PURPOSE: To illustratre how an astronomical telescope works.

DESCRIPTION: The TV camera functions as the eye, so what the eye sees can be viewed on a monitor or the rear projection screen. At the left below is a direct view of the object scale by the video camera as displayed on the monitor. A 10 cm or a 5 cm convex lens acts as the eyepiece and a 20 cm convex lens is the objective lens for the telescope, with the TV camera focused at infinity (relaxed eye). The rule is positioned across the lecture hall (at infinity) from the telescope. The image is seen to be inverted and to have a magnification M=f(ob)/f(eye). The magnification can be checked by viewing a two-meter stick with the TV camera directly (left) and then with the telescope. The image with a 10 cm eyepiece has a magnification of 20/10=2, seen in the center picture. The image with a 5 cm eyepiece has a magnification of 20/5=4, seen in the picture at the right.

To set up in practice, set the camera focus and lens positions and move the objective lens until the image is seen by the camera.

SUGGESTIONS:

See Question of the Week #124 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.)

EQUIPMENT: B&W TV camera eye, 5cm and 10cm focusing lenses on optical rail for eyepieces, large diameter 20cm lens for objective lens, two-meter rule for object, and TV monitor.

SETUP TIME: 5 min.