PURPOSE: To demonstrate off-axis astigmatism with a spherical lens.

DESCRIPTION: A bright point source is focused onto a small circular aperture which acts as the source. The 50cm focal length convex lens then focuses this spot onto a distant screen. If the lens is rotated about 30 degrees it will produce off-axis astigmatism, which can be seen as follows: Keep the screen perpendicular to the optic axis while moving it along the optic axis to see (moving outward from the astigmatic lens from left to right in the photographs below) (1) the beam immediately following the lens, (2) the horizontal focus (a vertical line), (3) the waist, (4) the vertical focus (a horizontal line), and (5) the expanding beam following the second focal point.

SUGGESTIONS: Compare this to Demonstration O1-23: ASTIGMATISM - ON AXIS.

REFERENCES: (PIRA 6A65.34)

EQUIPMENT: Bright point source with condenser lens, 5cm focal length lens, 2-mm circular aperture object, and 50 cm focal length lens on optical rail cart, ground glass screen.

SETUP TIME: 5 min.