. Stop sanding when the assemblies are just larger than the inside diameter of the corresponding tube.

9) Do not insert the lenses yet. Hold the two lens assemblies up and look through the lenses by placing the smaller lens (eyepiece) near you eye. Adjust their distance, while looking through both, until an far away image comes into a focus. Roughly, measure the distance between the two lenses when you capture a clear image of any object in class. Write the distance here ________cm.

10) Cut a segment from the outside and the inside tube that together equal 1.5 times the distance you just measured. Use sandpaper to smooth any rough edges.

11) Carefully, so as not to smudge the lenses, insert the objective lens (larger one) assembly into on end of the outside tube, and the eyepiece (smaller one) into the inside tube. Slip the inside tube into the outside tube so that the lenses are at opposite ends. Look through the eyepiece to focus on a distant object by simply sliding the tube, thereby changing the lens separation.

ACTIVITY: Determination of Magnification

You will first make a determination of the magnification of your telescope. Slide the inner tube from the outer one. Bring the single lens (eyepiece) assembly in front of a window by holding the lens end in front of a white piece of paper, form the image of outside on the paper. You may need to change the distance between the lens and paper until you form a focused image. Measure the distance from the lens to the image and record here. Repeat the same procedure for objective lens.

Distance of image to lens (this is also the focal length of your eyepiece lens)_______cm

Distance of image to lens (this is also the focal length of your objective lens)_______cm

To find the magnification, simply divide objective focal length by the eyepiece focal length. Record here, Magnification ____________

Compare yours with other in the class.

Observing distant/nearby objects:

Observe the nearby street signs, other signs around you, license plates of nearby cars in the lot.

Observe also close objects such as things in your classroom, and record what you observed.

What do you notice common in all of your observations?

Observing Moon:

During half moon, observe the moons surface with your telescope. Do you see part of the moon that is otherwise dark to naked eye?

What are your observations about the Moon? Record and bring to class for discussion.