Everyone needs to get a telescope kit from one of the instructors at the start of class, or before. One per customer. These are also available commercially from Learning Technologies, Inc., who sell all sorts of interesting and economical astronomy equipment.

You will build a simple, two-lens refracting telescope. The optics are described in, for example, Section 35-7 of "Fundamentals of Physics, Sixth Edition" by Halliday, Resnick, and Walker. The telescope focuses distant objects by creating an image with the first lens at its focal point, and then using the second lens to make the rays from this image parallel again. The trick is to adjust the distance between the two lenses so that it equals the sum of the focal length fOB of the "front" lens, called the "objective", and the focal length fEY of the "eyepiece" lens. The magnification of the telescope should then equal fOB/fEY. Higher magnification comes from having longer objective focal lengths, and/or shorter eyepiece focal lengths.

First, measure the focal lengths of the two lenses in your kit. It is easiest to do this by forming the image of a distance object projected onto some sort of "screen". (A blank sheet of paper does nicely.) For a distant object, the sun works very well, assuming the sky is clear. Adjust the distance between the lens and the screen until the image is focussed most sharply. Write down the focal lengths of the two lenses. What do you expect the telescope magnification to be? How long will the telescope be when it is in focus?

Using a cloth to hold the lenses to avoid smudges, carefully assemble the telescope as follows:

1. Insert the large lens into the red plastic holder, with the curved side facing out. Place the flat cardboard washer inside the holder, against the flat side of the lens.
2. Slip this assembly over the larger diameter cardboard tube. Make sure the fit is snug, with the lens firmly against the tube and washer.
3. Push the small lens into the grey foam holder. Insert the small cardboard washer into the foam, up against the flat side of the lens.
4. Insert the foam holder into the smaller diameter cardboard tube. It is important that the flat side of the lens be facing out, and be perpendicular to the axis of the tube.

Your telescope is now assembled. Look through it at some distant object and adjust the sliding tubes until the object is in focus. How does the distance between the two lenses compare to what you expect?

Consider the features of your telescope:

• Estimate the Light Gathering Power (LGP) of your telescope relative to your eye.
• Estimate the improvement in the diffraction limited angle of your telescope relative to your eye.
• Estimate the magnification of your telescope.How does this compare to what you expect?

The last one might be tricky. Try to find something nearby on campus that you can just read using the telescope. Estimate your distance to it using the campus map. Then, go out to it, and see how close you have to be to read it with your naked eye, and measure that distance. The ratio can be interpreted as the magnification.

Your telescope is good enough to discern that Jupiter is indeed a planet, not a star. You should also be able to see Jupiter's moons, although you'll probably need something to hold the telescope steady.