1. Aberrations

2. Spherical Aberration Problem: Lenses or mirrors whose faces are curved like part of a sphere are cheapest & easiest to make They refract/reflect light entering at the edges more than the center. Aberration: Light is not all focused to the same point Images are distorted…a “fish-eye” effect Solution • make the surfaces a parabolic shape $$$ • (Lenses only) Combine convex & concave lenses to make a compound lens (Image will be dimmer)

3. Chromatic Aberration Problem: Chromatic aberration is when a lens acts as a prism Aberration: Different colors focus at different places. The images become blurred and possibly fringed with rainbows Solutions: • Make the focal length as long as possible (This is why telescopes are so long) • Use a compound lens made of two different types of glass (Image will be dimmer)

4. Sag Problem: Lenses can only be supported at edges and glass flows. Over time gravity will warp the shape of the lens, it will sag…. The bigger the lens is the greater the problem is. This limits how big a objective lens can be. Aberration: Sagging shape causes distortion of the image No known solution! But… Sag affects mirrors much less than lenses. Because you can support them from below

5. Imperfections in the glass Problem: Specks or bubbles in a lens Aberration: spots and distortions in the image. Solution: use “optical glass” with almost no imperfections. $$$