If you do not have an excellent background in optics, complete Unit 8 (Lenses and Mirrors) before beginning this unit.

page 71. Questions 8 and 9 each have two answers.

page 73. At the top of the left-hand column. You are asked to take a set of exposures of a distant streetlight at this point, bracketing your exposures. Choose a streetlight that is distant enough to appear as almost a point source. You use the streetlight as an unknown "star."

For the star trail photographs, it is best to attach the camera solidly to a tripod, but if you do not have one, you may lay the camera on the ground (for overhead shots) or lean it against a rock, pointing it approximately to the area of the sky you wish to photograph. Using a cable release is strongly recommended so that the camera will not shake.

The text does not mention rewinding the film before removing it from the camera. Please do so.