An error (1/50th the thickness of a human hair) in the curvature of the main mirror caused major focusing problems for the Hubble Space Telescope. The 2.4 meter (94.5 inch) diameter mirror, was not able to focus all the light it collected to the correct point in the telescope. NASA suffered great embarrassment for this multi-million dollar mistake.

A reflecting telescope uses a mirror to gather light and focus the light toward the eyepiece. It usually consists of two mirrors: one large curved mirror at the end of the telescope to gather light and a smaller mirror used to direct the light to the eyepiece